the human cornea:

THE HUMAN CORNEA:A LIGHT AND ELECTRON MICROSCOPIC STUDY

OF THE NORMAL CORNEA AND ITSALTERATIONS IN VARIOUS DYSTROPHIES*

BY John W. McTigue, M.D.

THE DEVELOPMENT OF THE ELECTRON MICROSCOPE in the twentieth cen-

tury has led to a reevaluation of the normal and pathologically alteredtissue in all systems and has resulted in a vast new field with a greaterstorehouse of information on both normal structure and changes innormal structure. This information is now beginning to alter, or atleast affect, some of our ideas about disease processes and the waysin which they may begin. Such an increase in our knowledge aboutthese changes may eventually result in a more rational approach toour methods of therapy.The world of electron microscopy has developed its own special lan-

guage which, in many instances, may appear to confuse or obscurebasic changes which were first described by the anatomists and pathol-ogists of the past. In spite of this apparent difference in language,however, these later studies have often pointed out the accuracy ofthe observations made by many investigators using the compoundmicroscopes of 50 or more years ago.Much of the information on the studies of ocular tissues with the

use of the electron microscope has been published throughout thegeneral scientific literature as well as in the ophthalmic literature.The work which relates particularly to the cornea, the subject of thispaper, was begun by such researchers as Jakus,25-32 Smelser,6'Kayes and Holmberg,36,37 Garron and Feeney12'20'21 Teng,66-69 andothers. 3839'43-45 47 48 5 55-59,62, 77 The purpose of this study is to seekadditional insight into the structure of the normal cornea and its

*From the Ophthalmic Pathology Branch, Armed Forces Institute of Pathology,and the Department of Ophthalmology, the George Washington University,Washington, D.C. This investigation was supported by a research contract, ProjectNo. DA-49-193-MD2680, from the Medical Research and Development Com-mand, U.S. Army, Washington, D.C., and by Research Grant NB-05575 from theNational Institute of Neurological Diseases and Blindness, Bethesda, Md.

TR. AM. OPHTH. Soc., vol. 65, 1967

alterations in certain corneal dystrophies, and thereby to help toamplify some of our ideas about disease processes in this transparentocular tissue.The material presented here begins with a reexamination of the

layers of the normal cornea. These layers are identified by electronmicroscopy as the epithelium; the basement membrane of the epi-thelium; Bowman's layer; the stroma; Descemet's membrane (thebasement membrane of the endothelium); and the endothelium (meso-thelium) proper. From these observations the stroma appears to beseparable into two histologic layers which may also be observedclinically with the slit-lamp. Rather than regard the cornea in itsclassical arrangement as a five-layered structure, this report will con-sider six separate layers (the basement membrance of the epitheliumbeing the sixth layer) and, in addition, will regard the stroma asseparable into two sublayers. Following this material, recent examplesof some of the corneal dystrophies that have been examined will bedescribed in detail to show the type of changes that may occurseparately in the anterior, middle, or posterior parts of the cornea orsimultaneously in two or more of these parts.

Because this study is part of a continuing sampling procedure,which is not complete at this stage, and because of the volume ofmaterial which results from any electron microscopic study, themanner of presentation is a selective one. Nor have all dystrophicand degenerative disease changes been investigated at this time, sincesome of the diseases exhibiting the dystrophies are relatively un-common and have not been available for study as yet. Thus the avail-able material is presented as a series of examples of the various changeswhich can occur in any of the layers of the cornea or in any combina-tion of these layers in response to a disease process. An attempt ismade to correlate, wherever possible, these morphologic changes withthe sometimes transient pathologic alterations that can be observedclinically.

It is still not possible to point out the initiating factors in thesediseases. Only morphologic changes are described, and an attempt tosynthesize something about the pathogenetic mechanisms will bemade by combining these observations with existing clinical, histo-pathologic, and experimental information.

MATERIALS AND MIETHODS

The corneas examined in the present study were obtained either atthe time of surgical enucleation or during keratoplasty. In either

592 John W. McTigue

The Cornea and Its Alterations in Various Dystrophies

case the excised comeas were immediately placed into the fixative inthe operating room. The tissue was allowed to remain in the fixative(Dalton's solution) for 50 to 60 minutes, and was then dehydratedthrough increasing concentrations of alcohol and embedded in Eponas described by Luft.42 All sections for electron microscopy weretreated with one per cent uranyl acetate72 in 50 per cent ethanol for30 minutes and then examined with the electron microscope.7'13-'5'77In addition, 1.5-micron thick sections were made for the purposes oforientation and study with the light microscope, and these sectionswere stained with paraphenylenediamine.'0 All tissues were studiedby both light and electron microscopy.

In some instances, where the button from the opposite eye was notavailable for comparison, a single button or excised cornea was bi-sected so that one half could be prepared for more conventional lightmicroscopy (paraffin embedding, hematoxylin and eosin staining, andother special stains) and the other half for electron microscopy. Othercorneas were taken from the files of the Registry of OphthalmicPathology at the Armed Forces Institute of Pathology, Washington,D.C., to illustrate various stages of some of the dystrophies. Thesespecimens obtained from the Registry were all formalin-fixed andparaffin-embedded for conventional light microscopic study.The sampling of pathologically altered tissues presented here was

obtained from cases of corneal dystrophies, each of which was a well-defined example of the clinical entity described by other authorsunless there is some modifying statement to the contrary in the text.

RESULTS

The results are recorded as legends to the micrographs. Micronmarkers indicate one micron unless otherwise indicated.

DISCUSSION

Normal CorneaTraditionally, the cornea has been considered a five-layered struc-

ture.3 8'40'54 These layers are designated from the anterior to theposterior surface as the comeal epithelium, Bowman's membrane, thecorneal stroma, Descemet's membrane, and the endothelium or meso-thelium (Figures 1-4). This concept has been useful but does notreflect some of the more recent observations which have been reportedor which can now be made. For example, Bowman's membrane is nowclearly known to be a modified portion of the anterior stroma in whichkeratocytes are normally lacking. It is, therefore, better termed a layer

593

John W. McTigue

FE>

FIGURE 1

Light microscopic section from central portion of normal adult cornea. FE, theflattened cells of the superficial layers of the epithelium; CE, the cuboidal cellsof the deeper epithelium; BM, the basement membrane of the epithelium (almostimperceptible here); BL, Bowman's layer. Note the slight undulations of thesuperficial stromal lamellae. AFIP Neg. 65-12343, 1.5 micron, paraphenylene-

diamine, X 395.

..Z

FIGURE 2Light microscopic section from peripheral portion of normal human adultcornea. The epithelium thickens slightly toward the limbus. BM, the thickenedbasement membrane of the peripheral epithelium which apparently thinsmarkedly or ends at E; BL, Bowman's layer, which extends toward the limbusslightly beyond the thickened epithelial basement membrane. AFIP Neg.

65-12340, x 395.

594

- - . >C'¶.

a 'S

t..t "V. -, -

".-..--- -

* .

FIGURE 3

Light microscopic section of limbal area of normal human adult cornea. Theappearance of the cells in the deeper epithelium is similar to those seen inFigure 2. The basement membrane of the epithelium is either very thin orabsent in this area. The area of Bowman's layer, BL, is irregular and showsincreasing cell content as the sclera, SC, is approached. The stromal lamellaeare more undulating in appearance throughout the depth of the region shownhere. Blood vessels, BV, appear in the superficial stroma. AFIP Neg. 65-12341,

x 395.

FIGURE 4

Light microscopic section of central area of normal human adult cornea toshow the deep stroma. Note the very regular arrangement of these layers ofthe stroma as contrasted with those of the superficial stroma. Descemet's mem-brane, DM; the single layer of "endothelial" or mesothelial cells at EN; AC,

anterior chamber. AFIP Neg. 65-12342, x 395.

John W. AlcTiguethan a membrane,29'30'32 a name applied by light microscopists in theearlier days of histology. Bowman, in his studies, called this layer theanterior elastic lamina in contrast to the posterior elastic lamina ofDescemet. Descemet's membrane is now known to be completelydifferent in appearance, physical properties, reaction to injuries, andprobable origin. The endothelium is, in reality, a mesothelium. It doesnot resemble other endothelia which line the lutmina of blood vesselsor lymphatics but more closely approximates the cell lining of bodycavities such as the peritoneum, and, since its derivation is apparentlymesodermal, is more accurately called a mesothelium.

In recent years the concept of another layer, that of the basementmembrane of the corneal epithelium (Figures 1, 2, 8, 9) has becomemore widely appreciated.28-32'36,52,55,56,59'65-67 This layer is particularlywell brought out with special staining techniques such as the periodicacid-Schiff reaction (Figure 41) and with this stain it can often bedetected by light microscopy. Subsequent observations on patho-logic aberrations of this membrane have served to further support theconcept of a normal layer lying between the corneal epithelium andBowman's layer. Current cytologic terms as used in electron micro-scopy, together with recent electron microscopic observations, stronglysupport the designation of this basement membrane as an importantseparate layer.From these more recent observations, it has also become apparent

that Descemet's membrane is the basement membrane of the endo-thelium (mesothelium) (Figures 4 and 1.3-15). This idea is suipportedby current observations in cytology and by observations on pathologiccorneas in which a new Descemet's membrane-like layer is sometimesseen deposited beneath the normal endothelium, or its abnormal exten-sions into the angle, or onto the anterior surface of the iris stroma.A more modern concept of the structure of the cornea would

suggest the following layers: (1) the epithelium (Figure 6), witl(2) its basement membrane (Figure 7); (3) Bowman's layer, as amodification of the anterior stroma (Figure 7); (4) the stroma proper,consisting of collagenous fibrils in layers or lamellas, between whichlie the keratocytes (Figure 12); (5) Descemet's membrane-the largebasement membrane of the endothelium (Figure 13); and (6) theendothelium or mesothelium (Figure 14). To these layers one mightadd the tear film, which lies on the epithelial surface and has consider-able clinical significance in lubricating and wetting the surface of thecornea and, possibly, in assisting with the optical properties of thecornea. In recent years the tear film has been of more interest because

596

FIGURE 5

Electron micrograph showing the surface of the epithelium from a normalhuman cornea. A marked irregularity of the surface of these cells is seen.Between these irregularities on the surface lies a material which probably repre-

sents the mucinous residue of the tear film. X 14,500

FIGURE 6

A sample of corneal epithelium to show the tortuous intercellular spaces wheremany desmosome attachment plates are present (DE). Intracytoplasmic fila-ments typical of epithelial cells are present here in abundance as are clustersof granules interpreted as glycogen granules. N, nucleus of epithelial cell.

