EPIDERMIS (Cuticle; Cuticula; Scarf skin; Epithelial layer)
The epidermis (3, Fig. 1) is the external layer of the skin. It is a distinct formation, quite independent of the corium, having its origin in the ectoderm, and is not regenerated after complete loss of this matrix. It is entirely cellular in structure, contains no blood-vessels and only a few nerves in the innermost part. The junction between the corium and epidermis is nearly a straight line at about the middle period of fetal life. During the latter part of fetal period, the epidermis grows downward by linear processes into the corium and, as the capillaries in the papillae develop, centers of nutrition are established for the epidermis. The full development and interrelations of the papillae and epidermis, however, are not attained until after birth. Despite the intimate union and close relationship existing between the epidermis and the corium, their behaviour both in health and disease is widely different.
The minute furrows on the surface of the skin, especially noticeable on the back of the hand, are due to the depressions of the epidermis between the papillae. The coarser furrows, as seen upon the back of the neck, extensor surfaces of the joints, forehead and other parts of the face, are due to repeated tensions or muscular contractions of the skin. The epidermis is conveniently divided into four layers from within outward, the stratum mucosum, the stratum granulosum, the stratum lucidum and the stratum corneum.
The stratum mucosum (the rete; rete mucosum; mucous layer; germ layer; rete Malpighii) (M, Fig. 1) is an important layer of the epidermis, being concerned in most of the morbid processes of the skin. During embryonic life, the appendages of the skin are formed by the nutritive activity of the vitalized protoplasm of this portion of the epidermis, and after birth, it is the source of the organic substances of all physiological or pathological secretions. It lies immediately above the papillary layer of the corium and directly beneath the grandular layer of the epidermis, hence it is the deepest and most active stratum of the epidermis. This layer is composed of several layers or strata of nucleated cells, polyhedral in shape, rich in protoplasm and arranged in parallel rows. The cells of the lowest layer are columnar or cylindric in form with oval nuclei which are surrounded with granular protoplasm and often largely provided with pigment. Their long axis is perpendicular to the surface of the corium; they have no cell-wall and sometimes appear as a mass of protoplasm with scattered nuclei. The cells of the next few rows are larger, cuboid in form, with well-defined nuclei and a distinct cell-wall, and contain granular and pigmentary matter. The more superficial rows of the mucous layer are made up of still larger cells, more granular and flattened and their axes generally assume a horizontal position to the cutaneous surface. All the cells of the mucous layer have characteristic protoplasmic processes, called prickles, which unite the cells to each other. Hence the name, prickle-cells or prickle layer, given by some authorities to this part of the epidermis. These processes unite the cells firmly, but at the same time the body of epithelia are kept separated from each other by the so-called cement substance of the skin. This substance also permits the free passage of nutritive material from the papillae of the corium, the ingrowth of nerve-threads, the immigration of white blood corpuscles, and the counterflow of lymph inward to the interpapillary depressions, thence to the lymph-vessels of the corium. The epidermis contains no lymph-vessels proper and no blood-vessels.
The stratum granulosum (granular layer) (G, Fig. 1) is made up of one or two, rarely more, rows of coarsely granulated nucleated epithelia arranged directly above the prickle cells of the mucous layer. In disease, the cells of this layer may be increased to four or five rows. The cells are attached to each other by short threads, hence the intercellular spaces are much narrower than in the mucous layer and the nutrition of these epithelia is correspondingly poor.
This layer is often considered a part of the rete and its granular material, composed of some chemical substance, is generally believed to have an important place in the process of cornification. Waldever named the granular material keratohvalin; some authorities believe that there are two distinct substances; while still others believe that it is a nitrogenous substance (chitin). But whatever the true nature of this granular substance, it first appears as isolated granules near the nuclei of some of the cells of the mucous layer. It is greatly increased in and is characteristic of the granular layer. Unna believes that to this layer the white race owes the color of its skin and supports this belief by the fact that, before the appearance of the granular layer during fetal life, the outer portion of the skin is transparent, so the blood-vessels of the corium can be seen through it, and also, that the color at the border of the lips and the nail beds is due to the absence of the granular layer in these parts throughout life. The changes which occur in the cells of the granular layer, as they are progressively forced upward and outward from the mucous layer, are necessary to further changes in the external layer of the epidermis, known as cornification.
The stratum lucidum, so-called by Oehl, is a thin, ribbon-like, translucent or shining layer, situated immediately above the stratum granulosum and below the stratum corneum of which it is considered a part by most authorities. There is no particular reason why these two or three rows of cells which only represent a stage in cornification should be designated as a separate layer, except that the subject can be more clearly presented by so doing. Kaposi believes this layer to be due to some chemicobiologic change that must take place in the cells of the granular and mucous layers before they become horny cells.
The stratum corneum (horny layer; corneus layer) (C, Fig. 1) is the external layer of the epidermis and hence of the entire skin and extends from the stratum lucidum below to the outside atmosphere. It is essentially the dead layer or surface division of the epidermis and consists of several layers of flattened imbricated cells whose exact shape depends upon the degree of keratinization which they have undergone. By rapid changes, the upper cells of the granular layer are transformed by the apparent melting of their granules into the surrounding cell protoplasm, and clear transparent epithelia are produced. The lower cells of the corneous layer are arranged in polygonal plates with shrivelled nuclei and rudimentary threads, which still serve to connect the frame-work of cells with each other. Still nearer the surface, the cells become thin, flattened, dry scales, lying parallel to the cutaneous surface, the outer rows wrinkled and curled up, preparatory to being shed. According to Unna, cornification of the epidermis is not a complete process but consists of an alteration of the periphery and connecting threads of the cells into horny tissue which, after digestion of the central part of the cells, presents a honeycomb-like structure. No traces of the intercellular canals are found in the corneous layer. The hard, dry character of the cells of this layer is due to the presence of a very hard and resistant substance, known as keratin.
BLOOD-VESSELS
The epidermis has no vascular supply, but the corium and the subcutaneous tissue are liberally supplied with truncal and capillary vessels. Tomsa has shown that the arterial supply is derived from subcutaneous branches which pass through the fascia and, by division and subdivision, form three separate vascular districts. The deepest variety supply the subcutaneous fat with numerous capillaries in a net-like arrangement inside and between the fat lobules (A, Fig. 1). The middle district sends off arterioles to form a capillary plexus for the sweat glands (B, Fig. 1). These capillaries end in small veins, some of which pass upward with the ducts of the sweat-glands and anastomoses with the veins of the papillary region. The third or upper district is supplied from an ascending artery (F, Fig. 1), which sends off branches to form capillaries for the hair-follicles, the sebaceous glands and the papillae. Each papilla is furnished with one or two capillary loops. The papilla of the hair has its own arteriole and capillary similar to those of the papillae of the corium. The capillaries of the papillary layer anastomose freely with those of the upper part of the hair-follicles from which loops pass to supply the sebaceous glands.
Venous plexuses accompany the arterial in all portions of the skin and in general, it may be said that all the venous vessels are somewhat larger than the arterial. The most superficial veins of the skin are derived from the capillaries of the papillae (V, Fig. 1) and form narrow meshes which, together with the deeper and circular veins, form venous branches which anastomose with branches from the hair-follicles and sebaceous glands. These unite into larger vessels and, with the veins from the sweat-glands and fat lobules, merge into the venous sinuses which end in the subcutaneous veins.
American homeopath, he directed several hospitals in New York.
Professor of dermatology.
Served as Lieut. Colonel during the 1st World War.
See his book online: American homeopathy in the world war