anatomy,physiology,morphological lesions

8/3/2019 Anatomy,Physiology,Morphological Lesions

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Man's health, the functioning of the nervous, endocrine, cardiovascular and other systems of the body,

the character of the activity of separate internal organs, the activity and tendency of metabolism, and many

other factors cause a direct or indirect effect on the condition and function of the skin. There is a direct

dependence between the skin and the activity of the organism as a whole. Beginning the study of

dermatology with the anatomy and physiology of the skin, it should therefore be emphasized that the skin

is an integral organ closely related to all functions of the organism.

SKIN ANATOMY AND HISTOLOGY

Skin anatomy. The human skin (cutis) is the outer covering of the body and is continuous with the

mucous membrane in the region of the mouth, nose, urogenital organs, and the anus- In an adult, the skin

surface measures 1.5 to 2 m2 while the thickness of the skin (without the subcutaneous fat) varies from

fractions of a millimetre (on the eyelids, the external acoustic meatus) to 4 mm (on the palms and soles). The

thickness of the epidermis varies from 0.06-0.09 mm (on the eyelids) to 0.5-0.8 mm (on the palms and soles).

The thickness of the subcutaneous fat varies considerably: some areas are devoid of fat while in others (on

the abdomen and buttocks of obese persons) it is several centimetres thick., The mass of skin i an adult

accounts for approximately 5 per cent while together with the subcutaneous fat for about 16 to 17.7 per

cent of the total body mass.

There are numerous furrows, folds and depressions on the skin surface which form a complicatedpattern of triangular or rhomboid fields. The wrinkles on the face and the folds on the palms, soles and

scrotum are the coarse furrows of the skin. The ridges and furrows running parallel to each other on the skin of

the palms and the plantar surface of the fingers form diverse figures the pattern of which is very

individual and is an authentic distinctive mark of a person (dactyloscopy).

The skin has a mat tinge and a peculiar colour due to the colour of its component tissues, the

thickness of the granular and horny layers, the blood vessels visible through the skin, and the presence of

the pigment melanin. The colour of the skin may change because the amount of the pigment in it

varies under the effect of external and internal factors.

The skin surface is covered with hairs over a great area. The areas devoid of hairs are the lips

(vermilion border), the palms and soles, the palmar surface of the fingers and the plantar surface

of the toes, the glans penis, the inner surface of the prepuce, and the in-jier surface of the largeand small pudendal lips.

There are hardly noticeable pores on the skin surface which are openings of the sweat and

sebaceous glands. In some diseases (e.g. seborrhoea) these pores are seen with the naked eye.

The distal phalanges of the fingers and toes have nails on their dorsal surfaces.

Skin histology. In ontogenesis the skin develops from two germinative zones: the ectoderm (the

outermost embryonal layer) which is represented by the epidermis (the most superf icial skin layer)

and the mesoderm (the middle embryonal layer) represented by two-layers, namely the true skin, or

dermis (the middle layer) and the subcutaneous fat, or hypoderm (the deepest skin layer).

The boundary between the epidermis and dermis forms a wavy line because of the presence

of skin papillae (special outgrowths on the surface of the true skin) the spaces between which are

filled with epithelial processes.

EPIDERMIS, THE OUTER SKIN LAYER

The epidermis is stratified epithelium undergoing keratinization, it consists of the following

layers: (1) germinative layer, or stratum basale, or stratum germinativum; (2) prickle-cell layer, or

stratum spinosum; (3) granular layer, or stratum granulosum; (4) stratum lucidum; (5) horny layer,

or stratum corneum. All these layers are pronounced well in the skin of the palms and soles; the

stratum lucidum is not found on the face, chest or the flexor surface of the limbs, whereas the

stratum granulosum in these areas is formed of a single, sometimes interrupted row of cells.

There are many nerve endings in the epidermis but no blood vessels; the cells are supplied withnutrients by the lymph flowing in the intercellular slits.

The germinative layer (stratum basale or stratum germinativum) is the innermost layer of the

epidermis and borders directly upon the dermis, or true skin. It consists of a single layer of prismatic


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(columnar) cells arranged like a palisade; between these cells there are slit-like spaces called

intercellular bridges. Large round or val nuclei are seen mostly in the upper part of the cells. These

nuclei are rich in chromatin and stain deeply with main nuclear dyes and because of that they seem

darker than the nuclei of the cells of the overlying layers.

In addition to the prismatic cells, the germinative layer contains a few peculiar branching

(dendritic) cel ls with small dark nuclei a light protoplasm. The bodies of these cells lie on a level

with prismatic cells while their numerous processes entwine the neighbowing cells and penetrate

between the cells of the overlying layers.

With regard to function, the cells of the germinative layer have two features. First, they re the main

sprouting (cambium) elements of the epidermis, from which cells of all the overlying epidermal layers form.

The columnar cells arranged perpendicular to the basement membrane divide by mitosis. Second, the

protoplasm of the cells of the germinative layer contains a pigment, melanin, in the form of brown

granules of various size. It is now believed that the function of pigment-formation is inherent only in the

dendritic cells of the stratum basale which are the only true melanocytes. It is considered that there are 1155

melanocytes per 1 mm2, on the average, and it was established that the number of melanocytes in intensely

pigmented skin (e.g. the skin of Africans) is not more than that in people with a light skin and that the

amount of melanocytes at the site of freckles is 28.6 to 44.5 per cent that in the surrounding light skin.

Because of this, it is now accepted that the degree of skin pigmentation is determined by the functional

capacity of the melanocytes and not by their amount. Melanin forms in the melanocyte cytoplasm throughpolymerization of the products of tyrosine oxidation under the effect of the enzyme tyrosinase the activity

of which depends on the presence of copper ions. The function of the endocrine, glands influences the

formation of the pigment actively. Stimulation of the sympathetic nerve inhibits the production of the

pigment, while ultraviolet rays, ionizing radiation, and some chemical substances stimulate it. Vitamins,

vitamin G in particular, play an important role in melanin formation.

The prickle-cell layer (stratum spinosum) overlies the germinative layer and consists of five to ten

rows of cells which are cuboid in the deep parts of the layer but become flatter gradually as they approach the

next layer, the granular layer. The cells of the prickle-cell layer like the cells of the germinative layer are

separated from each other by intercellular bridges and come in contact by means of protoplasmic

processes. The nuclei of these cells are spherical and large and contain one or two nucleoli. Special

Langhan's cells are demonstrated (with the gold stain) in this layer. They have a poorly staining nucleus andnumerous branching processes stretching between other cells. These cells are devoid of pigment and are always

located above the germinative layer. The nature of Langhan's cells has not been disclosed. Some authors

believe that these cells are of nerve origin, others refer them to migrating leucocytes, still others claim them to

be of mesenchymal origin, while a fourth group identifies them with dendritic cells devoid of the pigment. The

cells of the prickle-cell layer are marked by the presence of specific tonofibrils in their cytoplasm. The

tonofibrils do not pass from cell to cell but terminate in the protoplasmic processes; in the cytoplasm of the

prismatic cells of the germinative layer they are demonstrated less clearly. As we approach the next layer, the

stratum granulosum, the cells of the stratum spinosum become flatter gradually and elongate parallel to the

surface of the epidermis and blend with the overlying layer without forming a distinct boundary.

The granular layer (stratum granulosum) contains one to two or four (on the palms and soles)

rows of cells elongated parallel to the epidermis; the nuclei of these cells gradually grow smaller andnumerous granules which take a deep stain with the main dyes appear in the protoplasm. Some authors

believe these granules to be the products of nuclear degeneration, others think that they are the result of

fragmentation of the tonofibrils. It was considered previously that they were formed of a special substance

called keratohyalin; it proved, however, that this substance is neither keratin nor hyalin but is related to

DNA in structure. The presence of the keratohyalin granules is the first visible stage, of the beginning of

the process of keratinization of the epidermal cells.

The epidermal germinative, prickle-cell, and granular layers are sometimes embraced under the name

of Malpighian layer.

The lucid layer (stratum lucidum) overlies the granular layer and is composed of elongated cells

containing a special protein substance which refracts light strongly. This substance resembles drops of oil andis called eleidin (Gr. elaia olive tree). Besides its main component, eleidin, the stratum lucidum contains

glycogen and fatty substances (lipoids, oleic acid).

With the commonly used staining methods, the stratum lucidum of skin areas that have a thick

epithelial layer (e.g. on the palms and soles) is seen as a colourless strip. It is also demonstrated well in some


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pathological processes (ichthyosis, porokeratosis). A substantiated opinion has been advanced to the effect that

impermeability of the epidermis to water and electrolytes is associated with the stratum lucidum and that it

consists of two layers, the upper one has an acid and the lower an alkaline reaction. There fore, this stratum is

a very complex epidermal layer.