X 21,200

IL

FIGURE 7

Electron micrograph of the deep layers of the epithelium and superficial Bow-man's layer. The interdigitating epithelial cells are seen with many desmo-somes. Hemidesmosomes (free arrows) line the basal membranes of theepithelial cells. The basement membrane of the epithelial cells is seen as a thingray line at BM. Bowman's layer, BL, is seen here to consist of interminglingof delicate fibrillar material. X 15,900. Inset: Higher magnification of the areaof the basement membrane, BM. The hemidesmosomes of the epithelial cellsare more easily seen at HD. There is a lucent zone between the basal plasmamembrane of the cell and the extracellular basement membrane. The irregularlyarranged collagen fibrils (CO) of Bowman's layer are seen in both longitudinal

and cross section. These fibrils measure approximately 300 A x 47,800.

AR

The Cornea anal Its Alterations in Vlrious Dystrophies

FIGURE 8

An electron micrograph to show the "thick" basement membrane that is presentbetween epithelium and Bowman's layer at the periphery of the cornea.11,50This thick basement membrane zone shows many areas of density and lucencywhich are better seen in Figure 9. The epithelial cells send deep villous projec-tions into this basement membrane layer and hemidesmosomes are present alongthese villous extensions (free arrows). EP, epithelial cells; BNI, basement mem-

brane zone; BL, Bowman's layer. X 15,900

of the important part it plays in the widespread use of contact lenses.This film or layer is not usually demonstrable by the conventionalhistopathologic techniques, but may, in suitably fixed preparations, beidentified by a delicate flocculent layer on the surface of the epitheliumthat may be preserved in tissue prepared for electron microscopy(Figure 5).To these modern concepts of corneal structure, some further obser-

vations might be added, which, although perhaps not entirely new,may give us more insight into the structure of the cornea as seen byhistologic methods and at the same time help to clarify some observa-tions that may be easily made by conventional slit-lamp examinationof the cornea.For these observations we must turn to both current light and

.599

John W. AlcTigtie

FIGURE 9Higher mnagnification of lbasement meml)rane zone seen in Figuire 8. A largevillotus extension of the epithelial cell is seen on the left (V) and the honeycomiib-like configuration of the basement membrane material tlhrouighout this basenmentmemiibrane is clearly seen. Embedded within this basemiient memlbrane zone aremiany pectuliar fibrils, some) of which possess a pectuliar periodicity which hasbeen described only in the amphibian dermal basement memibrane zone. X 26,500

electron microscopic observations which show that there is a distinctdifference in the arrangement of stromal lamellas between approxi-mately the anterior third of the cornea and the posterior two-thirds.The anterior part of the corneal stroma (excluding Bowman's layer)is composed of collagenous fibrils which are arranged more obliquelyto each other than are those in the deeper corneal stroma (Figures1, 4, and 17). This somewhat surprising observation was illustratedby the earlier histologists,54 but they did not call attention to thissubdivision of the corneal stroma in their illustrations. This subdivi-sion appears to explain the difference in appearance between theanterior and posterior stroma in slit-lamp observations that can bereadily seen on all normal corneas in varying degrees. Beneath thereflecting zone, which represents Bowman's layer, there lies an anteriorband of corneal stroma showing a much more pronounced reticulation

600


2?,' * U * W 46t= ,1

FIGURE 10

A nerve btundle is present within thebasal epitlhelial layer of the corneal epi-thelitum. The netirites are denselypacked and possess no myelin sheaths.The nerve fiber buindle is separatedfrom the adjacent epithelial cells by the

usual intercellular space. X 31,900

FIGURE I 1

Another nerve fiber bundle within thebasal epithelial layer to show thebundle of neuirites lying between adja-cent epithelial cells. Neurotubules(NT) cutt in cross-section as well asmitochondria (MI) are clearly seenwithin these non-myelinated neurites.BMN4, basement membrane of the cor-

neal epithelitim.

than that of the deeper corneal stroma which shows a more delicateuniform relucency.

This histologic and clinical observation did not appear at first tohave any ready explanation, but an explanation did develop uponreexamination of the cornea from the phylogenetic and anatomicpoints of view. As Walls73 has pointed out, the corneal epthelium is anextension of the epithelium found on the conjunctiva, and this, togetherwith the conjunctival substantia propria (collagenous layer), isrepresentative of the skin of the head, which doubles back at themargins of the upper and lower eyelids to form their inner linings,the fornices, and continues over the front of the globe to fuse with

601

John W. McTigue

.,*- --,.:----. ... ... -; ---

FIGURE 12

A typical area of corneal stroma to show the body of adjacent keratocytes withtheir nuclei (N) and adjacent patches of homogeneous and fine fibrillar material(FI) often found in the vicinity of the keratocytes' plasma membrane in thenormal cornea. Collagen fibrils cut longitudinally are clearly seen at L and cutsomewhat obliquely at 0. Small focal patches of dense homogeneous and finefibrillar material resembling that which is seen near the keratocyte are found

throughout the corneal stroma (free arrows). X 15,900

the cornea at the limbus. At the limbus it appears on superficialexamination that only the epithelial layer continues over the cornealstroma. Closer observation, together with an understanding of thephylogenetic derivation of these layers, indicates that more than justepithelium continues over the corneal surface. The conjunctival sub-stantia propria, which is a representative of the dermal substantiapropria, also continues as part of the corneal stroma. This dermal sub-stantia propria contribution is represented by the superficial layer ofcornea which possesses lamellas running more obliquely to each otherthan those in the deeper layers (Figures 1 and 4). A well-defined zoneis then established in the anterior corneal stroma by both histologicand clinical examination. In the human, this junction may be so wellblended that a precise line of histologic subdivision cannot be made.In lower animals, however, the division is quite clear. Walls pointed

602


out that under the slit-lamp some indication of this anterior contribu-tion of the dermis to the corneal stroma might be visualized, but thathistologic study did not make this contribution clear. The line ofdemarcation is, however, clearly observed in fishes60,"7 in which thelayers of the fresh cornea could be readily separated along this boun-dary within the corneal stroma. Walls suggested that this incomplete-ness of fusion between the original cornea and its dermal additionmade it easy for many fishes and some amphibians to produce "secon-dary" spectacles which were a regression to the anatomical conditionfound in the lamprey.A phylogenetic concept of the formation of the corneal epithelium

and its probable dermal contribution to the stroma stresses thisorigin and allows us to consider the possibility of relatingdermal-type problems to ophthalmic pathology. The transition ofcorneal epithelium to epidermidalization under conditions of dryingand chronic irritation is well known, as is the involvement of theanterior third of corneal stroma in some general and local skin dis-orders. The cornea may, therefore, be separated into two major divi-sions, the dermal and the "mesodermal" portions. The dermal part(i.e., derived from both parts of the skin) consists of epithelium, epi-thelial basement membrane, Bowman's layer, and superficial stroma,while the "mesodermal" part is made up of the deep stroma, Descemet'smembrane, and the endothelium (mesothelium) (Figure 17).As will be pointed out in this paper, the conditions of Meesman's

dystrophy, Reis-Biicklers' dystrophy, and keratoconus apparently beginas a disturbance in the area of the dermal contribution to the cornea.Franceschetti16 has pointed to a relationship between keratoconousand diseases having an allergic character and reported a statisticallyproven correlation with vernal conjunctivitis. Keratoconus may also befound with atopic dermatitis"9 and has been known to be associatedwith mongolism and with other congenital eye defects, such as retinitispigmentosa and infantile tapetoretinal degeneration.16

Lattice, macular, and granular dystrophies apparently traverse thedermal-mesodermal portions of the stroma and involve both partsindiscriminately. Fuchs' combined dystrophy is an example of aprimary mesodermal involvement with secondary dermal changes.

Corneal DystrophiesThe corneal dystrophies are a group of diseases representing changes

within the cornea which are sometimes difficult to separate, difficult tocontrol and treat, and often difficult to diagnose. They do notfall readily into a strict disease category, for there is difficulty in

603

604 John W. McTigue


distinguishing readily between a dystrophy and a degeneration. Froma general standpoint, degeneration is the appearance of clinicallyforeign substances due to metamorphosis of cell elements, and it ismost often found in a structurally impaired cornea. A dystrophy, onthe other hand, is described as a functional or morphologic changeof the original tissue, which is most often, but not always, due toan hereditary change in the normal corneal nutrition. It is obvious,however, that the trophic alterations of a dystrophy may precede theappearance of secondary chemical substances and deposition of thesesubstances, as in a degeneration, just as degenerative processes maymodify the nutritional state of the cornea and thus open the way forthe possible development of a dystrophy.4A dystrophy has been defined as a condition of deficiency or altera-

tion of tissue nutrition, whereas degeneration indicates a degradation,deterioration, or passage from a form of higher structure to a lowerform.'Hughes27 has described the term degeneration as implying the

conversion of elements of acquired tissue into some inert substancessuch as the changes that are found after inflammation, systemic dis-turbance, or aging processes, and he notes that degenerations of thecornea in older patients usually begin in the periphery. He states thata dystrophy is a developmental and frequently hereditary change inthe original host tissue which usually begins in the central corneaearlier in life. The word dystrophy itself is of Greek origin and meansdefective nutrition.7' Clinically, the corneal dystrophies occur mostfrequently bilaterally, manifest themselves first about the time ofpuberty, and progress slowly throughout life. They are characterizedby the development of hyaline material in one or more layers of corneaand are at least initially confined to the axial region.

