1. THE ANATOMY OF THE SKIN


A detail lecture on the anatomy of skin, including all the layers of epidermis and dermis, nails and hairs; the blood supply, nerves and pigmentary cells, sweat and sebaceous glands are all discussed….


SKIN DISEASES by M.E.DOUGLASS, M.D

In order to correctly understand the nature of the morbid changes that go on in the skin, and to comprehend correctly how and where these changes begin, it is necessary to have an accurate knowledge of the healthy skin in its different parts.

The healthy skin is, of course, the standard of comparison for all changes in the skin, and without the clearest perception of what the standard is the student can, of course, only fall into error from the inability to distinguish between what is normal and what is abnormal

The skin consists of an epithelium resting upon a connective tissue basis. The epithelium, which is composed of many layers of cells, is called epidermis, the connective tissue basis is called cutis vera, dermis, or corium. The surface of the dermis is thrown up into a number of elevations-papilla-which differ in size, form, complexity and arrangement in different regions of the body. Some are small, more or less conical elevations, simple papillae. In others, a broader primary elevation is divided at its summit into a number of secondary elevations; these are compound papilla. In many regions of the skin, as, for example, in the palms of the hands, the papillae are arranged in ridges separated by shallow furrows. The surface of the skin, that is, the contour of the epidermis, does not follow the papillary contour of the dermis; the papillae accordingly appear to plunge into and be covered up by the more even epidermis, the surface of which, however, is marked by the ridges and furrows spoken of above as well as by bolder creases and folds.

The surface of the dermis is not developed into a distinct and separable basement membrane, as is so often the case in a mucous membrane; but in the most superficial portions of the dermis the connective tissue shows little or no fibrillation and consists of a homogeneous matrix, in which are imbedded connective tissue corpuscles and extremely fine elastic fibres. This superficial portion of the dermis, which is especially well developed in the papillae, serves accordingly the purposes of a basement membrane, and sharply defines the distance from the overlying epidermis. At a very little distance from the epidermis, fibrillation makes its appearance, the bundles of fibrillae interlacing in a network which very closely set in the outer, more superficial layers, becomes more and more open in the inner, deeper parts, The connective tissue of the dermis thus passes insensibly into the subcutaneous connective tissue, in which thick interwoven bundles of fibrillae, bearing in transverse section a certain resemblance to sections of tendon bundles, form a tough open network, the larger spaces of which are frequently occupied by masses of fat cells of the subcutaneous adipose tissue. Elastic fibres are very abundant in the dermis proper, being very fine immediately beneath the epidermis and becoming coarser in the deeper parts; they are present also, though to a less extent, in the subcutaneous connective tissue. The skin, as a whole, is a very elastic structure.

Blood vessels are very abundant, forming close-set capillary outworks and looks immediately under the epidermis, especially in the papillae, and more open networks elsewhere; but no blood vessel passes into the epidermis. Lymphatic vessels and lymphatic capillaries are abundant in the dermis, being connected here as in other regions of the body with smaller “Lymph spaces.”

The consideration of the nerves of the skin will be deferred until we come to deal with the skin as an organ of sense; for though some of the cutaneous nerve fibres are efferent fibres distributed to the blood vessels, and probably to the sweat glands and other structures not directly connected with the sense of touch, by far the greater number are afferent fibres beginning in the distinct tactile organs, or otherwise serving as sensory structures.

The epidermis consists of two parts, separated by a fairly sharp line of demarcation; an inner soft layer, the Malpighian layer, or stratum Malpighii, and an outer harder horny, layer or stratum corneum. The skin, as is well known, varies in thickness in different regions of the body, and the differences are due almost exclusively to variations in the thickness of the horny layer which, as over the lips, may be extremely thin, or as on the heel, excessively thick; compared with the variations in thickness of the horny layer, the variations in thickness of the Malpighian layer or of the dermis may be disregarded.

The line of demarcation between the Malpighian and horny layers follows the contour of the surface of the skin, not that of the dermis, the papillae of which appear in sections as if imbedded in the Malpighian layer. When the skin after death is macerated, the horny layer is apt to peel off from the Malpighian layer below, which, originally soft and rendered still softer by the maceration, then appears as a layer of slimy tissue spread out between the sides of and covering the summits of the papillae of the dermis, somewhat after the fashion of a network; hence this layer was in old times spoken of as the rete mucosum.