The horny layer (stratum corneum) is the outermost layer of the epidermis, it comes in direct

contact with the external environment and is distinguished by resistance to a variety of external factors. It is

composed of fine, anuclear keratinized elongated cells. They are firmly attached to one another and are filled

with a horny substance (keratin) the chemical structure of which has still not been finally determined. It is

believed that this is an albunoid substance poor in water and rich in sulphur and contains fats and

polysaccharides.

The outer part of stratum corneum is less compact and occasional lamina separate from the main

bulk, i.e. the process of physiological desquamation occurs. The various areas of the skin differ in the

thickness of the stratum corneum which is especially thick on the palms and soles and very thin on the

eyelids and the external male genitals.

DERMIS, THE TRUE SKIN

The dermis, or true skin (derma, cutis, corium) is located between the epidermis and the subcutaneous

fat. Two layers are distinguished in it, the papillary (stratum papillare), or the subepithelial layer and the

reticular layer (stratum reticulare). The papillary layer is that part of the dermis which is found between theepidermis and the superficial network of blood vessels. The reticular layer merges with the subcutaneous fat

and is not demarcated from it sharply. The epidermis is molded into the dermis in the form of rounded

strands between which the papillae penetrate, which lends the boundary between the epidermis and the

dermis the appearance of an uneven wavy line; this ensures close joining of the epidermis and the true skin.

The height and shape of the papillae in the same layer but in different areas of the skin differ. On the palms

and soles, for instance, the papillae may be very high, whereas on skin areas with a thin epidermis they form

only a wavy line which can hardly be distinguished. The presence of the papillae probably improves the

conditions for the nutrition of the epidermis because in this way the area of contact between it and the

dermis is greatly increased as a result of which the supply of nutrients to the epidermis from the papillary

capillaries is easier. The basement membrane situated between the epidermis and the dermis plays an

important role in metabolic processes though its structure is still insufficiently known. Some researchers

believe that the epidermal protrusions of the membrane cells and the dermal argyrophil fibrils form a com-

mon thick network which has the appearance of a membrane, others contend that there is a true homogeneous

membrane containing small amounts of lipoids and mucopolysaccharides (mainly hyaluronic and

chondroitin-sulphuric acids).

The true skin is composed of a fibrous substance of collagen, elastic, and argyrophil (precollagenous)

fibres, and an astructural amorphous interstitial substance found between the connective-tissue fibres. The

dermal papillary layer consists of thin bundles of collagen fibres and many fine elastic and argyrophil fibres.

In the reticular layer the collagen bundles are more compact and thick and intertwine into a thick network

of loops.

Here also numerous elastic fibres, thicker than those in the papillary layer, lie between the collagenbundles and form a thick reticulum. The structure of the reticular layer determines on the main the strength

of the skin which differs with the skin area.

The reticular and particularly the papillary layer of normal skin have a small number of various cell

elements: fibroblasts, histiocytes, lymphocytes, mast and plasma cells, and peculiar pigment cells

(melanophages).

Hairs, glands (epithelial appendages of the skin), muscles, vessels, nerves and nerve endings are

located in the dermis.

THE DEEP PART OF THE SKIN (SUBCUTANEOUS FATTY TISSUE), THE HYPODERM

The subcutaneous tissue, or hypoderm, consists of thick bundles of collagen and elastic fibres

stretching from the reticular dermal layer and forming a wide-loop reticulum in which accumulations of

large fat cells, lobules of fatty tissue, are lodged. The fat cells are almost completely formed of a large

drop of fat which displaces the cell nucleus to the periphery, and a very small amount of protoplasm. The skin

fascia which is a thick connective-tissue plate is also part of the hypoderm; it often fuses with the


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underlying periosteum or the aponeurosis of the muscles to form a single structure. Bundles of connective-

tissue fibres stretch from the superior surface of the fascia to the dermis and form a sort of a lattice called

retinaculum cutis. The thickness and length of the elastic and collagen bundles determine the degree of

mobility of the skin over the underlying tissues and organs.

The thickness of the hypoderm is marked by considerable variability both in different persons and on

different skin areas of the same person. It is usually about 2 mm thick over the skull, 5 to 10*mm thick on

the back, while its thickness in the region of the abdojnen and buttocks may reach several centimetres. There

is no subcutaneous fatty tissue on the eyelids, under the nails, on the prepuce, small pudendal lips, and

scrotum, and there is very little of it on the nose, ears, and lips (the vermilion border).

The subcutaneous fat protects the body from mechanical injuries and cooling. Vessels and nerves are

found in its thickness and hair bulbs and coils of sweat glands may grow into its upper part.

Muscles of the skin. Muscles stretching obliquely in relation to the hairs and raising them on

contraction (mm. arrectores pilorum) and muscle fibres in the walls of vessels and sweat glands are the

smooth voluntary muscles of the skin. When the arrectores pilorum muscle contracts, it raises the hair and

squeezes out the secretion from the sebaceous gland. Smooth muscle fibres which are not connected to the

hair follicles are present in the skin of the scalp, forehead, cheeks, and dorsal surfaces of the hands and feet.

Considerable layers of these fibres are found in the papillary layer of the skin on the scrotum, the axillae,

the nipple, and in the regionthe anus and the prepuce. The muscles which lend expression and mobility

to the face are the striated muscles of the skin.

BLOOD SYSTEM OF THE SKIN

The blood system of the skin is formed of several networks of blood vessels. Large arterial vessels stretch

from the fascia through the subcutaneous fat and give of! small branches to the fat lobules. On the

boundary of the dermis and hypoderm, they divide into branches which stretch horizontally and anastomose

with one another. A deep arterial plexus of the skin forms, which gives rise to branches supplying the coils of

the sweat glands, the hair follicles, and the fat lobules. In addition, the deep arterial plexus gives off quite

large arteries which reach the subpapillary layer and form here a superficial subpapillary arterial plexus. The

small arterial branches originating from it supply the muscles, the sebaceous and the sweat glands, and the

hair follicles.The subpapillary plexus also gives rise to small arteries which do not anastomose (and are therefore

called end arteries) but pass parallel to the epidermis for some length. They give off capillaries which pass

into the papillae and form loops in them. These loops are continuous with loops of venous capillaries

which are wider than the arterial capillaries.

The venous capillaries stretching from the papillae, the sebaceous glands, the draining ducts of the

sweat glands, the hair follicles and muscles come together and form the first superficial subpapillary venous

plexus. In the area up to the boundary with the subcutaneous fat, there are four venous plexuses. The

veins arising from the fourth plexus pass through the hypoderm and drain into the subcutaneous veins.

The epidermis is devoid of blood vessels.

The most powerful network of blood vessels is located in the skin of the face, palms, soles, lips,

genitals and in the skin around the anus.

LYMPHATIC SYSTEM OF THE SKIN

The lymphatic system of the skin forms a superficial and deep networks. The superficial lymphatic

network arises in the papillary layer as blind rounded dilated capillaries between which there are numerous

anastomoses. The second network of lymph vessels is in the lower part of the dermis and already has

valves. This is a network of wide loops forming a lymphatic plexus which in deeper parts is continuous

with lymph trunks.

NEURO-RECEPTOR APPARATUS OF THE SKIN

The skin is richly supplied with nerve fibres and special nerve end apparatus or nerve endings whichform together a large receptor field of the skin as the result of which it can accomplish the function of a sense

organ.

Both the cerebrospinal and the vegetative (sympathetic) nerves contribute to the innervation of the

skin. The main nerve plexus is in the deep parts of the subcutaneous fatty tissue. The skin nerves originating


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from it ascend through the thickness of the true skin and give off along their length numerous small

branches to the hair follicles, the sebaceous and sweat glands, and the vessels of the skin.

A plexus of closely arranged nerve fibres is located in the subpapillary layer, from which separate nerve

branches penetrate the papillae and epidermis. On approaching the epidermis, the fine nerve fibres lose

their myelin sheath and penetrate the intercellular bridges of the germinative and prickle-cell layers as

demyelinated axial cylinders.

There are free (non-capsulated) and encapsulated nerve endings (receptors). The free nerve receptors are

either dendriform or coil-shaped, sometimes with a button-like or funnel-like thickening on the end of a very

fine fibre. Nerve endings (bodies, or corpuscles) are enclosed in a connective-tissue capsule.

The sensory nerves of the skin and the special end apparatus make it possible to perceive the sensation

of pain, warmth, cold, pressure, and touch.

There are several varieties of the encapsulated nerve bodies, depending on the structure of the end

branchings of the nerve (inner bulb) and the outer capsule. The following encapsulated end apparatus are

distinguished in the human skin: laminated Vater-Pacini corpuscles, Golgi-Mazzoni bodies, Meissner's

corpuscles, Krause's bulbs, and Ruffini's bodies. Some authors, however, claim that there are no Ruffini's

bodies in the human skin. represents schematically some of the end nerve apparatus.

