Epidermis

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 imbedded a relatively large nucleus; this, however, is, spherical, not oval. The 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, is 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 of 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, inasmuch 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 homy layer, pushed upward through which, by the new formation 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 1µ. 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 varies. 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-medul-lated; 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.