The lowermost innermost portion of the Malpighian layer resting upon the dermis, consists of a single layer of elongated, or almost columnar cells placed vertically, that is, with their long axis perpendicular to the plane of the dermis. This layer which preserves the original features of the epiblast of the embryo, and which may be followed over the papillae as well as along the intervening valleys, presents a characteristic appearance in vertical sections of the skin. Each cell, which is about as large as a leucocyte, consists of a relatively large oval nucleus lying in the midst of a coarsely granular cell substance, which stains readily with the ordinary staining reagents. The base of the cell abutting on the dermis often shows fine processes interlocking with corresponding processes from the dermis; the sides of the cells are in close contact, but merely in contact, no cement substance existing between them.

The rest of the cells of the Malpighian layer, much like each other are polygonal or irregularly cubical cells, resembling the vertical cells just spoken of in so far that each consists of a coarsely granular cell-substance in which is embedded a relatively large nucleus; this, however, is spherical, not oval. This surface of each cell is thrown up into short ridges, radiating somewhat irregularly from the centre of the cell and projecting at the surface and edges, so as to give the cell somewhat the appearance of being armed with a number of prickles. Hence these cells are often called “prickle cells.” The prickles of a cell do not interlock with those of its neighbors but touch at their points, so that the contact of two adjacent cells is not complete but carried out by the points of the prickles only, minute spaces being left between. Hence the whole Malpighian layer is traversed by a labyrinth of minute passages, along which fluid can pass between the touching prickles.

In dark skins, as that of the negro, pigment particles abound in the lower Malpighian cells, especially in the vertical layer. In such cases branched pigment cells, connective tissue corpuscles loaded with pigment granules, are to be seen in the dermis also; and occasionally similar branched cells may be seen in the epidermis between the Malpighian cells. Leucocytes also not infrequently pass out at the dermis and wander among the cells of the Malpighian layer.

The nuclei, not only of the vertical, but also of the other polygonal cells may, not infrequently, be observed in various stages of karyomitosis. Throughout life the cells of this Malpighian layer of the skin appear to be undergoing multiplication by division, the increase of population thus arising is kept down by the cells passing upward and outward, and becoming transformed into the cells of the horny layer.

The line of demarcation between the Malpighian layer and the horny layer is, as we have said, sharp and distinct. It is furnished by two peculiar strata of cells, more conspicuous in some regions of the skin than in others. The lowermost, innermost stratum consists of a single layer or of two or three layers of cells which are not unlike Malpighian cells, but are differentiated by their form, being extended horizontally so as frequently to appear fusiform in vertical sections, by the absence of prickles, by their staining very deeply with certain reagents, such as osmic acid, and especially by their cell substance being crowded with large discrete granules of a peculiar nature. Hence this stratum is called the stratum granulosum.

The stratum above this consists of two or even more layers of cells, elongated and flattened horizontally, the cell substance of which is homogeneous and transparent, free from granules and not staining very readily. In the middle of a cell may frequently be seen a rod-shaped nucleus placed horizontally. These clear transparent cells form a transparent seam, the stratum lucidum, between the stratum granulosum and Malpighian layer below and the horny layer above.

The horny layer, which is, as we have said, of variable but nearly always of considerable thickness, if formed of a number of layers of cells which, differentiated already in the lowest layers, have that differentiation completed as these pass upward. The upper, outer portion of this horny layer is continually being shed or rubbed off in the form or flakes of variable size. Each flake upon examination, as for instance after dissociation by maceration or with the help of alkalies, is found to be composed of elements which can no longer be recognized as cells, and which may be spoken of as scales. Every scale is a flattened mass or plate in which no nucleus can be seen, and which consists not of the proteids and other constituents of ordinary cell substance, but almost exclusively of a material called keratin. This is a body, the exact nature of which has not yet been clearly made out, but which has the general percentage composition of proteids, from which it is a derivative, with the exception that it contains a considerable quantity of sulphur (the keratin of hair contains as much as five per cent); this sulphur appears to be somewhat loosely attached to the other elements of the keratin since it may be removed by boiling with alkalies.

The lowermost portions of the horny layer are composed of elements which may still be recognized as cells, in as much as each contains a nucleus, though this obviously undergoing change and on the way to disappear. Each cell is, however, flattened and plate-like, and its substance already consists largely of keratin. In passing upward from the lower to the more superficial pairs of the horny layer such an imperfect cell loses its nucleus, and becomes the wholly keratinous plate just described. The whole horny layer consists of strata of elements, horny to begin with, but becoming more completely so in the upper parts. Below, in contact with the moist Malpighian layer, the horny layer is moist but the superficial parts become dry by evaporation; and here the strata delaminate from each other, the outer ones, as we have said being shed in the form of flakes, which seen in the dry condition under the microscope have often the appearance of irregular fibres.

