Uveal Tract Anatomy

Done by: Othman Al-Abbadi, M.D

Choroid • a thin, soft, brown coat lining the inner surface of

the sclera.• extremely vascular.• extends from the optic nerve posteriorly to the

ciliary body anteriorly.• thickest at the posterior pole (about 0.22 mm)

and gradually thins anteriorly (about 0.1 mm)• Firmly attached to the RPE… firmly attached to

the sclera at the optic nerve, post ciliary arteries, ciliary nerves & at the exit of the vortex veins.

• At the optic nerve the choroid becomes continuous with the pia and arachnoid.

• The Vessel Layer – loose connective tissue containing melanocytes in

which are embedded numerous large and medium-sized blood vessels. The arteries are branches of the short posterior ciliary arteries and extend anteriorly . .the veins are much larger and converge to join four of five vorticose veins that pierce the sclera to join the ophthalmic veins

Choroidal layers

• The Capillary Layer– a network of wide-bore capillaries with saclike

dilatations. They are fed by arteries from the vessel layer and drained by veins into the vessel layer. The capillaries are supported by delicate connective tissue containing melanocytes. They are lined by a continuous layer of fenestrated endothelial cells. It should be noted that the density of the capillaries is greatest and the bore is widest at the macula.

• Bruch's Membrane – 2 to 4 μm thick and consists of five different components: 1) the basement membrane of the endothelium of the

capillaries of the capillary layer, 2) an outer layer of collagen fibers, 3) a meshwork of elastic fibers,4) an inner layer of collagen fibers, 5) the basement membrane of the pigment epithelium of the

retina. The function of this membrane is not exactly known, although it is believed to play a role in the passage of tissue fluid from the choroidal capillaries to the retina.

• Blood supply– Post ciliary arteries & recurrent branches of the

anterior ciliary arteries– The four or five vorticose veins drain the choroid and

pierce the sclera to join the ophthalmic veins.• ·Nerve Supply– The choroid is innervated by the long and short ciliary

nerves. – The long ciliary nerves are branches of the nasociliary

nerves, a branch of the ophthalmic division of the trigeminal nerve. (sensory & parasympathetic)

– The short ciliary nerves arise from the ciliary ganglion and carry parasympathetic fibers and sympathetic fibers.

Functions

• Nourishment of the outer retina• Heat exchange• Absorption of excess light preventing

scattering• IOP regulation (suggested)

Ciliary body• 6.5 mm on the temporal side and 5.5 mm on the

nasal side.. forward to the scleral spur and backward to the ora serrata of the retina.

• On the outside of the eyeball, the ciliary body extends from 1.5 mm posterior to the corneal limbus to a point 7.5 to 8.0 mm post to this on the temporal side and 6.5 to 7.0 mm on the nasal side.

• Triangular on cross section.• The anterior surface or base is ridged or plicated

and is called the pars plicata. The posterior surface is smooth and flat and is called the pars plana.

• The pars plicata gives rise to the ciliary processes where the suspensory ligaments of the lens attach in the intervals between them.

• The equator of the lens is situated about 0.5 mm from the ciliary processes.

• The ciliary body is made up of – 1) the ciliary epithelium; – 2) the ciliary stroma, – 3) the ciliary muscle

• Ciliary Epithelium . – Two layers of cuboidal cells that cover the inner

surface of the ciliary body. The nonpigmented inner layer of cells is the anterior continuation of the neural retina. The pigmented outer layer of cells is the anterior continuation of the RPE. These cells rest against the stroma of the ciliary body. It is interesting to note that the basal surface of the non pigmented cells faces the interior of the eye, while the base of the pigmented cells is directed toward the stroma. As a result, the apices of the non pigmented and pigmented cells face each other; in places they are separated by small spaces called ciliary channels .