FIGURE 13

An area of deepest corneal stroma adjacent to Descemet's membrane (DE, X17,900). The stromal lamellae consisting of collagenous fibrils are very parallelin this zone. An extremely attenuated cytoplasmic extension of a keratocyte ispresent at K. Very small patches of homogeneous material are distributedthroughout the collagenous lamellae, and these materials become more heavilyaggregated in the zone between stroma and Descemet's membrane which isbetter illustrated in the enlarged inset. Here the superficial portion of Desce-met's membrane material is seen to infiltrate irregularly the deep collagen fibrilsfor only a short distance. Collagen fibrils are clearly seen cut in both longitudinaland cross section; presumably the cleavage plane, which is known to exist here,separates the collagenous lamellae slightly anterior to these irregular extensionsof Descemet's membrane leaving a few collagen fibrils within Descemet's mem-

brane. Inset X 53,700

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John W. McTigue

FIGURE 14A sample of corneal endothelium (mesothelium) from the central cornea.Descemet's membrane (DE) is in direct contact with the surface plasma mem-brane of the cell and represents the basement membrane of this cell layer. Thecomplex interdigitation of adjacent endothelial cells is clearly seen, and thecells contain typical mitochondria (M) and a few segments of granular endo-plasmic reticulum (ER). At the apex, or luminal side (i.e., anterior chamberside) of these endothelial cells, there is a tight junction resembling a terminalbar (free arrow). No observable material is present on the anterior chambersurface of these cells. N, nucleus of endothelial (mesothelial) cell. X 15,900

Many different classifications of varying complication and lengthhave been proposed, but a simple division of the cornea into its mainanatomic layers serves as the most reasonable basis for classifying thevarious forms of dystrophy. The most convenient classification isperhaps the one proposed by Franceschetti and Forni9 in which thedystrophies are listed as (1) those affecting the anterior limiting mem-brane, (2) those affecting the corneal stroma, (3) those affecting theposterior limiting membrane, and (4) those affecting some combina-tion of two or more of these layers. It is to be remembered that alesion which commences superficially may involve the deeper tissue,and the lesion which appears initially in the stroma may extend toinclude either limiting membrane.

606


FIGURE 15

Corneal endothelial cells (mesothelial) from the periphery of the cornea butcentral to Schwalbe's ring. What appears as a fracture (free arrows) in Desce-met's membrane is interpreted from other observations as deep channels inDescemet's membrane in which lie cytoplasmic villi from the endothelialcell. The dense bodies within these clefts are interpreted as degeneratedmaterials from this basement membrane. Exaggeration of such regionsproduces the well-known Hassal-Henle bodies of light microscopy. Theremainder of the endothelial cells, although somewhat thinned out, appearidentical in their content of mitochondria and granular endoplasmic reticulum.N, nucleus of endothelial cell. The cytoplasm along the anterior-chamber-limiting plasma membrane shows a band of slightly increased density, a zonein which no cell organelles are present and which may be interpreted (as inthe intestinal epithelia) as a terminal web (free arrows). No observable materialis present on the anterior chamber side of this plasma membrane. x 17,000

The histopathology of these various dystrophies was described inthe years before the turn of the century by Fuchs'7"8 and others. Thesedescriptions have held true through recent years, with the work ofJones and Zimmerman33'34 and others giving us the basis for thehistopathologic identification and classification of the various stromaldystrophies. Electron microscopic studies began to appear in the1950s with the result that the fine structure of the most importantdystrophies has been described by Jakus,29'70 McTigue and Fine,4345

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John W. McTigue

FIGURE 16

Light micrograph of typical Hassal-Henle bodies present atthe periphery of basement membrane in an aging cornea. Theendothelial cells (free arrows) are markedly thinned outover the apices of these bodies and often become difficult topreserve. AFIP Neg. 66-6699, 1.5 micron, paraphenylene-

diamine, X 395.

and others.38 39'48'62 These various electron microscopic studies haveemphasized and generally supported the observations which had beenmade previously by light microscopy. Light microscopy has shown thatthe dystrophies of the cornea affect different layers and differentcellular and extracellular elements separately or in combination. Theyare, therefore, not one separate class of disease entities, but are diseaseswith presumably different origins. The electron microscopic examina-tions have served to pinpoint the area of the primary site of the lesion,clarifying the layer of the cornea involved in such instances as Reis-Bucklers' dystrophy, keratoconus, and Fuchs' combined dystrophy, andthe structural element involved in lattice dystrophy where the detect-able histopathologic alteration was found to lie in the stromal collagenfibrils.A dystrophy involving each of the main divisions of the cornea-

anterior, stromal, and posterior-will be presented and discussed. Notall of the dystrophies have been examined by light and electron micro-scopy in this study, but where the dystrophy has been examined, thefindings will be presented in detail.

MEESMAN S DYSTROPHY

A juvenile epithelial dystrophy involving the corneal epitheliumwas first studied pathologically by Meesman in 1938.9 He describeda deposition of glycogen in the epithelial cells. The dominant heredi-tary character of this change was established by Meesman and Wilke

608

The Cornea and It.s Alterations in Various Dystrophies 609

. ~~~~~~~~~~~~~~~~~~~~~~~~~t

FIGURIE 17

Schiematic drawing to demionstrate the layers of the normaliaduilt cornea: ( 1) corneal epithelium; (2) epithielial basementmemil)irane; (3) Bowman's layer of corneal stroma; (4) cornealstroma, (4a) dermaiil portion of stromia, (4b) mesodermalportion of stroma; (5) Descemiet's membrane (endothelialbasement m-enmbrane); (6) corneal endothielial (miesothelium).

in 1939,46 and has been amply supported by the findings of the laterwriters. This disease was also reported as a rare form of hereditaryepithelial dystrophy by Stocker and Holt in 1955.Y More recently ithas been described by Kuwabara and Ciccarelli39 in their electronmicroscopic studies.The epithelial disturbance appears within the first year or two of

John W. McTigue

life with minute changes, visible only with the aid of the slit-lamp,that appear as fine punctate opacities in the epithelium between theanterior two lines of corneal relucency. The opacities are most promi-nent in the area of the palpebral fissure where they extend fromlimbus to limbus; the upper and lower portions of the cornea coveredby the lids tend to remain free of the lesions for some time. Theepithelial spots usually stain well with fluorescein and at a late stagethe corneal surface may become irregular. Repeated attacks of inflam-mation occur which may lead to corneal scarring. Vision is rarelyaffected seriously except in the aged where a lamellar corneal trans-plant may be necessary to restore vision or to give relief from frequentinflammatory attacks. Histologically, the lesion is confined to theepithelium where the cells are seen to be loaded with glycogen, andcysts form which contain degenerated cell products. Stocker and Holtdescribed peculiarly shaped, pedunculated excrescences extending intothe epithelium from the basement membrane. These excrescencesstained with PAS while Bowman's layer itself remained intact. It wasassumed that much of the PAs-positive material represented an excessof basement membrane.The more recent work of Kuwabara and Ciccarelli has shown that

the glycogen accumulations first noted by Meesman are truly locatedin the proliferating epithelial cells and not in the cysts which containPAS-positive but diastase-negative cell debris. Because of the presenceof many mitotic figures they felt that the glycogen accumulations werethose usually present in young or rapidly turning over cells, andprobably not related to a form of glycogen storage disease. Theirobservation of a thickened basement membrane containing fibrousstructures resembling collagen fibrils is of interest because of the simi-larity with thickened basement membrane normally seen at the peri-phery of the cornea (Figures 2, 8, and 9). As Bowman's layer was notincluded in their material no observations could be made on thisstructure.

It is of interest that even with repeated removal of this abnormalepithelium the replacement epithelium continues to be abnormal.

REIS-BUCKLERS DYSTROPHY OR BUCKLERS IV DYSTROPHY

Reis-Bucklers' dystrophy23'24'51 is an affection of the anterior limitingmembrane in the area of Bowman's layer. It was first described byReis in 1917.9 Bucklers reported the dominant familial characteristicsof the condition in 1949.9The disease begins about the age of five with painful inflammatory

610


FIGURE 18. REIS-BUCKLERS DYSTROPHY.

Artist's drawing of slit-lamp appearance of early changes inBowman's layer (from Griffith and Fine24).

episodes lasting many weeks, and these episodes recur frequently upto about the age of 20. Opacification of the cornea may result from therecurrent attacks of epithelial desquamation and may increase slowlyduring the early years of life. Usually the attacks of inflammation thendecrease in frequency between the second and fifth decades whenpainful episodes appear again which are frequently incapacitatingbecause of large erosions of the epithelium. The lesions resemblethose of neuroparalytic keratitis. In later clinical examination the sur-face of the cornea is uneven and appears to be diffusely cloudy withwhat has been described as the appearance of molten glass. Theopacities in the early case lie at the level of Bowman's layer (Figure18) and are arranged as irregular map-like patterns composed of fine

611

612 John W. McTigue

I,WM..WV, - 4..


Region from junctional area of corneal epithelium (EP) and superficial Bow-man's layer (BL). Epithelial hemidesmosomes and associated basementmembrane are present in some areas (free arrows). In other areas (BM) thehemidesmosomes are lacking but the epithelial basement membrane is stillpresent. There is patchy disorganization of the collagenous fibrils of Bowman'slayer creating foci of increased density. Accumulations of dense material (D)

are frequently seen (see Figure 21). X 16,500


Another area of epithelial cell-Bowman's junction. Normal interepithelialcell desmosomes are present (free arrows) but basal hemidesmosomes areabsent. The associated basement membrane is also lacking. Dense materials

(D) are present in Bowman's layer. x 16,500

-*st = ^kH{:.s* a R . + .. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. . ....:.FIGURE 21. REIS-BUCKLERS DYSTROPHY.

Foci of dense materials (pathologic accumulations) are present at all levels ofBowman's layer in areas where this layer has remained normal in thickness.

X 14,500

614 John W. AlcTigue

*L.............. W:

FIGURE 22. EARLY KERATOCONUS.