The karyomitosis seen in the cells of the Malpighian layer, not only in those of the vertical layer, but in the others as well, show, as we have said, that these multiply by division; we have no evidence of multiplication taking place elsewhere in the epidermis. The more superficial cells of the Malpighian layer, thrust upward by the newcomers, are transformed into the cells of the stratum granulosum; and although we do not as yet fully understand the exact nature of the transformation we may conclude that the peculiar granules of these cells are concerned in the manufacture of keratin. Changed by the consumption of their granules in this manufacture, the cells of the stratum granulosum become first the cells of the stratum lucidum, and then the cells of the distinctly horny layer, pushed upward through which, by the new formations continually succeeding below them, they pass to the surface and are eventually shed.

The papillae in the papillary part vary in size and aspect in different parts of the skin. They are pointed or thread-like about the fingers, and club-shaped or rounded over the general surface of the body. On the palm of the hand, about the nipple, and the sole of the foot, they are longest and largest, being often 66 to 1 m. They are shortest on the face. They are of two kinds-nervous, containing nerve fibres ending in tactile corpuscles, and vascular, containing blood-vessels in the form of a loop.

There are, in addition, spindle-shaped cells, or cells anastomozing by processes, amongst the connective tissue bundles in the substance of the corium and around the vessels.

The thickness of the corium various. It is thinnest about the eyelids and prepuce; on the face, scrotum, and body, generally, it is thicker. On the sole of the foot and palm of hand it is thickest, being from 2.25- 2.28 of a millimetre.

The corium is well supplied with vessels, lymphatics, and nerves. The vessels come up from the connective tissue below, give off branches to the fat and the glands, then constitute a network which sends off more or less obliquely twigs to form a longitudinal plexus along the base of the papillae, from whence finally capillary loops are supplied to most of the papillae. The lymphatics have much the same arrangement as the blood-vessels. The plexuses, however, are situated beneath those formed by the capillaries, but no lymphatics are supplied to the papillae. The spaces normally seen between the connective tissue fibres are supposed to be lymphatic spaces; they have no proper boundaries. The blood-vessels and lymphatics are thought to communicate, though this has not been proved, by means of peri-vascular spaces where the blood-vessels and lymphatics run together, the walls of these spaces being formed by the connective tissue around.

Nerves accompany the blood-vessels coming up from below, and are of two kinds, medullated and non-medullated; the former go with the tactile and Pacinian corpuscles, and the latter form, it is thought, a fine network below the site, in connection with the capillary plexus, and are in communication with those found in the site. Non-medullated fibres also run with the capillaries, which supply the vascular papillae.

The subcutaneous cellular tissue is nothing but a lax network of connective tissue. The bundles of fibres, “usually cylindrical, exhibit constrictions at various points like those of the arachnoid membrane, and consist of a number of sinuous fibres of connective tissue, between which lie numerous fusiform and connective-tissue corpuscles of various forms and dimensions.” It is this part in which the fat cells are collected in the meshes of the fibres, the only seats destitute of fat being the eyelids, penis, scrotum, and ears. The fat I need not describe. The fusiform cells that lie between the bundles and fibres of the connective tissue are connected with these fibres by means of processes given off one at either end, with other minor ones elsewhere. Other fusiform cells, without processes (the migrating cells), and small cells like blood-cells, are observed, the latter being contiguous to the vessels. There is no line of demarcation between the corium above and the subcutaneous tissue below.

The Sweat Glands.-A sweat-gland, like other glands, consists of a secreting portion and a conducting portion. The secreting portion is a long tubular alveolus coiled up in a knot and placed in the subcutaneous connective tissue at some distance from the epidermis. Generally the gland is formed of one such tubule only, but sometimes two tubules unite into a common duct. The duct beginning in the knot, in the convolutions of which it shares, runs a somewhat wavy but otherwise straight course vertically toward the surface of the skin onto which its lumen opens.

Through the epidermis the duct is nothing more than a tubular passage excavated out of the epidermis with a remarkable corkscrew course, the turns of the screw becoming more open and the canal wider in the upper part as it approaches the surface. In the Malpighian layer the cells bordering on the passage are flattened and inclined downwards so as to afford a more or less definite lining; there is a similar arrangement, but not so well seen, in the corneous layer. Reaching the dermis in a valley between papillae, the passage becomes a regular duct, with an independent epithelium of its own, a distinct basement membrane continuous with the upper surface of the dermis, and an outer coat of connective tissue strengthened, in the case of some of the larger glands, such as those of a the axilla, with plain muscular fibres. The epithelium consists of two or three layers of small rounded cells, each with a relatively large but absolutely small nucleus, generally staining deeply. The cells leave a narrow tubular thread-like lumen which is lined with a very characteristic distinct cuticle.