– The basement membrane of the non pigmented cells faces the posterior chamber and is continuous with the inner limiting membrane of the nervous part of the retina. The basement membrane of the pigmented cells faces the stroma and is continuous with the basement membrane of the pigmented epithelium of the retina

– The structure of the two layers of ciliary epithelium appears to suggest that both layers are involved in producing aqueous humor. The presence of numerous cell attachments suggests that the activities of the two cell layers may be coordinated.

Ciliary stroma

• consists of bundles of loose connective tissue, rich in bood vessels and melanocytes, containing the embedded ciliary muscle. The connective tissue extends into the ciliary processes, forming a connective tissue core.

• The blood vessels consist of the ciliary arteries, veins, and capillary networks.

• At the peripheral edge of the iris is the major arterial circle, formed mainly by branches of the long posterior ciliaiy arteries. The endothe

Ciliary muscle• Forms the bulk of the substance of the ciliary body and

consists of smooth muscle fibers. • Divided into three main groups;

– 1) The longitudinal or meridional fibers; the most external and closest to the sclera, pass posteriorly into the stroma of the choroid.

– 2) The oblique or radial fibers radiate out from the scleral spur. – 3) The circular fibers, the most internal, run around the eyeball like a

sphincter. They lie close to the peripheral edge of the lens.

• It’s the contraction of the ciliary muscle, especially the longitudinal and the circular fibers that pulls the ciliary body forward in accommodation. This forward movement is responsible for relieving the tension in the suspensory ligament, making the elastic lens more convex and thereby increasing the refractive power of the eye.

• The ciliary muscle is innervated by the postganglionic parasympathetic fibers derived from the oculomotor nerve; The nerve fibers reach the muscle via the short ciliary nerves.

• Functions:– Suspension of the lens and with the process of

accommodation. – The anterior surface of the ciliary processes produces

the aqueous humor. – The posterior surface faces the vitreous body and

probably secretes glycosaminoglycans.

Iris • A thin, contractile, pigmented diaphrngm with a central aperture,

the pupil, lt is suspended in the aqueous humor between the cornea and the lens.

• The periphery of the iris, which is attached to the anterior surface of the ciliary body, is called the ciliary margin, or root of the iris… while the pupil is surrounded by the pupillary margin of the iris.

• It is thickest about 2 mm from the pupillary margin and thinnest at the ciliary margin

• Measures about 12 mm in diameter.• Varies in diameter from 1 to 8 mm, and in 25% of normal subjects

the pupils differ slightly in size.• It divides the space between the lens & the cornea into ant & post

chambers.• The color of the iris varies from light blue to dark brown; the color

may vary from one eye to another in the same person and in different parts of the same iris.

• The anterior surface of the iris is divided into a central pupillary zone and a peripheral ciliary zone. The line of demarcation is formed by a circular ridge, the collarette, which lies about 2 mm from the pupillary margin.

• The anterior surface of the iris is devoid of epithelium and has a velvety appearance.

• Near the outer part of the ciliary region are a number of contraction furrows; which become deeper when the pupil dilates. They’tr merely caused by the folding of the iris as the pupil dilates.

• At the pupillary margin, the pigmented posterior epithelium extends anteriorly around the edge of the pupil for a short distance. When the pigmented epithelium of the iris extends around the pupil margin anteriorly to an excessive degree, it is called ectropion uvae and can be an important sign of abnormal traction on the iris tissues, induced by tumor or other significant pathologic processes.

• The posterior surface of the iris is black and shows a number of radial contraction folds

• Microscopically, the iris consists of two layers: – the stroma, situated anteriorly and derived from

mesenchyme,– two epithelial layers located posteriorly and

derived from the neural ectoderm

Stromal layer

• The stroma of the iris consists of highly vascular connective tissue containing collagen fibers, fibroblasts, melanocytes, and matrix.

• It also contains nerve fibers, the smooth muscle of the sphincter pupillae, and the myoepithelial cells of the dilator pupillae

• The loosely arranged collagen fibrils have a diameter of ~60 nm, and a periodicity of 50 to 60 nm. There are no elastic fibers.