A light micrograph of a thin (1.5 micron) paraphenylenediamine-stained, Epon-embedded section of the lesion. The inset shows the full thickness of the corneain the region of this early lesion. The deeper layers of the cornea appearmorphologically normal. The destructive process is observed only in the regionof Bowman's layer and the superficial stroma laamellace. The larger micrographshows the intact corneal epitheliunm overlying the area of the lesion, which con-sists of a very small disruption in Bowman's layer and a disturbance in thenormal alignment of the underlying superficial corneal lamellae. Some of thisdisturbance of superficial lamellae is probably due to their mechanical displace-ment anteriorly following disruption and anterior displacement of Bowman'slayer and also, in part, to a folding of the tissue during preparation. There isno clear-cut evidence of increase of stromal keratocytes in this disturbed region

of early keratoconus. AFIP Neg. 65-4496, x 305.

white strands arranged in angular form.A Bowman's layer appears tobe disintegrating and projecting into the epithelium, and small dot-like opacities appear in the projections. The corneal sensitivity isdiminished and visual acuity is considerably reduced as a result of theirregularity in the anterior limiting membrane.Both light and electron microscopic observations in this condition

(Figures 19-21) indicate that the early disturbance lies within Bow-man's layer. The disturbance has been observed to begin as a realign-ment of the irregularly distributed collagen fibrils of Bowman's layerinto small groupings accompanied by foci of degenerating materialwithin Bowman's layer. Portions of cells which may represent invad-ing cells from the superficial stroma are also present. Other changesobserved are the loss of areas of the epithelial basement membraneand its associated epithelial cell, hemidesmosomes. In this regard, thedystrophy resembles that of the early changes found in Fuchs' com-



The inset shows a region slightly lateral to the area seen in Figure 22 illustratinga more normal region of comeal epithelium, Bowman's layer, and superficialcorneal lamellae. AFIP Neg. 65-4497, paraphenylenediamine stain, X 305. Thelarger figure is an electron micrograph of this region showing the basal cytoplasmof the relatively normal epithelial cells (EP), the very delicate basement mem-brane (BM), and the normal band of irregularly arranged collagenous fibrilsthat make up a normal Bowman's layer (BL). A few collagenous lamellae ofsuperficial corneal stroma and portions of two keratocytes are present beneath

Bowman's layer. X 15,650

bined dystrophy (Figure 57). The similarity in two such apparentlydissimilar entities suggests that the basement membrane and thehemidesmosomal loss is a secondary change which follows a distur-bance beneath them.

Reis-Bucklers' dystrophy differs considerably clinically from Mees-man's heredofamilial dystrophy. Meesman's dystrophy is recognizedclinically, under the slit-lamp, by a fine stippling between the first andsecond zones of relucency which represents changes within the epi-thelial layers themselves or at least changes anterior to the relucent

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The Cornea and Its Alterations in Various Dystrophieszone wN7hich represenits the suirface of Bowman's memnbranie. The. wlhitedots of Reis-Biicklers' dystrophy occur on and are seen to be attachedto this second zone of relucency only in early cases. In Meesman'sdystrophy there are whitish dots lying within the epithelium andchanges in the second zone of relucency appear later in the disease,presumably as a secondary chalnge in the epithelial basementmembrane.The three dystrophies affecting the anterior limiting membrane can

be readily distinguished in their early clinical forms as well as histo-pathologically by both light and electron microscopy, the pure epi-thelial disturbance of Meesman, the almost pure Bowman's layerdisturbance of Reis-Biicklers', and the endothelial, subepithelial dis-turbance of Fuchs' combined dystrophy (see p. 646).

KERATOCONUS

Keratoconus is an obscure condition which has been known sincethe reports of Mauchart in 1748 and Taylor in 1766.8 It was first ade-quately described and distinguished from other ectatic conditions ofthe cornea by Nottingham in 1854.8 It is considered by some writersas a late developmental anomaly or aberration of growth and byothers as a dystroplhic condition. The two elements are probablycombined. Terson,'" in discussing a report of this condition in 1909,said that keratoconus was an affection resembling only itself, butFranceschetti'6 states that this is a vtie d'esprit rather than a realanalogy.

Clinically, keratoconus involves the stretching of the cornea in itsaxial parts. It usually becomes manifest in youth or adolescence andresults in visual impairment due to the development of a high degreeof irreguilar astigmatism. It is a non-inflammatory ectasia, or, in asense, an anterior myopia. Slit-lamp examination shows the typical

layer is grossly disruipted in the region between the two large free arrows.Superficial stromiia-like collagenous material fills this defect. Two keratocytes(K) are clearly visible in this region. Extending into the region of the basalcorneal epithelial cells, there is a large cellular mass which, on closer examina-tion, shows the presence of nerve elements (NE). On higher magnification(inset) these nerve elements are obviously tubules that are typical of neuro-tubules wben fixation is carried out with glutaraldehyde.15 The adjacent largecellular mass, which differs considerably in its cytoplasmic composition fromthe adjacent epithelial cells, is therefore interpreted as a Schwann-like cellalthough most nerves here are non-myelinated (see Figures 10 and 11).

X 20,300; inset X 101,700.

617

John W. McTigue


This electron micrograph shows a region of Bowman's layer (BL) adjacenlt tothe apical zone of keratoconus in this lesion. Bowman's layer is thickest on theleft and much thinner on the right toward the apical portion of the lesion. Theanterior part of Bowman's layer appears normal, while the posterior or deeperlayers are grossly disrupted and occupied by stroma-like material containing

keratocytes (K). x 11,700

signs of this condition: thinning of the cornea, easy visibility of theendothelial reflex due to the increased and irregular posterior curva-ture of the cornea, increased visibility of the nerve fibers, vertical linesin the deeper layers of the parenchyma, Fleischer's pigment ring inthe epithelium, ruptures in Descemet's membrane, linear superficialband-shaped opacities due to ruptures in Bowman's layer, and secon-dary scarring. In rare cases thinning may progress to ultimate rupturein the cornea.The first clinical sign is an asymmetrical astigmatism which is most

easily determined by distortion of the mires on the keratometer. Thecondition at this early stage may also be determined in some cases bythe presence of a disk-shaped area of thickening of the dermal portionof the anterior corneal stroma as shown in Figure 42B. This disk-shaped area of exaggeration of the relucency of the dermal portion ofthe cornea has been observed in six cases before the presence of the

618



Higher magnification of the deeper part of Bowman's layer shownin Figure 25. Dissolution and replacement of this deepest portionof Bowman's layer by stromal lamellae, containing more highlyoriented collagenous fibrils, is clearly seen. These collagenous fibrilsare cut in longitudinal section at L and are more cross section at X.A dense keratocyte is present within these superficial lamellae.

x 31,300

distortion of the mires was seen with the keratometer. The crescent ordisk-shaped area of increased relucency is usually found slightly awayfrom the optical center of the cornea and is presumed to be in the

619

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An electron micrograph of the extreme periphery of the area of the lesionobserved in Figure 22. In this zone, the homogeneous disorder of collagen

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FIGURE 28. MIODERATE KERATOCONUS.

A light micrograph of a thin ( 1.5micron), paraphenylenediamine-stainedsection of Epon-embedded tissue froma case of keratoconus of clinicallymoderate severity. There are multipleinterruptions in Bowman's layer, oneof which is clearly observed here.Bowman's layer (BL) is attenuated andreplaced by a fibrillated material whichclosely resembles stroma. Normal ker-atocytes are present in this fibrillatedmaterial, and they are difficult to dis-tinguish from presumably activatedkeratocytes. The basement membraneof the corneal epithelium is intactthroughout, but in the region of Bow-man's layer it is somewhat irregular.The epithelial cells appear normal.

AFIP Neg. 65-13104, x 380.

FIGURE 29. MIODERATE KERATOCONUS.

A light micrograph of a thin (1.5micron), paraphenylenediamine-stainedsection showing another lesion froma case of moderately severe keratoconus.Bowman's layer (BL) is grossly dis-rupted and replaced by stroma-liketissue, which is heavily infiltrated perunit area (when compared with normalstroma) with keratocyte-like cells. Thecorneal epithelium is slightly artifac-titiously separated in this preparation.More keratocytes are also present inthe adjacent superficial corneal lamellaethan are normally seen in this layer.

AFIP Neg. 65-13103, X 380.

fibrils that normally make up Bowman's layer are present on the left, while onthe right, the deepest zone of Bowman's layer clearly shows early organizationof collagen fibrils into a more lamellar arrangement. Corneal epithelium appearsnormal (EP) as does the underlying basement membrane (BM) in this region.Presumably, this slight change of uniformly disarranged collagenous fibrils intoa more lamellar arrangement represents the earliest morphologically detectable

change in keratoconus. X 45,000

621

622 John W. McTigue

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FIGURE 30. MODERATE KERATOCONUS.

An electron micrograph of the anterior region in a zone in which there hasbeen complete dissolution of Bowman's layer. Altered corneal epithelium (EP)and thickened areas of basement membrane (BM) are present. Bowman'slayer (BL) is seen on the left, and a fragment of keratocyte is present in themid-zone of this layer near its site of termination. A large keratocyte extendsfrom Bowman's layer to well within the stroma-like tissue, which now pro-trudes anteriorly beneath the corneal epithelium in front of the zone previouslyoccupied by Bowman's layer. On the apex of this stroma-like protrusion thebasement membrane of the corneal epithelium is much thickened in an

irregular fashion. X 15,900

SS


area of the ultimate tlhinning of the corniea which forms the coneshape.

Amsler'16 has classified the different degrees of keratoconus. The firstand second degrees can be considered as abortive forms of keratoconuswhich are rarely diagnosed. They are characterized by a depressionand angulation of the horizontal axis, which is visible with the Placidodisk and the Javal apparatus. The vision in the first and seconddegrees is generally normal with a cylindric correction in ordinaryglasses. In the third and fourth degrees, according to Amsler, classicalkeratoconus with the typical slit-lamp findings appears, and the fourthdegree is complicated by superficial scarring of the cone which israrely bilateral, since Amsler found only 41 instances of bilateralscarring in 143 cases. He states that the keratoconus and astigmatismbegin and grow together-that is, the keratoconus is not secondary tothe astigmatism.