Laminated Vater-Pacini corpuscles are the largest encapsulated receptors, which may measure 3 mm and

more. They are usually located in the subcutaneous fatty tissue and are thought to be the receptors of the sense

of deep pressure and proprioceptive sensations. They are found in especially large amounts on the palms,soles, and genitals. Fine, connective-tissue laminae, between which there is tissue fluid, form capsule of the

laminated corpuscle. The capsule has a cylindric cavity in its centre (inner bulb). Blood and lymph capillaries

are supplied to the capsule. The nerve fibre approaching the corpuscle loses its sheaths and enters the centre

of the capsul as a demyelinated axial cylinder. It stretches along the entire cavity of the inner capsule, gives

off fine lateral branches, and ends as a mace-like thickening.

Golgi-Mazzoni bodies are now considered to be a variety of laminated corpuscles although they are

much smaller than these corpuscles and are located not in the hypoderm but in the dermis almost directly

under and in the papillae.

Meissner's tactile corpusclesare situated in the papillae and have an elongated-oval shape. Their

outer connective-tissue capsule is tnin and special 'tactile cells' are arranged inside it horizontally inrelation to the long axis. The myelinated nerve fibre approaches the lower pole of the capsule, loses its

sheaths, and as a naked axial cylinder penetrates the capsule in which it forms meniscus-like thickenings

adjoining the tactile cells. Meissner's corpuscles are developed especially well in man, in whom the tactile

function is very important in every day activity, in work in particular. The corpuscles are present in

especially large numbers in the finger-tips, lips, and tongue mucosa.

Krause's bulbs (the receptors for the sensation of cold) are situated in and under the papillae. They are

composed of a fine connective-tissue capsule and are oval. The nerve fibre loses its sheaths, enters the

capsule and forms a thick coil in it.

Ruffinis bodies (receptors for the sensation of warmth) resemble Krause's bulbs in structure but are

located much deeper, in the deep parts of the dermis and in the upper parts of the subcutaneous fat.

GLANDULAR APPARATUS OF THE SKIN. SEBACEOUS AND SWEAT

GLANDS

The glandular apparatus of the skin is varied and consists of the sebaceous and sweat glands of

different structure and a diverse principle of functional activity.

The sebaceous glands (glandulae sebaceae) are related to holocrine glands in the character of

secretion, i.e. glands in which the formation of the secretions is associated with physiological degeneration

and decomposition of the cells of the gland. This occurs as follows. The intensively dividing epithelial

cells of the sebaceous gland are arranged in a single row on the periphery of the sac of the gland. They

are gradually displaced from the wall of the gland to the centre and undergo physiological degeneration

as a result of which many drops of a fat-like substance form in them while the nuclei of the cells shrink.

The cells that had undergone fatty degeneration swell and rupture, and the secretion of the gland, sebum,

together with the cell fragments enters the wide draining duct of the gland.

The sebaceous glands are found in all skin areas with the exception of the skin of the palms and soles.

In most cases they are connected to the hair follicles, one or more (up to six or eight) sebaceous glands


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surrounding a follicle. Contraction of the mm. arrectores pilorum promotes the discharge of the secretions.

In the skin of the vermilion border, glans penis, inner surface of the prepuce, small pudendal lips, nipples,

and the eyelid margins the sebaceous glands open directly on its surface without any association with the

hair follicles, of which these areas are devoid. Sebaceous glands of the inner surface of the prepuce are

called Tyson's glands (they produce smegma), those on the margins of the eyelids are known as

maibomian glands. The amount of sebaceous glands not connected with the hair follicles varies greatly

in man. The size of these glands is also just as variable; they are largest on the skin of the nose, cheeks, in

the region of the sternum, and above and between the shoulderblades.

The composition of the sebum is complex. It includes free and bound lower and higher fatty acids,

cholesterol and isocholesterol esters, lipoids, protein products, phosphates, chlorides and other extractive

substances among which are those which have been insufficiently studied. In the atmosphere, the sebum

turns into a thickening mass.

The sweat glands (glandulae sudoriberae) are simple tubular glands which secrete sweat by

reflex and play an important role in thermoregulation. In the human skin their number exceeds 3

million.

According to the character of secretion, eccrine and apocrine sweat glands are distinguished. In the

process of secretion, the cells of the eccrine glands are preserved while the apical part of the cells of

the apocrine glands is destroyed and turns into the secretions.

The sweat glands secrete 300 to 800 ml of sweat daily; in unfavourable conditions its amountreaches 1500 ml and more.

The eccrine sweat glands are found in all skin areas with the exception of the vermilion border, the

glans penis, the inner surface of the prepuce, and the outer surface of the lesser pudendal lips. They are

present in particularly large numbers in the skin of the palms, and soles and there are many of them on the

forehead, chest, abdomen and forearms. The eccrine gland is a long epithelial tube which has a coiled end

(the body of the gland) located in the deep layers of the dermis, less frequently in the upper layers of the

hypoderm, and a long draining duct. The wall of the body, or the secretory part, of the gland consists of

glandular cuboidal or columnar epithelium, a basement membrane, and a connective-tissue capsule. At

rest, the secretory cells have a columnar shape and contain many basophil inclusions in the form of

granules of various size, drops of fat, and vacuoles; after the secretion is discharged they become flatter.

There is a layer of longitudinally stretching muscle fibres between the epithelium and the basementmembrane. They are called the myoepithelial cells and contract under the effect of nerve impulses and

thus contribute to the secretion of sweat.

The draining duct of the eccrine sweat gland stretched upward from the body of the gland almost

perpendicular to the epidermis. In the dermis, the wall of the duct is formed of epithelial cuboidal

cells, in the epidermis the duct has a twisted corkscrew shape and preserves its own walls, which was

proved by G. Pinkus and K. Kalantaevskaya independently of each other.

The apocrine sweat glands are located predominantly in the axillae(where they form a whole layer 0.5

to 4 mm thick) and there are very many of them around the anus, in the region of the nipples, the external

female genitals, in the groin, on the pubis, and around the umbilicus. It was mentioned above that when

these glands discharge their secretions part of the secretory cells die. The apocrine glands are larger than

the eccrine glands, their draining ducts open into the hair follicles, while their body lies deep in thesubcutaneous fat. The activity of the apocrine glands is linked with the activity of the sex glands and

they therefore remain underdeveloped until the period of sexual maturation. In the elderly their function

weakens.

The body (coil) of the sweat glands is surrounded by many capillaries forming a thick network.

Innervation is mainly accomplished by branches of the sympathetic nerve, which are located in the

connective-tissue capsule.

HAIR AND HAIR FOLLICLES

Several types of hair are distinguished: long (on the head, beard, moustache, in the axillae, pubis,

genitals); bristly (eyebrows, eyelashes, the hair in the nose and external acoustic meatus); downy (onthe face, trunk, and limbs).

The length, thickness of localization, rate of growth, and shedding of hair vary considerably in the

different skin areas. There are 300 to 320 hairs per 1 cm3 on the crown of the head, 200 to 240 on the

back of the head, 44 on the chin, 24 on the dorsal surface of the forearm, and 18 per 1 cm 2 on the dorsal


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surface of the hand (Brunn). According to different authors, there are 30 000 to 150 000 hairs on the

scalp. The rate of hair growth varies from 0.1 to 0.5 mm per 24 hours and increases or decreases

depending on the general condition, the function of the nervous system and the endocrine glands, the

value of nutrition, and many other factors which exert an effect on tissue trophies. The duration of the life

of hairs varies from several months to four years and longer; under normal conditions, an adult loses 30 to

100 and more hairs daily.

The hair consists of a shaft rising above the skin surface (the outer or free part of the hair) and a root

buried in the dermis. The hair root enclosed in membranes and a connective-tissue capsule is called the

hair follicle, orsacculus. The lower expanded part of the root is known as the hair bulb, this is the site

from which the hair grows. The hairpapilla conveying nerve fibres and blood vessels supplying nutrientsto the hair protrudes into the bulb.

The hair follicle is cylindrical in shape and terminates on the skin surface as a peculiar expansion

called the infundibulum of the follicle, through which the hair shaft passes. The draining ducts of the

sebaceous glands are connected with the follicle on the boundary between its upper and middle third. The

wall of the hair follicle is lined with epithelium which preserves all its layers in the region of the

infundibulum but is represented only by the cells of the stratum basale and stratum spinosum below it (the

outer root sheath). As this sheath penetrates deeper and comes nearer to the bulb it thins out gradually

and deep in the follicle it merges with the cells of the hair bulb.