The duct continues to possess these characters after it has entered the knot and begun to pursue a twisted course, but soon changes suddenly into the secreting tubule. This may be distinguished from the duct by being wider, and by being lined by a single layer of cubical or columnar cells larger than those of the duct, bearing larger nuclei, and behaving differently toward various staining reagents. The lumen though fairly distinct is not lined by any cuticle as in the duct. Lying between the basement membrane and the epithelial cells, or rather imbedded in the basement membrane, are seen a number of plain muscular fibres disposed longitudinally or in an elongated spiral, and often forming a distinct coat beneath the epithelium.

As in the case of other glands, we are unable to make any statement as to the work carried on by the epithelium lining the duct, but we may probably assume that the sweat is mainly secreted by the larger cells of the terminal coiled part of the tubule. These cells, therefore, like other secreting cells, are probably “loaded” and “discharged”, but as yet no structural changes in the cells corresponding to these phases have been satisfactorily ascertained, though after the administration of pilocarpine, which causes sweating, the cells of glands hardened in alcohol stain more deeply than usual with carmine. It must be remembered, however, that the sweat contains normally neither mucus nor proteid substances, and we should, therefore, not expect to observe granules in the cells.

The peculiarly placed fibres have been supposed, by their contraction, to assist in the flow of sweat along the tubule. In certain cutaneous glands of the frog, of a relatively simple nature, there is evidence that the secretion is ejected from the comparatively large lumen by the contraction of plain muscular fibres in the wall of the gland, or by a contraction of the wall itself, which is contractile without being distinctly differentiated into muscular tissue. And this rather supports the above view, but the matter is at present by no means clear.

The coil of a sweat gland is well supplied with blood- vessels in the form of capillary networks, and nerves have been traced to the tubes; but the exact manner in which these end is not as yet known.

Though present in all regions of the skin (of man), the sweat glands are unequally distributed, being more abundant in some regions, such as the palms of the hand, than in others. In the axilla are glands of very large size, and in these the ducts possess distinctly muscular coats.

The sebaceous glands are appendages of the hairs, and are seated in the corium; their ducts open into the hair follicle at the neck in the case of the larger hair follicles; but in the case of the smaller, or downy hairs, the relative position of the glands and hair follicle is altered, so that the minute hair follicle leads into the duct of the sebaceous gland, which opens directly on the surface. These glands are absent from the palm of the hand, the sole of the foot, and the dorsum of the third phalanges of the fingers and toes, and there are few about the penis. The largest are found about the nose, scrotum, anus, and labia.

Just where the corneous layer abruptly leaves off in the upper part of the hair follicle, a sebaceous gland opens into the cavity of the follicle, on each side of the hair. Each gland consists of a short rather wide duct which divides into a cluster of somewhat flask-shaped alveoli. The basement membrane both in the alveoli and in the duct, is lined with a layer of rather small cubical cells continuous with the layer of perpendicularly disposed cells which form the innermost layer of the outer root sheath, as of the Malpighian layer of the skin generally. This layer of cells leaves a wide lumen both in the alveoli and in the duct; this lumen, however, is occupied not as in other glands with fluid, but with cells. Both alveoli and duct, in fact, are filled with rounded or polygonal cells which may be regarded as modified cells of the Malpighian layer. The whole gland, indeed, is a solid diverticulum of the Malpighian layer.

In the alveoli the cells next to the layer of cells immediately lining the basement membrane, though larger than these, resemble them in so far that each consists of ordinary cell substance surrounding a nucleus of ordinary character. The more central cells are different; their cell substance is undergoing change; numerous granules or droplets, some of them obviously of a fatty nature, make their appearance in them, and the nuclei are becoming shrunk and altered. The cells are manufacturing fatty and other bodies and depositing the products in their own substance, which, however, is not being removed, but is dying. These changes are still more obvious in the cells lying within the duct; the cells as indicated by the breaking up of the nuclei are dead, and the whole of the cell substance has been transformed into the material constituting the secretion of the gland called sebum, which is discharged on to the surface of the skin through the mouth of the hair follicle.

Melford Eugene Douglass
M.E.Douglass, MD, was a Lecturer of Dermatology in the Southern Homeopathic Medical College of Baltimore. He was the author of - Skin Diseases: Their Description, Etiology, Diagnosis and Treatment; Repertory of Tongue Symptoms; Characteristics of the Homoeopathic Materia Medica.