In some cases the ectasia appears as a primary dystrophy as issuggested by its hereditary occurrence. The nature of its transmissionis not clear and it is not common, perhaps because the abortive formsare not recognized in the members of a pedigree. A recessive trans-mission is suggested by a number of cases showing a familial instancein which consanguinity has sometimes been demonstrated. Occasion-ally, transmission through two or three successive generations in adominant fashion has been shown.The median frequency of keratoconus is one in 143 cases. It is

generally more frequent in females than in males and is progressivein only 22.5 per cent of the cases. Progression is more pronouncedduring and after puberty.The etiology of keratoconus has not been determined. Endocrine

disturbances have been considered to be the most important factor,and it is found in the presence of other hereditary constitutionalaffections. 1 6

Histopathologically, the condition will be considered as anotherform of corneal dystrophy involving the anterior limiting membrane.This disruptive change in the anterior cornea appears to bridge thegroup of dystrophies which involve the epitheliuim and Bowman'smembrane and those which involve the stroma.Many histopathologic studies have been carried out in keratoconus,

and the general consensus today is that the anterior layers of thecornea are the site of the primary involvement. Light microscopicexaminations have localized the early changes to multiple disruptionsin Bowman's membrane (See Figure 41). Recent work with the

623


An electron micrograph of the zone just lateral to the fracture in Bowman's layershows the considerable papillary-like thickening of the basement membrane ofthe comeal epithelium (BM). Within this basement membrane there are manysmall, dense bodies and circular profiles of lucent material, indicating thepresence of some spherical bodies. The underlying Bowman's layer (BL)appears slightly more organized in the direction of corneal stroma than does

normal Bowman's layer. EP, fragments of epithelial cells. X 15,900


A higher power electron micrograph in the region of a fracture zone in Bow-man's layer in which the corneal stroma has protruded anteriorly beneath thecorneal epithelium (EP). The intraepithelial filaments normally present incorneal epithelial cells can be clearly seen in the upper left-hand corner. Fociof thickened epithelial basement membrane (BM) are seen along the surfaceof this stromal herniation. A more normal Bowman's layer is present on the

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Another region of Bowman's layer near a fracture zone in a case of moderatekeratoconus showing the considerable papillary-like thickening of the epithelial

basement membrane (BM). x 17,500

electron microscope suggests that an earlier change lies in the basal celllayer of the epithelium.5'29'30'68 The observations reported here whichinclude early, intermediate, and late cases of keratoconus may help topinpoint the time sequence of these changes more precisely. Theobservations, primarily by electron microscopy, show the severity ofchange as the condition progresses.

In the earliest case examined, the normal periphery of the cornealbutton removed at operation helps to locate the early changes andmakes it apparent that the first detectable morphologic disturbanceoccurs in the junctional region between Bowman's layer and thesuperficial stroma (Figures 22, 23, 25, and 26). There is, apparently,a reaggregation of the irregularly arranged collagen fibrils of Bow-


An electron micrograph of tissue beneath Bowman's layer at the periphery of oneof the interruptions in Bowman's layer shows a large keratocyte-like cell with itsmany cytoplasmic extensions, which penetrate deeply into the adjacent cornealcollagenous lamellae. Keratocytes do not usually show such a wide extension ofcytoplasmic processes when the cornea is cut normal to the surface (see Figure12). Bowman's layer (BL) is seen in the upper left-hand comer of this micro-graph, and its collagenous fibrils are becoming more lamellarly arranged as itapproaches the region of this keratocyte. Segments of granular endoplasmic reticu-lum are prominent throughout the cytoplasm of this keratocyte, but they are

better illustrated in Figures 37 and 38. N, Nucleus of keratocyte. X 15,900

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FIGURE 37. LATE KERATOCONUS.

An altered keratocyte is present in the region of disturbed superficial cornealstroma of the keratoconus lesion. The large cytoplasmic content of granularendoplasmic reticulum is clearly seen. The adjacent stromal lamellae show

early loss of their lamellar arrangement. X 10,600

man's layer into more definite aggregates (Figures 27, 40, and 42A),together with an alteration of the keratocytes into active fibroblast-likecells (Figures 34, 37, and 38). This change is followed by the invasionof Bowman's layer by these "activated" superficial keratocytes whichprobably move through the zone undergoing fragmentation. Basementmembrane (Figures 30-33) is produced on the surface of the involvedarea of Bowman's layer by the epithelial cells in an irregular fashionwith thickening at the immediate edge of the ruptured area ofBowman's layer. This is presumably an attempt on the part of theepithelial cell to repair the damage which is going on beneath it, suchas may occur in Meesman's dystrophy. Such an attempt at repair isaccompanied by an increase in cellular activity in the superficialstroma which results in scar formation and the obliteration of shortsegments of Bowman's layer (Figures 28, 29, 35, and 36). The appear-ance of the multiple foci of fragmentation and scarring of Bowman's

629

John W. McTigue


Another area from the region of altered superficial stroma showing twoportions of altered keratocytes containing a rather large amount of granularendoplasmic reticulum. The collagenous lamellae between these two cells areno longer uniformly arranged. The inset shows a few fragments of this endo-plasmic reticulum at a higher magnification, as well as the presence of intra-cellular filaments. These criteria together with the sequence of observationsmade of early to late keratoconus lesions all support the interpretation oftransformation of a normal resting keratocyte into an active fibroblast-like cell.

Inset X 21,200.

layer with clusters of activated keratocytes* is the only "typical" histo-pathologic picture of keratoconus (Figure 41). Since the destructiveprocess involves a layer of the stroma beneath it, keratoconus may beconsidered as a dystrophy which involves not only the immediate areaof Bowman's layer but also the dermal portion of the cornea (Figure42, A and B).

*These "activated" stromal cells are believed to be predominantly altered kera-tocytes. Experimental evidence74-76 indicates that invasion by a monocyte-likecell from the periphery is also a possible source for fibroblast-like cells in thisarea. A third possibility, that of the migration of fibroblasts from the periphery,is not likely in an acute corneal wound but becomes a possibility in this slowlyprogressing lesion. However, the absence, to date, of "activated" or foreign-appearing cells in the periphery of the corneal button seems to indicate an altera-tion of the keratocyte in the area of the lesion as the source of these cells.

630


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Occasionally, a cell may be found, in this zone of superficial stromal disturbancecontaining large pockets of a relatively homogeneous electron-dense materialthat resemble the acid mucopolysaccharide mixtures observed in maculardystrophy.48,69 This suggests that these cells which show morphologic signs of

fibroblastic activity also produce a mucinous component. x 18,300

The disturbance is detectable clinically by the slit-lamp when thenormal dermal layer is disturbed to such an extent that the slightlyrelucent band of dermal stroma is deepened to form a posteriorlydirected cup-shaped area. This observation is probably, togetherwith keratometry, the earliest detectable clinical change in kerato-conus, and it is the earliest detectable slit-lamp change. In kerato-conus this sign is best seen when examining the second of a pair ofeyes in which the more typical changes of the cornea are evident withthe slit-lamp in one eye, but detectable only by keratometry inthe second eye. (As stated previously, it has been observed in sixsuch second eyes before keratometric changes could be determined.)The histopathologic disturbance of the anterior or dermal layers may

appear slight, but the change may be great enough to interfere withvision significantly. It is well known that any slight disturbance in theanterior layers of the cornea produces much more disturbing opticaleffects than equally severe disturbances in the posterior layers.2'6'49

631

*.


A region of Bowman's layer far to the periphery of the main lesion of kerato-conus where stroma herniates through Bowman's layer beneath the epithelium.The epithelial layer has torn away from this layer artifactitiously, but the basal


Since keratoconus apparently begins in the anterior layers of thestroma, these changes may then have a profound effect on the opticsof the eye.

It is only with the final involvement of the mesodermal portion ofthe corneal stroma and the posterior limiting membrane-Descemet'smembrane and the endothelium-that sudden edema of the stromaoccurs.

STROMAL DYSTROPHIES

Groenouw, in 1890, described a form of corneal degeneration whichhe termed noduli corneae. In 1898 he amplified this report with adescription of the histologic findings and reported, in 1917, the domi-nant trait of this disease.9 Biber, in 1890, described a reticular dystro-phy, and the dominant transmission of this dystrophy was establishedby Haab in 1899 and by Dimmer in the same year.9 Biicklers, in 1938,classified the extensive literature which had been reported on theseconditions and established a standard nomenclature. He concludedthat the cases reported by Groenouw represented two different typesof dystrophy: one, described as nodular or Groenouw's Type I witha dominant transmission and hereinafter referred to as granular dystro-phy; and the other Groenouw Type II, which had a recessive trans-mission and has been called a macular dystrophy in more recentreports.9 The third major type of stromal dystrophy is then thatdescribed by Biber and later by Haab and Dimmer and is referredto as lattice dystrophy. Other rarer forms of dystrophies have beendescribed by Schnyder, Pillat, Maeder and Danis, Franvois, Fran9oisand Neetens, and Waardenburg and Jonkers.9Duke-Elder states that these conditions, or at any rate, some of

them, are not independent morbid entities but are evidence of thephenotypic variability of the same morbid gene. This view was stronglyadvanced by Fleischer in 1905 and by others who argued that anyprecise morphologic differentiation was blurred by the occurrence ofdifferent appearances in the same cornea or of different appearances

cytoplasm and the basal cytoplasmic membranes bearing normal hemidesmo-somes remain adherent to the relatively normal basement membrane layer(BM ). The superficial portion of Bowman's layer in this region appears normal,while the deeper layers of Bowman's zone show increased prominence of thecollagenous fibrils and their transition into a more organized or lamellar fashionfrom left to right in this micrograph. The apex or region of maximum dis-turbance of this lesion is at the far right of this illustration. That is, at theperiphery of the lesion can be seen transition from "Bowman's" of normal(disarranged) composition and arrangement to the deeper portion near the

corneal strorna where Bowman's is more highly organized. X 15,900

633

B "affi.,1

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... _ ........... - _

FIGURE 41

A, Light micrograph of a conventional section stained with the PAS reaction toshow the very thin basement membrane that lies beneath the corneal epitheliumand Bowman's layer (BL). Compare this thin layer of PAS-pOsitive material thatis present in the central (i.e., toward the pupil) portion of the cornea with thatfound at the periphery of the cornea (Figure 41B). AFIP Acc. 101049, X 900.B, The zone between corneal epithelium and Bowman's layer (BL) produces an

intense PAS reaction indicating a wider basement membrane which is normallypresent beneath the corneal epithelium. This thick basement membrane is typicaland normal in the periphery of the adult cornea. The difference between thethickness of corneal basement membrane from central to peripheral cornea is alsowell appreciated by examining the electron micrographs (Figures 7 and 8).AFIP Acc. 101049, X 900. C, A conventional section of cornea taken from acornea from a case of keratoconus and treated with the periodic acid-Schiff