The hair bulb is formed of polygonal cells which multiply continuously and contain a great amount ofpigment. The cells of the bulb give rise to the hair and to several rows of cells located between the hair root

and the outer root sheath. They form the inner root sheath which almost reaches the site where the ducts of

the sebaceous glands open. It consists of three layers: (1) the inner root-sheath cuticle formed of a single layer

of keratotic cells lying close to the hair cuticle; they are poorly distinguished on sections; (2) Huxley's

layer (one or two rows of polygonal cells, those located deep in the layers of the hair follicle, have a

shrunken pyknotic nuclei while cells found in its upper parts are devoid of nuclei);Henle's layer, the

outer layer of a single row of keratotic cells devoid of nuclei. At the site of the orifice of the sebaceous

gland, the cells of Huxley's and Henle's layers become squamous, disintegrate, and mix with the sebum.

Three layers are also distinguished in the hair root: (1) the central layer, called the medulla,

consisting of keratotic polygonal cells; it is found only in long hair; (2) the bulk of the hair, the cortex,

formed of spindle-shaped keratic elements containing a large amount of pigment; (3) the hair cuticle formedof keratic laminae devoid of pigment and arranged like tiles.

The cuticles of the hair root and inner root sheath are connected to each other and ensure strong

fixation of the hair to the walls of the hair follicle.

The connective-tissue capsule of the hair follicle contains many fine elastic and argyrophil fibres

which form the basement membrane on the boundary with the outer root sheath.

Like the hair bulb, the hair follicles are surrounded by many nerve fibres. The hairs are supplied with

smooth muscles (mentioned above) shaped like a band, one end of which is attached to the compact dermis

layer and the other to the outer connective-tissue hair sheath slightly below the orifice of the sebaceous gland.

On contracting, the muscle raises the hair which thus compresses the sebaceous gland and facilitates the

excretion of the sebum.

NAILS

The nail (unguis) is a horny plate on the dorsal surface of the distal phalanx of the fingers and toes

(nail bed). The proximal end and sides of the nail plate are covered with skin folds called the nail folds.

The nail has a body (corpus unguis), a root (radix unguis) which is its proximal part, and a distal free

margin (margo liber) that freely protrudes forward. The proximal nail fold covers the proximal part of the

nail plate like an arch and forms a thin horny plate of the epidermis, the nail cuticle (eponychium). The

area of the nail bed covered by the nail root is called the matrix (matrix unguis); this is where the nail

grows. In the proximal part of the nail body (which corresponds to the distal part of the matrix) there is a

whitish crescent area called the lynula; it is particularly noticeable on the thumb and big toe.

The nail plate is formed of hard compact horny masses the outer surface of which is smooth, whereasthe inner surface is rough because it has horny ridges which ensure close fitting of the nail plate to the nail

bed.

Large cells with a homogeneous light protoplasm, unclear outlines, and abnormal nuclei are found in

the region of the matrix near to and under the root of the nail. They are called onychoblasts and are thought to


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be concerned with nail formation. It is believed that in the region of the matrix the onychoblasts are

responsible for growth of the nail plate in the direction of the free margin, whereas in the region of the nail

bed they simply contribute to the thickening of the nail. The nail plate moves gradually along the nail bed

and is completely renewed in 90 to 115 days.

MAIN ANATOMICAL AND HISTOLOGICAL FEATURES OF CHILD'S SKIN

Like the whole organism, the skin in children is in a state of organic and functional development from

birth to puberty, which is reflected in the physiology and pathology of the skin in the different periods ofearly childhood (in the newborn, in infants, in young children) and at a more mature age. A marked effect is

produced on skin physiology and pathological conditions in the different periods of childhood by hereditary

and constitutional factors, allergic and immunological reactivity of the child's organism, the activity of the

nervous and endocrine systems, and the metabolic processes in the growing organism. The social and

life conditions, the care of the child, and the character and assimilability of the child's diet are factors

which also influence the pathological processes in the skin.

The skin begins to form from two embryonal buds in the first weeks of intrauterine life. The

epidermis forms from the ectodermal germinative layer, ectoderm, and the dermis and subcutaneous fat from

the mesoderm. By the end of the second month of uterine life, there are two layers of polygonal cells in

the epidermis, the buds of elastic and collagen fibres form in the dermis, and the formation of the hair

follicles and the sebaceous and sweat glands begins. By the end of the fourth and in the fifth month ofintrauterine life, all the layers of the epidermis are clearly outlined, the formation of the elastic and

collagen fibres is completed, and the formation of the nails, hair, and fat lobules in the hypoderm begins.

The buds of the sweat glands are distinguished in the skin in the second month of intrauterine life,

the buds of the apocrine glands between the second and third months, while the buds of the sebaceous

glands are revealed much earlier, between the second and third weeks, and their formation is completed

by the third or fourth month that is why the skin of the newborn is entirely and abundantly covered with an

unctuous lubricant (vernix caseosa). The first hair buds appear in the region of the eyebrows on the second

or third month of intrauterine life; the formation of hair buds over the whole skin surface is completed by

the fourth or sixth month. This hair appearing first is unmodulated, atrophic and is rapidly lost. Formation

of the nails begins in the third month of embryonal development. The nail grows very slowly and reaches

the tip of the finger only at the end of pregnancy and the maturity of the newborn may therefore be

judged by the length of the nails.

Complete structural formation of all main anatomical skin components occurs in the second half of

intrauterine life.

Children grow and develop so intensively and with such significant differences in the structural

functional features of the whole organism, and of its separate organs and systems, that from the didactic

standpoint six main periods in the development of children are commonly distinguished: the first period

(from birth to three or four weeks), the period of the newborn; the second period (from three or four

weeks to 12 months), the period of breast-feeding, or the period of infancy; the third period (from one to

three years), the period of early childhood, or nursery age; the fourth period (between the ages of three and

seven,) the period of preschool age; the fifth period (between the ages of seven and twelve), the period ofyoung school age; the sixth period (from twelve to eighteen), the period of older school age, or the period

of adolescence, or the period of puberty.

Underdevelopment of the central nervous system, lability, imperfect immunity, and variability of

metabolism are especially revealed before the age of three years. These conditions together with the

abundance of vessels in the child's skin, its looseness, a high content of water, increased permeability of

vessels and tissues lead to a more frequent and more turbulent development of allergic reactions in

children, particularly those of the first three or four age groups.

The epidermis of the newborn is covered with a skin lubricant, the periderm, and only three layers

(stratum basale, stratum spinosum, and stratum corneum) are found in the epidermis over a long

distance. Stratum granulosum and stratum lucidum in this age group begin forming only on the palms

and soles. At the same time, the epidermis on the palms and soles is much thinner in children than inadults, while the papillae and epidermal strands are still poorly developed. This explains the smooth

velvety appearance and indistinct pattern of the child's skin. The rapid physiological change of the

epidermal layers in children and the loose arrangement of the cells that are undergoing keratinization are

attributed to intensive mitotic division occurring not only in the stratum basale, but also in the stratum


9/26

spinosum and stratum granulosum.

The dennis in children, like the epidermis, has marked structural specific features which distinguish

it from the dennis of an adult. In a child, for instance, there is a prevalence of cell elements while the

elastic and collagen fibres are insufficiently developed. During the entire period of childhood, changes

are revealed in the structure, compactness, and extent of the fibrous substances. The papillary layer is

flattened out or is seen as a wavy line, except that on the palms, soles, the dorsal surface of the hands and

feet, and the lips (vermilion border) where the papillae are well developed by the time of the infant's

birth. All the fibres (elastic, collagen, and argyrophil) in a child are poorly developed, insufficiently

differentiated, short, unclearly outlined, and abnormally thin or thick. For example, in the papillary layer

the elastic fibres, which are prevalent here, have the structure of thin fibrils and are arranged perpendicular

to the skin surface. In the reticular layer, the collagen fibres are scattered at random, they do not form the

regular rhomboid-triangular network and have a fibrillar structure; there are many elastic argyrophil

fibres among them. The argyrophil fibres in a child's skin are short and weakly convoluted, which is

believed to be associated not with their coarsening but with the unclearness of their outlines because the

process of their formation into fine thin fibres is still not completed. The collagen tissue in this period is

also composed of very fine non-hyalinized fasciculi and only with the gradual growth of the organism do

the collagen fibres become hyalinized and thickened (in the aged this process leads to clear collagenization

of the whole dennis, its sclerosis, drastic thinning of the argyrophil fibres, and the formation of gross

fasciculi of fibres in the elastic network with the conversion of elastin to elacin; it is possible that thisprocess is linked with a shift of the acid-base equilibrium in the acid direction).