• The sphincter pupillae muscle is located in the pupillary zone of the iris forming a ring of smooth muscle fibers around the pupil, measuring about 1 mm wide.

• Electron microscopy shows that within muscle bundles, group of smooth muscle cells, 6-8 in number , are connected to each other by gap junctions.

• Nerve fibers usually end on one muscle cell situated at the periphery of each muscle group.

• The nerve supply of the sphincter pupillae is from the parasympathetic postganglionic fibers in the short ciliary nerves. They are derived from the oculomotor nerve

• The dilator pupillae muscle is a thin layer of myoepithelium that extends from the iris root as far as the sphincter pupillae.

• The myoepithelial cells are derived from the anterior layer of the iris pigment epithelium that covers the posterior surface of the iris.

• The apical processes form the pigment cells, while the basal processes form the muscle fibers.

• The muscle processes measure about 4 μm thick and 60 μm long. The myofilaments are present throughout the cells but are more concentrated in the muscular processes. The muscular processes are joined by gap junctions and are surrounded by a basement membrane.

• Nonmyelinatd nerve fibers terminate close to the plasma membrane. • The nerve supply of the dilator pupillae is from the postganglionic

fibers of the superior cervical sympathetic ganglion via the long ciliary nerves.

Endothelial layer• There are two posterior epithelial layers. • The cells of the two layers are apposed to each other apex

to apex; between these lies a potential space that can, under certain circumstances, fill with fluid and become a real space, forming an iris cyst.

• The anterior (epithelial) layer lies in contact with the stroma of the iris and is closely associated with the muscular processes of the dilator pupillae. The anterior layer contains relatively few melanin granules that are found in the apical cytoplasm.

• This anterior layer is continuous with the outer pigmented layer of the ciliary epithelium.

• The posterior (epithelial) layer is bathed with aqueous humor and faces the posterior chamber.

• The cells are larger than those of the anterior layer and are cuboidal in shape.

• They are packed with melanin granules.• It is continuous with the inner nonpigmented layer of

the ciliary epithelium.• The apical plasma membranes of both epithelial

layers have numerous microvilli that project into small intercellular spaces.

Blood supply• The arterial supply of the iris is provided by radial

vessels that lie in the iris stroma. The arteries arise from the major arterial circle which is formed from the two long posterior ciliary arteries and the seven anterior ciliary arteries. The radial arteries converge in a spiral pattern toward the pupillary margin and form the radial ridges seen on the anterior surface of the iris. The spiral pattern of the arteries permits adaptation to the movement .of the iris as the pupil dilates or constricts.

• The veins follow the arteries and form a corresponding minor venous circle.

• The radial veins do not drain into a major venous circle but converge and drain into the vorticose veins.

• The endothelial lining of all the blood vessels of the iris, including the capillaries, is nonfenestrated and there are tight junctions between the endothelial cells which makes them less permeable.

Nerve supply

• The iris receives its sensory and autonomic nerve supply from the long and short ciliary nerves. The long ciliary nerves are branches of the nasociliary branch of the ophthalmic division of the trigeminal nerve. These fibers innervate the dilator pupillae.

• The short ciliary nerves arise from the ciliary ganglion and contain postganglionic parasympathetic nerve fibers. This pathway originates in the Edinger Westphal nucleus of the oculomotor nerve. The parasympathetic innervates the sphincter pupillae. A few sympathetic fibers also travel in the short ciliary nerves.

• Function:– Controlling the amount of light entering the eye &

impinging on the retina… However, it should be pointed out that the actual contribution is fairly small and represents only a tenfold change in the amount of light, that is, one log unit

– During accommodation, it constricts restricting the incoming light to the central part of the lens thus diminishing spherical aberration.

Reference

• Clinical anatomy of the eye, Richared S. Snell, second edition.