The regional anatomy of the human integument with special reference to the distribution of hair follicles, sweat glands and melanocytes

The regional anatomy of human skin is discussed in terms of ( a ) the regional variation of the architectural pattern of the basal layer of the epidermis, ( b ) the regional variation in the distribution of hair follicles and eccrine sweat glands, and the regional variation in the distribution of melanocytes. ( a ) The architecture of the basal layer is regionally specific. The epidermis of the cheek is almost flat between the numerous hair follicles. Regions under tension have parallel ridges that end abruptly (neck, breast, abdomen); regions with a thick keratin or mucous layer have deep ridges with circular imprints of tall dermal papillae (sole, palm, knee, heel and oral mucosa). Elsewhere in the epidermis the creases of the skin surface divide the pattern of the basal layer into diamond-shaped areas where the imprints of the dermal papillae are to be seen. ( b ) There is great individual and regional variation in the distribution of hair follicles and sweat ducts:700 + 40 hair follicles per cm 2 were counted on the face, but only 65 + 5 in the rest of the body. The corresponding density for eccrine sweat glands was 270 + 25 in the face and 160 + 15 in the rest of the body. There are altogether about two million hair follicles and three million sweat glands in the integument. The epidermal appendages are symmetrically distributed; there is no significant difference between male and female in the density of hairs or sweat glands. The density of appendages is much higher in the foetus and in the infant than in the adult. Numerical estimates have shown that the differential rate of growth of the body surface may be solely responsible for regional differences in the density of appendages. A uniformly distributed foetal population of appendages would become ‘diluted’ three times more on the trunk and extremities than on the head during postnatal growth. The numerical ratio of sweat ducts/hair follicles is the same throughout foetal and postnatal life. ( c ) On the average there are about 1500 epidermal melanocytes/mm 2 of skin surface, excluding those in hair follicles. The total number of epidermal melanocytes in an adult is about 2000 million. They occur consistently in the basal layer of the epidermis of ‘white’ human skin (including the oral and nasal cavities). Their absolute number and their proportion to the keratinizing basal Malpighian cells are constant and characteristic in given regions. The distribution of melanocytes is also bilaterally symmetrical and their regional frequency is the same in male and female. The individual and regional variations of melanocyte distribution are, however, great. There are two or three times as many melanocytes per unit area in the epidermis of the cheek or forehead as in the other regions of the integument. Because melanocytes are mostly located on ridges, the numerical ratio of Malpighian cells/melanocytes is lower on than between the ridges. The cause of the great regional variation of melanocytes is not known. The regional differences are smaller in foetal than in adult skin. Regional differences in the degree of expansion of the body surface by growth cannot, however, explain the regional variation in the adult. Melanocyte density in the foetus is lower than in the adult, and in old epidermis a decrease in melanocyte density is one of the manifestations of ageing. Comparisons of the frequency distribution of melanocytes reveal no significant difference between the various human races. The degree of melanization of skin therefore depends not only on the number of melanocytes, but, more particularly, on their physiological activity in melanogenesis. The absolute number of melanocytes and the ratio of Malpighian cells/melanocytes are high enough to allow melanocytes to make contact with every Malpighian cell and so to disseminate melanin through the entire basal layer of the epidermis.