The dermis in the period of the newborn and the nursery period is marked by the presence of a large

number of various immature connective-tissue cells among which the most common are histiocytes,

fibroblasts, reticulocytes, lymphocytes, melanophores, melanoblasts, and mastocytes. The last are also

called Ehrlich's mast cells and play an important role in delayed allergic reactions; during the antigen-

antibody reaction they produce great amounts of histamine, heparin, hyaluronidase, protease, and other

biologically active factors which are conducive to an increase in the permeability of the vascular walls and

tissues and, therefore, to the frequent occurrence of delayed allergic reactions.

In the subcutaneous fat of children, especially the newborns and infants of nursery age, there is an

abundance of fatty lobules and, as a result, looseness of the pronounced hypoderm. The connective tissue

alveoli separating the accumulations of fat cells are formed by processes of collagen fibres which are stillimmature and unclearly outlined. The fatty lobules are composed of small incompletely differentiated fat

cells containing large nuclei. As compared to the fat cells of adults, those of children contain liquid

oleic acid in a lesser proportion but larger amounts of high-melting denser acids such as stearic and

palmitic acids, which determines a high turgor of the subcutaneous fat and rapidly developing

thickening of the dermis and hypoderm in sclerema and scleroderma of the newborn.

The characteristic features of the child's blood system are a single layer of endothelial cells forming

the walls of most vessels and increased permeability of vessels which are visible through the thin

epidermis. This, and the insufficiently developed dermis, are responsible for the 'physiological

hyperaemia in the newborn and the peculiar pink-mother-of-pearl colour of the skin in older children.

By the time of the child's birth the sweat (eccrine) glands are already formed but their functional

activity is low because the cerebral centres controlling sweat production are still insufficientlydifferentiated and there is no physiological equilibrium between the cortical and subcortical structures.

Sweating increases gradually in the first two years of the child's life. Insensible perspiration (perspiratio

insensibilis) predominates in child-type perspiration; it is especially intensive in the first year of life. It

is replaced by adult-type perspiration during puberty. The apocrine sweat glands develop completely within

the first year of life, but they begin functioning only in puberty. The sebaceous glands of children are

larger than those of adults and are found in large numbers on the face, scalp, back, and in the region of the

anus and genitals. With the growth of the child, the intensity of the sebum-producing function of the

glands decreases and some of them atrophy completely.

By the time of the child's birth, secondary, or infantile, hair grows, while the lanugo which had

appeared during intrauterine life is shed rapidly. Down hair (lanugo) covers almost the entire skin of

the newborn, with the exception of the lips (vermilion border), palms, soles, lateral surfaces of the

fingers and toes, the dorsal surfaces of the distal phalanges, nipples, the glans penis, the inner surfaces

of the prepuce, clitoris, and the small pudendal lips. During the first year of life the downy hair is shed

several times and new hair grows. Long hair grows on the baby's scalp. Bristly hair forms the eyebrows


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and eyelashes. During puberty hair appears in the axillae and over the pubis; at this same time it begins to

grow on the face of boys.

The muscles of the skin are underdeveloped in children. For instance, the smooth-muscle fibres of the

skin of the scrotum, around the anus, on the nipples and in the axillae are somewhat atrophied, thin, and

loose. Their formation is completed only in the pubertal period.

Because of the thin epidermis and the underdeveloped connective-tissue fibres of the dermis,

irritability of the nerve receptors and special nerve terminal apparatus of the skin in children is increased.

With the child's growth, the skin nerve apparatus becomes less accessible to the external stimuli and, in

addition, the regulating and coordinating influence of the central nervous system is noticeably

intensified,

SKIN PHYSIOLOGY

The skin and external mucous membranes separate the human organism from the environment and

accomplish a variety of functions. Normal functioning of the skin and its appendages is of high significance

for the organism's activity as a whole and has a positive influence on its general condition.

The skin not only responds by its adaptative reactions to the different effects of the external

(exogenic) environmental factors, but is also very sensitive to changes in the various body organs

and systems and is often the first to signal the development of a pathological condition by different changes

in its function. Consequently, though the skin is an independent organ, it at the same time is in aconstant dynamic connection with the external environment and with all the organs and systems of an

adult and child. The skin communicates with the organism by means of the nervous system, circulation and

endocrine glands. The skin takes an active part in protein, carbohydrate, fat, water-mineral, and vitamin me-

tabolism.

PROTECTIVE (BARRIER) FUNCTION OF THE SKIN

Many properties of the skin, subcutaneous fat, and skin appendages allow us to speak of the barrier

function of the skin in a broad sense, a function which contributes to the protection of the underlying

organs and tissues from the pathogenic effect of the diverse environmental factors.

The skin is resistant to mechanical effects (blows, friction, compression, etc.) because of the tightness

of its epithelial covering and the presence of a large amount of resilient elastic and collagen fibres andsubcutaneous fat.

The stratum corneum of the skin, being a poor conductor of heat, protects the underlying tissues from

drying. The thicker the stratum corneum, the more is the skin capable of withstanding the effect of

electric current. In moist skin, resistance to electric current is sharply reduced.

The skin protects the organism from the damaging effect of sun rays because it contains the pigment

melanin which absorbs ultraviolet rays. That is why brunettes, who have more pigment in the skin,

tolerate solar insolation better than fair-haired individuals.

The skin surface is covered with an acid (pH 5.0-6.0) water-lipid mantle, which attenuates or

neutralizes the damaging effect of chemical substances and prevents penetration of micro-organisms into

the skin. Chemicals occurring on damaged skin or those which are soluble in the epidermal lipoids

penetrate the deeper skin layers and from there may be disseminated in the body by way of the blood and

lymph vessels. The sterilizing properties of the skin are due to the bactericidal properties of sweat and

sebum and the continuous desquamation of the upper layers of the stratum corneum. It is believed that the

chemical composition of the sebum contributes greatly to the bactericidal properties of the skin and,

therefore, degreasing of the skin with alcohol or ether as well as its cooling reduce its protective functions.

Heating, in contrast, increases them.

The definite role of the skin in the control of the entry of infections into the body is also a

protective function.

SKIN AS AN ORGAN OF SENSE

The section dealing with the anatomy and histology of the skin describes the numerous nerve endings

and terminal nerve apparatuses which make the skin an enormous receptor field which is directed towards

the external environment and transmits to the central nervous system stimuli from various environmental

factors.


11/26

Four types of skin sensitivity are distinguished: pain, tactile, heat, and cold. The last two types are

embraced under the common name of temperature, or thermal sense. The sense of touch, pressure, and

vibration are related to tactile sense.

The various types of receptors responsible for this or that skin sensitivity surface are localized

irregularly. It has been established that on 1 cm8 of skin surface there are approximately 100 to 200

pain points, 25 tactile points, 12 or 13 cold points, and only one or

two heat points.

Temperature and tactile sensitivity are aroused by definite specific stimuli acting on the skin. The

pain effect may be induced by various factors which, on reaching a definite stimulation threshold, are

perceived as the sense of pain. The sensation of itching may also be considered a form of sensitivity.

Pain is a specific sensation with a pronounced emotional colouring. It is perceived by free nerve

endings in the epidermis and dermis. Because of the different emotional colouring of pain sensitivity,

acute and dull, stabbing, cutting and aching, dragging, and other types of pain are distinguished. As the

result of pain a reflex act occurs which is of a defensive, or protective, character and directed against the

stimulus. Pain sensitivity may be greatly disturbed. Increased pain sensitivity is called hyperalgesia and

hyperpathia; in hyperalgesia the sensation of pain is aroused even by a weak stimulus because the

excitation threshold in such patients is decreased, whereas in hyperpathy the patient's sensitivity is increased

and super-threshold stimuli are perceived as long-term very sharp pain. Diminished sensitivity to pain is

called hypalgesia, while its loss is known as analgesia. Hyperalgesia of various skin areas isencountered very often in diseases of the viscera. In such cases increased skin sensitivity is encountered in

only definite metameres which receive afferent fibres from the same spinal segment that contains the

sensory fibres of the diseased organ (e.g. hyperalgesia of the skin on the neck, chest, upper part of the

abdomen and back in diseases of the heart and lungs, hyperalgesia of the skin of the lower abdomen and

in the region of the lower vertebrae in intestinal disease). These skin areas are called Zakharyin-Head's

zones.

Patients with skin diseases, like some patients suffering from neuroses, often complain of a sensation

of itching which at times is so unbearable that the desire to scratch becomes irresistible. Both exogenous

and endogenous factors induce the itch sensation; the itch is quite often of psychogenic origin. A sensation

of itching on skin areas previously subjected to analgesia gives reason to assume that it is conducted by

nerve fibres other than those concerned with the sense of pain.The tactile sensitivity of the skin is perceived by two types of receptor apparatuses: the nerve

plexuses around the hair follicles (hair sensitivity) and Meissner's corpuscles (particularly in skin areas

devoid of hair). The sense of touch is aroused when the skin comes in contact with objects and when it is

slightly compressed. This type of sense makes it possible to judge the properties of the objects which the

skin touches and orient oneself in the environment. The acuity of touch is determined by measuring the

smallest distance between the legs of a pair of compasses (mm) at which twostimuli are still discriminate.

