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.

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.