John W. McTigue

in members of the same family. It is well recognized, however, that allheredodegenerative conditions tend to show a considerable variabilityin their expression, but the relative constancy maintained by thesedystrophies within a pedigree, both in their clinical appearances andin their evolution, leaves little doubt that they represent separateclinical entities. There is no satisfying evidence that they are secondaryin nature, either neurotrophic in origin as has been suggested, conse-quent upon corneal disease such as tuberculosis or herpes, or anexpression of impaired nutrition.9

Histopathologically, these three conditions were clarified and sepa-rated by the reports of Jones and Zimmerman33'34 in 1959 and 1961. Intheir reports, histologic examinations were made on portions of corneasremoved from patients who were suffering by clinical determinationfrom either granular, macular, or lattice dystrophy. In the histologicsections from all three types of dystrophy, a variety of non-specificalterations were found in the epithelium and Bowman's membrane.The lesions of diagnostic significance were found in the stroma in allof the cases included in the study. While the stromal lesions of granu-lar and lattice dystrophy have certain staining characteristics in

FIGURE 41 (CONT.)reaction to show the changes that take place in the region of the basement mem-brane in an area that may be considered an "early" lesion. The PAs-positive base-ment membrane in the region that is seen here is in part due to the basementmembrane and in part to glycogen that is presumably present in the epithelialcells. The PAs-positive material is grossly disrupted centrally as is Bowman's layer.Superficial corneal lamellae in somewhat irregular fashion occupy the zonefrequently located in Bowman's layer. AFIP Neg. 65-12188, X 900. D, A lightmicrograph of another region from a case of keratoconus to show gross disturbanceof both Bowman's layer which has undergone extensive fibrillation centrally (freearrow) and the disturbance of the basement membrane that lies between thislayer and the epithelitum. The nucleus of a stroma-type cell is present within thisfibrillated Bowman's layer. Cells are not normally present in this region. Theseparations of the stromal lamellae deep to Bowman's layer are mostly artifactitious.AFIP Neg. 65-12186, x 900. E, Light micrograph of a PAS-treated conventionalsection to show the changes that occur in the region of Bowman's layer andepithelial basement membrane in a case of keratoconus. Bowman's layer has beengrossly interrupted (free arrows) and the interruption filled not only with stromallamellae in disarray but with cells that have invaded this area. The basementnmembrane region is thickened and has been pushed forward through the defectirn Bowman's layer. AFIP Neg. 65-12186, X 900. F, A conventional section stainedfor iron at the mid-periphery of the cornea from a case of keratoconus to showthe Fleischer's ring which may be observed clinically as a light-brown line at thebase of the cone. Here the iron stain indicates iron-positive material presentwithin the deeper corneal epithelial layers (free arrows). This iron-positivematerial therefore is the basis for the brown Fleischer ring observed clinically.

AFIP Neg. 65-1212187, x 530,

636

The Cornea andl Its Alterations in Various Dystrophies 637

common, they could be differentiated on the basis of their morphologicfeatures, by their relative birefringence, and by the intensity of stainingof the lesions with the periodic acid-Schiff reaction. Entirely differentstromal lesions were seen in the cases of macular dystrophy in whichthe most conspicuous alteration was the presence of large accumula-tions of material which gave positive staining reactions for acid muco-polysaccharides (MPS) insensitive to hyaluronidase. The endotheliumand Descemet's membrane were normal in all of the cases of latticedystrophy. All of the cases of macular dystrophy- showed accumula-tions of NPs in the endothelium and there were frequent changes inDescemet's membrane. In one of the six cases of granular dystrophy,there were excrescences on Descemet's membrane. The endotheliumappeared normal in all cases of granular dystrophy. From their studiesJones and Zimmerman concluded that granular, macular, and latticedystrophies of the cornea could be clearly distinguished from oneanother by relatively simple methods of histologic examination.

Lattice DystrophyIn 1964, McTigue and Fine43'44 reported the electron microscopic

examination of a corneal button obtained from a clinically diagnosedcase of lattice dystrophy of the cornea. They found the typical-lesion(Figure 43) to consist of two distinct parts. One part was seen byelectron microscopy to be a mass of delicate filaments of approximately1oo A in diameter with no special orientation. The presence, of theselong delicate filaments together with -an intermingling of a number offibrils of more normal dimensions (Figures 44, 45) suggested that thelesion developed as a result of a breakdown of the larger fibril into itsaggregated constituent filaments (fibrillary degeneration).The second part of the lesion consisted of a highly oriented area of

fibrils of normal diameter but increased density. This difference indensity was clearly seen in both longitudinal and cross se&tion.43 Theirobs.ervations suggested a transition from the fibril of normal diameterand-low density, through a fibril of normnal diameter and high density,to a final separation of the fibril into delicate component filamentshaving approximately one-third of the diameter of the normal collagenfibril. This idea was supported by the presence of mixtures of cross-sectioned fibrils of normal lucency with those of increased densitywithin and at the periphery of the collections of delicate filaments(Figture 45).The separation of fibrils of normal diameter into more delicate fila-

ments of smaller diameter might be explained in part by a change in,

I~~~~~~~~~~~~~~~~~~~~~~~~ . i'.C1 .

. C

FIGURE 42

A, Diagrammatic representation of the presumed sequence of changes in keratoconusas interpreted from the light and electron microscopic observations made in (a) normaladult cornea, (b) early keratoconus, (c) moderate keratoconuis, and (d) late kerato-conus. In the normal cornea (a) the epithelial intercellular desmosomes or attachmentplates are represented by two dense surface spots on adjacent cell membranes while thehemidesmosomes are represented only along the basal cell membrane by a single density.The resting keratocytes beneath Bowm-an's layer become "active"; (b) develop amore prominent granular endoplasmic reticuLlum (resembling fibroblasts); (c) and


or loss of, a binding polysaccharide. Such a possibility is compatiblewith the characteristic loss of stainable mucopolysaccharide in theselesions when examined by light microscopy as reported by Jones andZimmerman.34One of the criteria used to separate lattice dystrophy from the other

stromal dystrophies in Jones and Zimmerman's report was the propertyof increased relative birefringence exhibited by the lesions of latticedystrophy. Examination of sections from the case of lattice dystrophywere studied by conventional light microscopy under polarized lightand revealed that only certain portions of the lesions showed increasedrelative birefringence. Repeated electron microscopic examination ofthe same tissues showed that this area of increased birefringenceapparently corresponded to the lamellae containing the oriented fibrilsexhibiting increased density (Figure 46). The dense lamellae werethen interpreted as the basis for the increased birefringence previouslydescribed in the lesions of lattice dystrophy. The birefringent portionsmay be seen completely enveloping a non-birefringent portion, or mayappear to be interwoven through the non-birefringent mass, dependingupon the plane of section of a particular lesion. Re-examination of thelesions by polarized light in conventional microscopy showed clearlythat the lesions did indeed consist of two portions (Figure 47).Although other investigators79'80 using methods of light microscopy(silver impregnation) feel that these lesions are produced by alhyalinization of the corneal nerves, no evidence in support of theseideas could be found on electron microscopic examination.

Granular DystrophyJones and Zimmerman34 characterized the typical lesion of granular

dystrophy as a large aggregation of distinct eosinophilic granules(Figure 48). Frequently, these aggregations presented a somewhatirregular outline and their edges were usually irregularly lobulated.The lesions sometimes contained markedly argyrophilic fibers which

finally perforate through a disrupted Bowman's layer on which the epithelium hasdeposited an excess of basement membrane (d). B, Artist's drawing of the slit-lampappearance of the zones of relucency in the normal human cornea (A,B). Figure Bemphasizes the relucency of the "dermal" band or anterior portion of the normalcorneal stroma which is easily seen with the slit-lamp, is present anatomically, but israrely illustrated in drawings of the optical section of the cornea as in A.3 C illustratesthe earliest detectable slit-lamp change that may be observed clinically in keratoconus.This consists of a focal exaggeration of the band or relucency which protrudes

posteriorly toward the mesodermal portion of the stroma,

639

FIGURE 43. LATTICE DYSTROPHY.

Typical appearance of stromal lesions from a case of lattice dystrophy of thecornea. The collagenous lamellae stain more deeply with the eosin stain withinthe lesions which appear more compact than adjacent corneal lamellae.

AFIP Neg. 64-3522, hematoxylin and eosin, X 300.


Electron micrograph from the edge of a small lesion from a case of latticedystrophy of the cornea. Collagen fibrils of normal appearance and dimensionsare present at C. The free arrows point to a tangled mass of delicate interwovenfilaments. This mass of tangled filaments represents the non-birefringent portionof the lesion (see Figure 47). A fragment of a cell is present at CL.43 X 14,300


Higher power of a portion of Figure 44 to show the tangle of delicate filaments(FIL) more clearly. Collagen fibrils of normal diameter are present in longi-tudinal section at L and in cross section at X. Presumably the normal sizecolkigen fibrils are undergoing separation into these finer filaments ("fibrillary

degeneration") .43 x 46,000

John W. McTigue


Another area from the edge of a lesion to include an area of "fibrillary degenera-tion" (FD) as well as adjacent collagen fibrils of normal diameter butincreased density cut in cross section (X). It is presumably these latter alignedbut dense fibrils of normal diameter which account for the area of increased

birefringence (see Figure 47) within the stromal lesion.43 X 13,000

sometimes appeared to be balled up like a tangled mass of yarn. Inthe periodic acid-Schiff reaction, the characteristic stromal lesion ofgranular dystrophy was less intensely stained than the surroundingstroma and the lesion did not show increased birefringence in polar-ized light. A previous report of an electron microscopic study froma case of granular dystrophy, made by McTigue in 1965,45 describedthese lesions at magnifications of up to 71,000 as continuing to appearas dense homogeneous masses. The clinical, light, and electron micro-scopic appearance was then that of a granular formation at any levelof magnification.