The acuity of touch is most pronounced on the palmar surface of the fingertips, the tip of the

tongue, and the vermilion border. Tactile stimuli may be distinguished not only according to the

acuity of touch (with Weber's compasses) but also by the strength of the stimulus, for which purpose

special ly designed instruments are used. The senses of touch and pres sure are sensations similar in

quality but differing in force. The sen sation of deep pressure is perceived by the laminatedcorpuscles.

Temperature sensitivity (the sensation of heat and cold) is very important in the life of man

because it makes it possible to regu late body temperature by reflex. It is believed that Ruffini's

bodies perceive heat stimuli and Krause's bulbs cold stimuli. It is mentioned above that there are

much more cold points than heat points on the skin. The skin on the face is most sensitive to

temperature, where as the skin on the limbs is least sensitive. It should be pointed out that the

strength of the temperature sense is greater the larger the stimulated area (0.5 to 0.7 C is the

minimum temperature fluc tuation appreciated by the skin). It is of interest that mechanical,

chemical, and electric stimuli cause a sensation of warmth or cold when they act on the

temperature points.

Dissociation (or splitting) of sensitivity is encountered in clini cal practice, when one type ofskin sensitivity is lost, while the others are preserved.

The conduction pathways of the skin sensory systems in the cen tral nervous system are

distinguished according to the functional sign. The central, or cerebral part of the skin


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temperature or tac tile analyser, for instance, is the region of the postcentral gyrus.

SKIN THERMOREGULATING FUNCTION

Participation of the skin in the process of body thermoregula tion is one of its most important

physiological functions. The body temperature in healthy man is usually maintained at a constant

level irrespective of the environmental temperature, whether high or low. This is achieved by

chemical and physical thermoregulation. The production of heat, which forms in the body as the result

of metabolism and the consequent release of energy, belongs to chemi cal thermoregulation. This is

heat production. An opposite process is heat emission which is related to physical thermoregulationand is accomplished by the skin through three mechanisms: heat radia tion (heat is radiated as radiant

energy, namely infrared rays), heat conduction (heat is given off into the external environment

because of the difference in temperature between the body and environment and because the

environment conducts heat), and evaporation of water from the skin surface . The body gives off

heat by radiation and conduction only when the environmental temperature is lower than the body

temperature. Heat conduction occurs as the result of changes in circulation in the skin blood vessels:

when the environ mental temperature is high, they dilate, the volume of blood flow ing in them

increases, and the emission of heat intensifies. With the increased heat emission the activity of the

sweat glands inten sifies greatly and, consequently, still more heat is emitted through the evaporation

of sweat: in the period of intense sweating, up to 10 litres of sweat and even more may evaporate

from the skin sur face within 24 hours. When the environmental temperature is low, the skin vesselsare drastically constricted, circulation in them de creases (but increases in the viscera) and as a result

heat emission reduces.

Heat emission is regulated by reflex (on stimulation of the skin temperature receptors) and by

direct stimulation of the thermoregu lation centres in the tuber cinereum and lateral wall of the third

ventricle (hypothalamic region). The skin vascular reactions and the secretion of sweat, which are

constituents of the process of heat emis sion, are controlled by sympathetic nerves and fibres arising

from the sympathetic ganglia.

SECRETORY AND EXCRETORY FUNCTIONS OF THE SKIN

The sweat (eccrine and apocrine) and sebaceous glands are great ly responsible for the secretoryand excretory functions of the skin.

The eccrine sweat glands produce sweat which has a weak acid reaction, relative density of

1.004-1.008. It consists of 98-99 per cent of water and 1-2 per cent of inorganic compounds such as

phosphates, sulphates, sodium and potassium chloride, calcium salts and organic substances (uric

acid, urea, creatinine, creatine am monium, amino acids, carbohydrates) dissolved in it. The chemical

composition of sweat may alter depending on its amount and the organism's general condition. In some

diseases attended with met abolic disorders, substances may appear in the sweat which are usually

not found. Diabetes mellitus in which sugar is detected in the sweat may serve as an example.

Various drugs given to the patient (arsenic, mercury, quinine, iodine, bromine, and others) may

also be excreted in the sweat. This is the excretory function ot the sweat glands. Under normal

conditions, the excretion of wa ter by the sweat glands and its evaporation from the epidermal surtace

is even and imperceptible; this is called insensible perspiration, m tne period of increased heat

emission, perspiration is visible profuse, and continuous whereas under normal conditions sweat

excretion is pulsatile. Increased perspiration leads at first to copious now of water from the tissues into

the blood, after which the water from the plasma is excreted by the sweat glands. The oral mucosa

becomes dry and there is a feeling of thirst.

The apocrine sweat glands, whose function is linked with the endocrine, especially the sex,

glands contain in their secretions, besides the common components of sweat, glycogen, cholesterol

and its ethers, and iron. Their secretions have a neutral or weak alkaline reaction. The functional

role of the apocrine glands is still not clear to a great measure. Evidently, they do not play an

important role in thermoregulation but, like the sweat glands of the palms and soles, they increasetheir activity sharply during emotional reactions of the organism.

The sebum, the secretion of the sebaceous glands, has a complex chemical composition. Its main

components are free lower and higher fatty acids, neutral fats, nitrous and phosphorous compounds,

carbohydrates, various stearins, steroid hormones, and cholesterol compounds. On the surface, sebum


13/26

mixes with the sweat and forms a fine film of water-fat emulsion. This film plays an important part

in maintaining a normal physiological condition of the skin. The excretory function of the sebaceous

glands consists in the excretion with the sebum, like with the sweat, of some drugs (iodine, bromine,

salicylic acid, antipyrin, etc.) that had been given to the patients and some toxic substances which

form, in particular, in the intestine.

As it is pointed out above, the function of the eccrine glands is regulated by the sympathetic

(cholinergic) nerves, whose centres are in the spinal cord, medulla oblongata, and the

diencephalon (the higher centres). The cerebral cortex has a regulating effect on the activity of the

sweat glands, that is why perspiration increases in fear, anger, fright, and other emotional

experiences. The secretion of the sebaceous glands is regulated by the nervous system and the

gonadal, pituitary, and adrenocortical hormones.

The process of keratinization of the epidermis with gradual conversion of the cell protein

substance to keratohyalin, eleidin, and keratin is now considered to be the secretory function of the

epidermis.

RESPIRATORY AND RESORPTION FUNCTIONS OF THE SKIN

The skin takes little part in respiration, i.e. the absorption of oxygen and elimination of carbon

dioxide; in children the diffusion of gases through the dilated skin capillaries is more pronounced.

As compared to pulmonary exchange, the skin absorbs 1/180 of oxygen and eliminates1

/flo-1

/8B ofcarbon dioxide. Besides, the skin discharges water vapours (up to 800 g daily, which is twice to

three times the work performed in this respect by the lungs).

Hardly any water or solids are resorbed through the healthy skin, but substances easily dissolved

in fats and lipids (resorcinol, sulphur, salicylic and boric acids, lead oxide, ferric chloride, iodine,

mercury, chloroform, pyrogallol, etc.) are readily resorbed by it. The degree of resorption on a skin

area depends on the condition of the water-lipid mantle, the buffer capacity of the skin surface,

and the presence of the sebaceous follicles. Therefore the skin of the palms and soles, for example,

which is devoid of sebaceous glands, has low resorption capacity, the more so since there is a dense

corneal layer.

In some cases, however, resorption is intensified. Factors conducive to this are swelling of

macerated epidermis, long-term rubbing into the skin of finely ground substances, application to theskin of solids dissolved in volatile fluids (e.g. salicylic acid in ether), hyperaemia of the skin.

It should be borne in mind that in some dermatoses (eczema, psoriasis, dermatoses attended with

the formation of vesicles and erosions) the resorption by the skin of resorcinol, tar, pyrogallol,

chrysarobin, salicylic acid, and other substances increases sharply. In such cases, the prescription of

these substances in high concentration is not advisable and, besides, the condition of the whole

organism and of the kidneys must be continuously checked when tar, chrysarobin, pyrogallol, and

salicylic acid are used. High doses of salicylic acid may cause sharp meningeal symptoms (headache,

nausea, vomiting, loss of consciousness).

PARTICIPATION OF SKIN IN METABOLISM

Some biochemical processes take place only in the skin. They include the formation of keratin

(the horny substance), melanin, and vitamin D. They are complex and have been insufficiently

studied to date.

The skin takes an active part in the metabolism of water, minerals (sodium, potassium, calcium,

etc.), fat, proteins, and carbohydrates in the body. It is also concerned with the metabolism of

hormones, enzymes, vitamins, and trace elements because, on the one hand, it is a spacious depot

and, on the other, some of these substances are removed from the body with the secretions of the

sebaceous and sweat glands.

The water content in the body of an adult ranges from 60 to 65 per cent, whereas in the skin it

may reach 71 per cent. The skin takes second place after the muscles as a large water depot. The wa-

ter content in the subcutaneous fat is much less (up to 10 per cent). In some dermatoses (pemphigus,acute eczema, erythroderma, etc.) the amount of water in the skin increases. It is stored within and

outside the cells because of the hydrophilia of the connective tissue cells, the elastic and collagen

fibres, and the subcutaneous fat.

The total amount of mineral substances in the skin may account for as much as 1 per cent of the


14/26

skin mass; the amount of these sub-stances in the hypoderm is about half that in the dermis. The

amount of some of the minerals alters and interrelationship between them is disturbed in various

disorders of mineral metabolism. In pemphigus and Duhring's dermatitis herpetiformis the content of

sodium chloride (like the content of water) in the skin grows considerably. In tuberculosis of the

skin, the amount of potassium increases in the foci of affection and in areas free of the lesions.

Protein metabolism in the skin occurs at the expense of collagen (98.8 per cent of the total skin

proteins), albumins, globulins, mucoid, elastin, keratin, and protein metabolites (amino acids, urea,

creatine, uric acid, ammonia, purine bases, and pigments). Tests for non-protein nitrogen make it

possible to judge the intensity of protein metabolism which grows in acute inflammation, burns,

tuberculosis and other pathological processes of the skin.

The content of sugar in healthy skin ranges from 50 to 75 per cent. In diabetes mellitus there is

much more sugar in the skin than in blood. It is believed that disorders of carbohydrate metabolism

(whether obvious or latent) lead to the development or persistence of some dermatoses (furunculosis,

acne, rubromycosis, candidiasis, urticaria, eczema, etc.). A. I. Kartamyshev established that

unpleasant emotions induced by hypnosis led to changes in the skin sugar content, and thus proved the

effect of the nervous system on metabolic processes, carbohydrate metabolism in particular.

Vitamin metabolism plays a very important part in the skin biochemical processes. Vitamin C,

for instance, contributes to the production of melanin and the conversion of glucose to glycogen; vi-

tamin A participates in the formation of the horny substance; vitamins A, E, and D activate proteinmetabolism in the epidermis and the assimilation of sulphur. Vitamins of the vitamin B complex, i.e.

BL(thiamine), B2 (riboflavine), B5, B8, B12, and others participate in the oxidation-reduction processes.

Vitamin PP (nicotinic acid) improves various functions of the liver and produces a vasodilative and

photodesensitizing effects. Lack of vitamin A causes the development of phrynoderma, ichthyosis,

xeroderma, acne vulgaris, and dystrophy of the nails. Pellagra, cheilitis, stomatitis, and glossitis

are linked with lack of the B complex vitamins in the body. Reduced vitamin C content leads to the

development of scurvy.

INTERACTION OF THE CENTRAL NERVOUS SYSTEM AND THE SKIN

Nobody doubts today that the central nervous system has an enormous effect on the skin and its

appendages under normal conditions and in various pathological conditions. The nervous system is re-sponsible for the interaction of the systems and organs of the whole human organism, the

connection with the external environment, and the character of the organism's reactions to the effect

of the numerous and diverse exogenic and endogenic factors. The reflex principle of reaction,

expounded and substantiated in the works of Sechenov, Pavlov and Vvedensky and the scientific schools

they founded, is of decisive importance for the vital activity of the whole organism. Research conducted

by Petrova at Pavlov's laboratory proved to be of direct importance for dermatology; in dog

experiments she demonstrated the role of nervous trauma ('collisions') of the cerebral hemispheres in

the development of eczematous foci and trophic ulcers of various localization.

Among the Russian and Soviet dermatologists Polotebnov, Pospelov, Nikolsky, Kartamyshev, and

Zheltakov were the first to emphasize the effect of the cerebral centres and the sympathetic nervous

system on the origination of some dermatoses. They showed in hypnotized persons the effect of thesecond signalling system on the biochemical processes in the skin, the state of the skin capillaries, the

morphological composition of the skin, the regenerative processes in the epidermis and dermis, and

on the character of allergic and inflammatory reactions of the skin to the administration of a

stimulus into the body. By exerting an effect on the second signalling system (treatment by suggestion

under hypnosis, electric-sleep therapy, treatment by indirect suggestion) it proves possible to treat

patients suffering from lichen ruber planus, warts, and sometimes those with urticaria, to bring relief

to patients with some form of eczema or neurodermitis, to alleviate some symptoms (e.g. itching) and

mitigate an inflammatory skin reaction (Pototsky, Zheltakov, Skripkin, and others).

At the same time, the condition of the skin, particularly in various pathological processes, has

a direct effect on the activity of some of the nerve centres and the cerebral cortex. Persistent derma-

toses, particularly if they are attended with severe and excruciating itching and various unpleasant

sensations, may lead to diverse disturbances in the activity of the nervous system at its different lev -

els and to the development of neurotic states and even psychotic reactions of definite severity. This

may ultimately lead to the creation of a vicious circle. A break in the chain of this circle at any level


15/26

has a favourable effect on the course of the whole morbid process.

MAIN SPECIFIC FEATURES OF SKIN FUNCTION IN CHILDREN

The anatomical deficiency of the structural components of a child's skin (non-differentiated cells of

the connective tissue, very rich vascularization and hydrophilia, etc.) results in marked imperfection

of its protective components. Other contributing factors are the lowered bactericidal property of the

secretions of the sebaceous and sweat glands and the neutral or weakly alkaline pH of the water- lipid

mantle (in adults it has an acid reaction). Because of all this, the skin of children, especially in thevery young, is very vulnerable and is marked by a tendency to the formation of cracks and abrasions,

easier development of pathological processes, and rather frequent development of extensive pyoderma,

sometimes with symptoms of general sepsis. In this respect one should not ignore the weakness of the

pituitary-adrenal system, which is conducive to diminution of oxidation-reduction processes in the

child's tissues, the immaturity of the cerebral cortical cells, and the non-differentiation of the

vegetative nervous system in childhood. On the other hand, the rich vascularization of the child's

skin, the high intensity of the metabolic processes, and the marked function of the thymus favour

the activity of the regenerative (restorative) reactions, while the weak heat conductivity of the skin

protects the child's organism from overcooling and overheating.

The neonatal period and the period of infancy are also more favourable in respect of some

infectious diseases because the level of natural immunity is directly dependent on the state ofimmunity of the mother and the immunobiological properties of her milk. The infectious disease

incidence (dermatomycoses among others) increases at the ages between one and eight years because

after the age of 12 months congenital immunity weakens while acquired immunit y is stil l not

adjusted.

The secretory function of child's skin is marked by a higher than in adults production of

cholesterol by the sebaceous glands and calcium and phosphorus compounds by the sweat glands.

At the same time, vitamin D synthesis and keratinization of the epidermis are weaker in the

newborn and infants, which lowers the protective properties of the skin, i ts bactericidal effect in

particular.

Because of the fine, thin horny layer of the epidermis, the dilated blood and lymphatic vessels

and increased permeability of their walls, the excretion of water through the skin of children occursmainly by means of insensible perspiration. Minerals are excreted in the water (in amounts several

times those excreted through the lungs). This process must be continuously corrected by means

of a rational regimen of feeding (including the intake of fluids) and bathing of the child. In febrile

conditions or when the child is clothed too warmly, much more water is lost through profuse

sweating, which often leads to the development of heat rash (miliaria crystallina), especially in

infants of the first months of life.

The same factors (the thin epidermal horny layer and the dilated vessels with increased

permeability of their walls) are responsible for the fact that gas exchange through the skin of

children accounts for 1 per cent of the total gas exchange of the body. Besides, the child's skin

possesses high resorption capacity which grows still more during inflammatory processes.

Metabolic processes are more active in child's than in adult's skin, and the fact that morewater is contained in children's skin than in the skin of adults is important in this respect (all

metabolic processes occur in the water phase). For instance, water constitutes three fourths of the

body mass of infants and three fifths of the mass of adults. With age, the percentage content of

water in the skin, like in the whole body, decreases. The water requirements per kilogramme of body

mass vary accordingly. In a newborn infant they are 150-200 ml, in the first six months of life 100-

150 ml, by the age of 12 months 80-90 ml, under the age of five years 60-70 ml, under the age of

ten 15-60 ml, and under 15 years of age 40-50 ml per kilogramme of body mass.