This study reports the examination of an additional case meetingthe requirements for a diagnosis of granular dystrophy in whichsimilar conclusions may be drawn. The granular lesions were foundto be present throughout the depth of the stroma and within Bowman's

642



A single stromal lesion examinedunder polarized light reveals its bi-partite composition, birefringent andnon-birefringent portions.43 AFIP Neg.

......_..*. 63-4653, hematoxylin and eosin,x 750.

i,lL <t ~~~j .a>' i:.-#9

F E 4. G A DST|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ __liii....- & .; ; s |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

e..e. .-g;- -ssb, =~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~S -*i_z X -* *~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

J; L },~~~ ~~~. > . , , A 8 - X _ . .

FIGURE 48. GRANULAR DYSTROPHY.

Light micrograph of a formalin-fixed, paraffin-embedded cornea from a caseof granular dystrophy. Except for the small fragment of normal thickness (freearrows) most of Bowman's membrane in this region is missing. Foci ofgranular degeneration of the nearby stroma are present at each end of thisfragment of Bowman's layer as well as deeper within the stroma. Notice thatthe epithelium is of normal thickness only over the fragment of Bowman's

layer. AFIP Neg. 66-6694, hematoxylin and eosin, X 145.

layer to the extent that Bowman's layer was elevated. In some areas,it was elevated into the overlying epithelium (Figure 49).The lesion was again composed of irregular fragments of a dense

material, the fine structure of which could not be determined at theresolutions employed (Figures 50 and 51). Stroma cells, presumably

643


A thin (1.5 micron) section of Epon-embedded cornea from another case ofgranular dystrophy. Subepithelial granular material has replaced most ofBowman's layer. In one region a fragment of Bowman's layer has been elevatedby this granular material into the overlying epithelium. Another granularlesion is present deeper within the comeal stroma. AFIP Neg. 66-8682,

paraphenylenediamine, X 300.


Electron micrograph of the edge of a granular lesion deep within the cornealstroma. (Inset X 300 shows similar typical lesions by light microscopy for

The Cornlea ancl Its Alterations in Various Dystrophies

altered keratocytes, Nwere frequently seen at the edge of these"granules." In some instances, the cell process appeared to be club-like with a number of clefts present within the granular materialsuggesting that an attempt was being made on the part of the cell todisrupt and ingest the homogeneous mate-rial of the granule (Figure51). In no instance, however, was there any morphologic evidencefor phagocytosis of this material by the altered keratocytes.

Alacular DystrophyThe electron microscopic examination of a case of macular dystrophy

of the cornea has recently been reported' by Klintworth and Vogel.35These investigators found intra- and extracellular accumulations ofmaterial which they identified as acid mucopolysaccharides, and thematerial was reported as insensitive to hyaluronidase. The intracellularaccumulations were observed to be due to dilatations within thegranular endoplasmic reticulum. The authors felt that these observa-tions were supported by the demonstration of deposits of colloidaliron along the border of the intracellular vesicles. According to theirreport, however, similar deposits of colloidal iron did not occur alongthe extracellular deposits.

In this same patient the authors also studied fibroblasts in the skinand found them to be entirely normal; no cellular abnormality couldbe found which might support the contention that this defect waspart of a general malfunction of fibroblasts. They concluded, tbherefore,that the pathologic process was apparently localized to the cornea.They fouind no evidence for phagocytosis of the extracellular mate-

rial by the keratocytes, buit this material was fixed only secondarily forelectron microscopy and such fixation has been found lacking inpreservation quality even for the study of normal tissue. Observationson the corneal endothelium were not reported in their study.The possibility remains that the vacuoles observed here are in large

part autolytic changes. This is supported by a report of similar vacuoleformation in keratocytes in a case of granular dystrophy of the corneareported by Sornson in 1965.62 The studies reported here have failedto suipport Sornson's observations on macular dystrophy (Figures 50and 51).

comparison.) The lesion is composed of irregtilar fragments of a dense material,the substructure of which cannot be seen at this magnification. A stromal cell(presumably an altered keratocyte) has cytoplasmic processes between theedges of these "granuiles" at the periphery of this lesion. There is no morpho-logic evidence here for phagocytosis of this granular material. The adjacent

collagenous stroma appears normnal. X 17,600

645

John W. McTigue


Another area of stroma where a cell butts on a fragment of granular material(C). The cell process is club-like (free arrow) and there are a number ofclefts within the granular material as if to suggest that an attempt is beingmade on the part of the cell to disrupt and ingest the homogeneous material

of the granules. X 16,500

Light microscopic examinations34 do, however, indicate some formof intracellular accumulations (either production with stasis, or phago-cytosis of mucopolysaccharides produced elsewhere). More recentelectron microscopic observation by Teng69 and Morgan,48 in whichthe fixation for electron microscopy was more satisfactory, indicatethat these inclusion are, in all probability, produced intracellularly,but either have not been discharged or are produced in excessiveamount. The granular reticulum was found to be swollen with a deli-cate lucent material resembling that occasionally seen in other"activated" keratocytes (Figure 39), and there was no evidence forphagocytosis or production of a "phagosome." No description of endo-thelial changes was made, and, in one report,69 no degenerativechanges of the collagen fibrils could be seen.

In an interesting addendum to his paper Morgan48 stated that he

646


had had the opportunity of examining a donor corneal disk used threeyears previously in a keratoplasty operation for macular dystrophyand had found the typical changes of macular dystrophy in the kera-tocytes of this donor graft. He interpreted this as evidence of invasionof cells from the host cornea and as evidence in favor of this conditionbeing a primary cellular disorder. The possibility that the donorkeratocytes may have survived25'53 and subsequently developed thesedystrophic changes was apparently not considered.

FUCHS COMBINED ENDOTHELIAL EPITHELIAL DYSTROPHY

This disease process, characterized by an edematous epithelium withimpaired sensitivity of the cornea occurring in elderly people, was firstdescribed by Fuchs in 1910.9 It was speculated as early as 1902 andthen again in 1916 that the endothelial lesion was responsible for theproduction of a curious condition of the cornea simulating that seenin glaucoma.9 Sometime later, Kraupa9 observed the priority of endo-thelial change in cases which developed into the typical clinicalpicture described by Fuchs. Vogt9 confirmed the observation that theprominent excrescences of Descemet's membrane which characterizedthe endothelial picture resembled Hassal-Henle bodies. He named thesyndrome occurring in the deeper parts of the cornea, cornea guttata.Graves in 19249 described the same condition in greater detail as abilateral chronic affection of the endothelial face of the cornea inelderly persons, and Kirby and Gifford in 1925 and 19269 referredto it as endothelial dystrophy of the cornea (Figure 52).

Attention having been thus attracted to the two conditions, thesequence of endothelial to epithelial dystrophy became amply con-firmed by following the progression in individual cases of endothelialand epithelial changes in the more affected eye and the less affectedeye. The two diseases may be separated to the extent that the progressof the degenerative changes may be extremely slow and the one maynot seem to develop into the other, but Duke-Elder9 stresses that thecomplete clinical picture appears to be an endothelial degenerationwhich allows the intraocular fluid access into the cornea followed bythe development of dystrophic changes in the epithelium and eventuallyin the substantia propria.

In the cases of endothelial dystrophy, the clinical changes of thecondition are so fine as to be visible only by the slit-lamp. It is usuallybilateral but may affect one eye more than the other. The earlieststages are evidenced by glints of golden hue on the posterior surfaceof the cornea visible by specular reflection and by the appearance of

647

FIGURE 52. FUCHS' COMBINED DYSTROPHY.

Light micrograph of the markedly thickened Descemet's membrane coveredby an attenuated endothelium. Some of the endothelium is lost, due, presum-ably, in part, to its friability during the preparation of the tissue for study.The excrescences on the anterior chamber side do not appear as pronouncedhere as those referred to as Hassal-Henle bodies, which are present at theperiphery of a normal aging cornea (see Figure 16). AFIP Neg. 67-147,

periodic acid-Schiff, X 395.

___n

.. ... ...

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~X... .r

..........i

AA AC' 's

FIGURE 53. FUCHS' COMIBINED DYSTROPHY.

An intraepithelial vacuole containing a small amount of preserved material.A slightly widened intercellular space is present at IN. Other bullae werefound to lie between adjacent endothelial cells (intercellular). Both varietiesof swelling formations were infrequent in this case. DE, Descemet's membrane;

AC, anterior chamber. X 13,700



Sample of stroma from the case of Fuchs' dystrophy to show the normalappearance of the stromal lamellae, their relationship to one another, and theusual spacing between collagen fibrils. Normal appearing keratocytes (K) arepresent between the lamellae. No edema or swelling of the stroma can be

detected. X 16,500

small dark areas lying in the mosaic of the epithelial cells. The condi-tion appears first in the central portion of the cornea and thengradually travels toward the periphery. The pigmentation increasesuntil the posterior surface of the cornea appears to have been dustedwith powdered bronze. The bronzing is due to changes in the endo-thelial cells and not to pigment deposited on the endothelial surface,which indeed may also be present.9

Pathologic studies of rather mild cases show that the non-reflectingglobules are excrescences or drusen-like formations on Descemet'smembrane which are similar to Hassal-Henle bodies except that theyare larger and found in the center as well as the periphery of thecornea. The endothelial cells show signs of degeneration and theircytoplasms contain pigment granules. The bodies of the cells areextremely attenuated over the excrescences.63 In an eye excised in the

649

John W. McTigue

---------E ----S>, -_i.i0eMM':::.if': ^ x-:x:::r:e: .i:.. .'''::}r .::: 1:if:B f: .:f..::i' ----k __


Another area of stroma. The collagenous lamellae appear normal as do theportions of the two keratocytes (K and K). Patches of dense (D) interfibrillarmaterial are present here as in the normal corneal stroma (see Figures 12, 13).

x 16,500.

late stage of the dystrophy, von Hippel9 found the endothelium to bealmost entirely lacking, an observation supported by the silver impreg-nation techniques of Wolter et al.78

Fuchs' epithelial dystrophy is a condition occurring most often infemales well over 50 years of age and is usually bilateral. The epi-thelial changes start with the appearance of a fine edema first centrallyand then peripherally. The vacuoles are small at first but later maycoalesce in groups; when they burst minute superficial gray opacitiesremain in the epithelium, and at the same time the corneal sensationis diminished or absent. The opacities multiply and the changes pro-ceed centrifugally just as the associated endothelial changes do. Finegray stria and dots appear in the parenchyma, and, after an extremelyslow course, the epithelium becomes completely opaque and insensi-tive and large superficial areas may be lost.9The condition of Fuchs' combined dystrophy has been divided

650

CN

0U

r:4

652 - JohaW. AMcTigue

*

652

.............

to


generally into four clinical stages: (1) an uncomplicated degenerationof the endotheliurn-cornea guttata; (2) edema- of -the -stroma and theepithelium with bulla formation; (3) subepithelial connective tissueformation with vascularization and scarring; and (4) complications,particularly glaucoma or infection:- -

Electron microscopic investigations into this corneal disturbancehave been limited. -The earliest examinations were those of Jakus,29and Feeney and Garron.12 They pointed out, in support of the pre-vious light microscopic observations, that -Descemet's membrane wasthickened, and that on this thickened membrane lay bodies somewhatresembling the Hassel-Hlenle bodies--common to the periphery of thecorne,a (Figure 16). They -stated that these Hassel-Henle-like forma-tions differed considerably in Fuchs' dystrophy in that they did notshow the fissures which are usually found iri Hassal-Henle bodies(Figure 15). Similar observations were- reported by other investiga-tors including the more recent work of Kayes and -Holmberg.36These observations are supported- in this study in the evaluation of

a moderately early case of Fuchs' .combined dystrophy. . Previousreports may have been made on rather-late cases; but the electronmicroscopic studies reported here -are from a -case in which there wasa decrease in visual -acuity but -no clinizal evidence of- stromal orepitheliaJ edema. -The basic changes are believed- to occur in -tlre zone of Descemet's

membrane and the endothelial cells. Descemet's membrane undergoes-considerable thickening (Figure 52), but there is some disagreementabout the manner in which the membranfe und-ergoes such thickening.At the deepest or innermost layers of the newly formed dystrophic

FIGURE 57. FUCHS COMJBINED DYSTROPHY. -

The electron micrograph is a sampling from the epithelium _( EP ) -Bowman's layer (BL) zone of this case of early Fuchs' combineddystrophy. Invader cells (ICY are present along the- plane of the anteiiorsurface of Bowmqrf's' layer. These cells- contain numerous delicate intra-cellu.ar filaments. They are separated from the-epithelial cells by a layerof delicate extraQellular fibrillar material '(presumably collagenous). Theepithelial basement membrane ( BM ) - encs abriuptly at -th&e; large singlefree arrow, and this abrupt termination is accompanied by an associated-loss -of epithelial cell hemidesmosomes (HD)- Associated with this lossof basal hemidesmosomes is a marked 'reduction in the intracellularfilamrint of -the immediarely adjacent cell cytoplasnm. x 24,000. The insetis aq--ht micrograph of a thin section (approximately 1.5 microns) toshow invasion of the subepithelial region by cells deep to Bowman's layer,presumably by passage of altered keratocytes through a neural passageway.

AFIP Neg. 67-146, paraphenylenediafm>ine, -x 395. '

653

bodies there are small accumulations of material resembling that ofthe normal anterior portion of Descemet's membrane (Figures 13, 56).These deposits may indicate that the dystrophic bodies are producedas a sort of "younger" or embryonic formation of Descemet's mem-brane. Before these excrescences are formed it appears that there isa zone of more typical collagen-like material lying posterior to thenormal banded layer of Descemet's membrane, which has itselfbecome layered over with a "normal," non-banded Descemet-likematerial. It appears that, at one point in time, the endothelium, whichis supposed to produce Descemet's membrane, has produced an"abnormal" formation resembling stromal collagen fibrils (Figure 56,insert A). The thickened, multi-layered Descemet's mebrane containsa layer which seems to have collagen fibrils of normal diameter withinit. This area would then resemble the peripheral cornea where nor-mally there is a split in Descemet's membrane with collagen fibrilsbetween the two layers.30 Apparently, the endothelium has at sometime after this fibril formation regained its ability to produce a morenormal Descemet-like material, but this production is not uniformover the posterior surface of the cornea so that localized excesses areproduced containing foci of banded material resembling that formedby young endothelial cells in embryonic or fetal life.29'30'47The formation of these bodies might relate to Jakus'29'31 suggestion

that in the chick embryo Descemet's was first formed of islands ofmaterial deposited by the primitive endothelium and that these islandseventually fused. Her suggestion would be consistent with the recentconcept that the Hassal-Henle-like bodies are, in reality, a young orembryonic form of Descemet's material. The increase in number ofthese bodies in Fuchs' dystrophy might then represent an attemptto age in a proper manner and an attempt to form a proper posteriorsurface. It is presumably because of these enlarging bodies that theendothelial cells are attenuated.The internal structure of the endothelial cells appears quite normal

except for a number of intracellular vacuoles (Figure 53). The vacu-oles are few in number in the early case reported here, but they areknown to occur in larger numbers in later cases. The observations bylight microscopy that these enlarging obdies on Descemet's membraneare freely in contact with circulating aqueous have not yet foundeither support or denial by electron microscopy. It is extremely diffi-cult to preserve corneal endothelium perfectly for microscopicstudies.29'78The changes that occur in the Descemet's membrane area, presum-

654 John W. McTigue

The Cornea and Its Alterations in Various Dystrophiesably produced by an abnormal or changing endothelium, have beeninterpreted by Kayes and Holmberg:17 as an attempt on the part of thecorneal endothelium to make Descemet's membrane less susceptible tohydration so as to prevent the corneal stroma from becoming edema-tous. With the failure of this protective device, fluid enters the stromacausing epithelial damage and changes. However, in this study ofearly changes there was no stromal edema by clinical, light, or electronmicroscopic study (Figures 54, 55). There was, however, evidenceof a subepithelial production of a fibrous layer and an invasion ofcells into this area (Figure 57), a change which was not identifiedclinically at preoperative examination. (Even with this histologicknowledge at hand such changes could not be found clinically on sub-sequent examination of the involved fellow eye.) The invasion of cellsappears to emanate mostly from the most superficial keratocytes whichpenetrate through Bowman's layer (Figure 57). These cells may passthrough normal channels such as those through which the nerves pass(Figures 10, 11, and 24). The cells then travel along the plane betweenthe epithelium and Bowman's layer, but deep to the basement mem-brane of the epithelium (Figure 57). A deposition of delicate collagen-like fibrils oceurs between the epithelial basement membrane and theinvading cells. These are apparently the first changes beneath theepithelium that are detectable.Accompanying the early pannus-type formation is a patchy loss of

the basement membrane and the associated hemidesmosomes of theepithelium (Figure 57). This loss resembles the basement membranehemidesmosomal one described previously in Reis-Biicklers' dystrophy(Figures 19 and 20). Reis-Biicklers' dystrophy has been shownrepeatedly to begin in Bowman's layer and then to be followed byepithelial disturbances. Fuchs' dystrophy, however, appears to leaveBowman's layer intact for a considerable period of time. The hemi-desmosome basement membrane loss also appears to be associatedwith accumulating glycogen and a retraction of the intracellular fila-ments from the basal cytoplasm of the epithelial cells and is probablya generalized phenomenon accompanying the reduction in the adher-ence of the epithelial cells to Bowman's layer. (See Meesman's dys-troplhy for accumulating glycogen, Reis-Biicklers' dystrophy forbasement membrane hemidesmosomes loss.) Presumably, therefore,this early cellular invasion results in lowered adherence of the cornealepithelium, followed by epithelial edema and subsequent desquama-tion, resulting in the usual consequences of erosion and superficialscarring.

655

John W. McTigue

In the case reported here, Descemet's membrane has beeni increasedto approximately four times its normal thickness. The normal thicknessof Descemet's membrane in the mid-portion of the cornea is five toseven microns (Figure 4). The measurement, in this early dystrophy,is approximately 30 microns (Figure 52). The corneal epitheliumdoes not show a change in thickness.

In conclusion, one might re-evaluate the stages in this dystrophyon clinical and histopathologic grounds. It would appear that thereis (1) an uncomplicated disturbance of the endothelium with vacuoli-zation and thickening of the basement membrane-cornea guttata; (2)an activation of the superficial keratocytes with migration of thesecells along the plane between the epithelium and Bowman's layer,and light subepithelial tissue formation; (3) edema of the stroma orepithelium with bulla formation; (4) heavy subepithelial tissue forma-tion, and secondary vascularization and scarring; and (5) the stage oflate complications such as glaucoma or infection.

SUMMARY

1. Light and electron microscopic stuidies on the normal corneA andits alteration in various corneal dystrophies are presented.

2. Studies on the normal cornea have resulted in a division of thecornea into six layers. The basement membrane of the epithelium isadded to the conventional five-layered concept of the cornea, and theimportance of this basement membrane discussed.

3. The separation of the stroma into dermal and mesodermal por-tions is demonstrated clinically, and by light and electron microscopy.The development of this separation from the phylogenetic viewpointand its possible clinical significance are discussed.

4. Light and electron microscopic observation on the corneal dystro-phies available for study are reported. It is felt that electron micro-scopy has helped to pinpoint the site of the early pathologic changesin some of these dystrophies and to clarify the sequence of thesechanges. It is hoped that a better understanding of the pathologic pro-cess may provide a clearer clinical understanding of the diseaseprocess.

ACKNOWLEDGMENT

The author wishes to express his thanks to Dr. Ben S. Fine for hisassistance and advice throughout this study and in the preparation ofthis manuscript.

656

The Cornea and Its Alterations in Various Dystrophies 657

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John W. McTigue

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the human cornea:

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