Though the child's skin perceives cold and warmth and possesses tactile sensitivity, in infants

the development of the nerve endings is not completed and the epidermis and dermis are

underdeveloped. Because of the insufficient differentiation of the peripheral analysers and the

centres in the brain, the nerve stimulation in children is not converted to a realized, clearly located

sensation. Functional immaturity is therefore typical of the receptors, conduction pathways, and

cortical cells. The insufficient differentiation of a large flow of skin stimulating impulses arriving

from the external environment induces a state of protective inhibition in the child's central nervous


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system, which is displayed by long periods of sleep, especially in infants.

HISTOMORPHOLOGICAL CHANGES IN THE SKIN

Many skin diseases are marked by an inflammatory character. According to the severity and

duration of the reaction, acute, sub-acute, and chronic inflammation is distinguished which

occurs in response to the effect of various external and internal stimuli. The type of the response

of the organism and skin to the action of the stimulus is determined by many factors: the condition

of the receptor apparatus, the character of higher nervous activity in the person, the organism'sreactivity, and others. Allergic reactions play a very important part in the type of the response of

the skin as a part of the entire organism to the ef fect of the stimulus.

Though the combination of the stimulation and the response of the organism, the skin in

particular, depending on numerous factors, may result in the development of an inflammatory

reaction differing in severity and duration, various degrees of manifestation of alteration, exudation,

and proliferation are distinguished in the microscopic picture of each inflammation. Alteration is

the manifestation of tissue damage (dystrophy and necrosis of the tissue elements); exudation is the

escape of fluid and formed elements from the vessels because of increased permeability of the vascular

wall; proliferation is the multiplication of tissue elements. In acute inflammation, occurring within a

short period of time, vascular-exudative phenomena prevail and the inflammatory process is charac-

terized by great intensity. In chronic inflammation, which is marked by a prolonged course, proliferative phenomena prevail, the vascular-exudative component is much less manifest and the

inflammatory process is not intense.. According to the intensity of the inflammatory reactions,

subacute inflammation takes an intermediate place between acute and chronic inflammation.

Because of the specific anatomical features of the epidermis, the pathological processes in it have

a peculiar course. Three main types of inflammatory changes are distinguished in the

epidermis.

The first type of serous inflammation is marked by intracellular oedema, or vacuolardegeneration (alteration cavitaire), in which vacuoles forming in the protoplasm of the cells of the

Malpighian layer are arranged in the vicinity of or around the nucleus and displace it to the periphery.

In such cases the nucleus is distorted and often has all the signs of pyknosis. The oedematous fluid

gradually dissolves the cell and causes its death. If the vacuoles occur within the nucleus, it swellsand transforms into a spherical vesicle filled with fluid in which a nucleolus is sometimes

preserved.

The second type of serous inflammation is characterized by spongiosis, or intercellular oedema(status spongioides), in which the oedematous fluid separates the intercellular spaces of the Malpig-

hian layer and ruptures the intercellular bridges, as a result of which the connections between the cells

are lost, the cells themselves become oedematous, and epithelial vesicles begin to form. Spongiosis

is typical of eczema and other dermatoses.

The third type of inflammation of the epidermis is ballooning degeneration occurring innecrobiotic and degenerative changes in the cells of the Malpighian layer. Besides the deep changes

in the epithelial cells, destruction of the intercellular bridges leads to the loss of the connections

between the cells which swim freely in the serofibrinous contents of the vesicle and take a spherical

shape. Such changes are found, for instance, in herpes. A combination of the first and second types

of serous inflammation is usually encountered in skin diseases marked by an inflammatory process.

Polymorphonuclear leucocytes (neutrophils, eosinophils) predominate in the infiltrate in acute

inflammation. In chronic inflammation the infiltrate mostly contains lymphocytes which are

scattered or arranged around the vessels. The infiltrates contain many histiocytes representing all

transitional stages, from non-differentiatedround cells to fibroblasts. The reticular cells are the

same histiocytes, but are la rger in size.

The plasma cells have a well developed basophil protoplasm and an eccentric nucleus and are

larger than the lymphocytes. The epithelioid cells are elongated and have a large spherical or oval

nucleus and abundant protoplasm. Large spherical or oval multinucleate cells with irregularcontours are called giant cells.

Besides serous inflammation, the following peculiar pathological changes may be found in the

epidermis.

Acantkosisis intensified proliferation of the prickle-cell layer in the form of projections into


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the dermis to this or that depth.

In acanthosis the number of mitoses in the basal layer is increased and the connective-tissue

papillae of the dermis are elongated accordingly, the condition is known as papillomatosis.Acanthosis is encountered in psoriasis and eczema.

Acantholysis is melting of intercellular epithelial bridges, the impairment of firm connectionsbetween epithelial cells with the result that cells are easily separated and form more or less large

layers of the desquamating epidermis. Such a process is observed in pemphigus.

Dyskeratosis is abnormal cornification of the cells of the epidermis; it may occur as

hyperkeratosis, i.e. thickening of the horny layer without changes in the structure of the cells, or

parakeratosis, i.e. the presence in the horny layer of the epidermis of cells with stained rod-shapednuclei (the granular and clear layers are absent in such cases).

Granulosisis thickening of the granular layer of the epidermis.

MAIN INFORMATION ON THE AETIOLOGY AND PATHOGENESIS OF SKIN DISEASES

Aetiology is the science of the cause of the disease; pathogenesis is the mechanism of the

origin and development of the disease and the routes by which the morbid process spreads;

sanogenesis are the mechanisms protecting the organism from disease.

There are many causes of the development of various skin diseases. Sometimes these causes

(factors) may be related to unconditioned stimuli because their action always and in all

individuals evokes a definite reaction of the skin and is a local response of the whole organism to

the damaging effect of the stimulus. Examples of such stimuli are concentrated acid and alkaline

solutions, which produce chemical burns, large doses of radiant energy, e.g. X-rays, which cause

specific skin lesions, the action of high temperatures on a limited skin area (thermal burns) or the

effect of low temperatures (frostbite), etc.

In most cases, however, a combination of several factors inducing the pathological process is

necessary for a skin disease to develop. For instance, it is general knowledge that microbial

associations, staphylococci among others, are always present on the skin. In some cases the

staphylococcal strains are highly virulent, but despite this, additional factors are needed for the

development of pyoderma. Among such factors, for example, are disturbance in the defence propertiesof the skin (changes in the water-lipid mantle of the skin, microtraumas) or in the organism's reactivity

because of hypovitaminosis, diabetes, and other endogenous factors.

The separation of the aetiolbgical factors of skin diseases into exogenous and endogenous would

therefore be conditional to a certain extent in such cases. Exposure to the effect of exogenous factors

usually gives rise to a morbid process when endogenous factors weaken the organism's physiological

defence mechanisms and in this manner create a background, as it were ('predisposition') against

which skin diseases develop.

It should be borne in mind that the condition of the human organism depends greatly on the

conditions of life and work. A disease may, therefore, be considered a social phenomenon.

The exogenous aetiological factors of skin diseases include physical, chemical, and bacterial

stimuli and plant and animal parasites.

1.Mechanical stimuli, thermal injury, and the effect of radiant energy are distinguishedamong the exogenous physical aetiological factors. Long-term pressure, friction, contusion, and

other mechanical effects may cause an inflammatory process of various intensity and haemorrhage

and may promote the development of corns and callosity. The thermal factors are the action

exerted on the skin by low or high temperatures as a consequence of which chilblain, frostbite,

burns, etc. may develop. Exposure of the skin to radiant energy (in particular ultraviolet rays, X-

rays, ionizing radiation, etc.) may lead not only to the development of dermatitides of various

severi ty but also to severe dystrophic changes with necrosis of skin areas which heal poorly.

2.Chemical factors capable of producing dermatosis are very diverse. They may be of an

occupational character (occupational chemical irritants) or may be encountered in every-day lifeor as various medicinal agents. The pathological changes induced in the skin by chemical factors

are also diverse.

The degree of sensitization of the organism, its allergic condition plays an important role in the

pathogenesis of dermatoses caused by these agents, as a result of which it is sometimes difficult to


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interpret the pathogenesis in a concrete case.

3. Bacterial factors, pathogenic microbes, are the cause of pyo-derma, tuberculosis of the skin, lepra; protozoa cause leishmania-

sis and other diseases; filtrable viruses induce lichen planus pernphigoides, herpes zoster, warts,

pointed condylomata, molluscum contagiosum.

4.Plant parasites (pathogenic fungi) cause trichophytosis, microsporosis, favus, and other

dermatomycoses.

5. Animal parasites (the scabies mite, the gadfly larvae) may penetrate the skin and

anatomy,physiology,morphological lesions

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