The Micro-Anatomy of the Mammalian Epidermis

The greater part of the superficial epidermis of mammals is of binary embryological origin, and consists of cells belonging to two distinct division lineages: that of the Malpighian or keratinizing system, and that of the melanocyte or pigmentary system. The melanocyte system can be artificially destroyed, or denied access to the epidermis in development, without impairment of any but the pigmentary activity of skin. The epidermis as a whole is a reproductively self-contained system, i.e. it is perpetuated by the division of cells that reside within the epidermis itself. It is argued that melanocytes as well as Malpighian cells are squamous in character, and that the functional melanocytes of the basal epidermal layer undergo a characteristic sequence of involutionary changes in the course of moving towards the skin surface to be flaked away. It follows that the entire epidermis, and not merely the Malpighian system, undergoes a continuous process of cellular renewal. Cell-divisions certainly occur in the basal epidermal layer; a number of difficulties of interpretation must be overcome before it can be held certain that divisions occur in more superficial layers as well. The Malpighian system has a cellular organization: there is no satisfactory evidence for the existence of intercellular cytoplasmic bridges that would endow it with a syncytial character. ‘Tonofibrils’ probably owe their origin to an artificial coarsening of a fine fibre-protein system within the cytoplasm of prickle cells. Elastic fibres probably play some part in anchoring the epidermis to its substratum; but there is evidence that the epidermis can be freed from the corium by disengaging the downwardly directed processes of Malpighian cells of the basal layer from the concentrated connective tissue ground-substance that forms the inner boundary of the dermo-epidermal interface. All the natural pigments of mammalian skins are melanins; melanocytes are the only seat of melanin formation, and pigmentary activity is the only function they are known to possess. Branches that arise from the perikarya of melanocytes are so arranged that each ends in close apposition to the superficial pole of a Malpighian cell and in some unknown manner causes pigment granules to enter into it. Tyrosine is probably the parental substrate in melanogenesis, and it is unlikely that more than one oxidase is responsible for the enzyme-mediated activities that lead to the formation of melanin. The fine structure and physiological activity of the Malpighian system varies from place to place on the body: many of the differences which appear to be due (and which, it is shown, could have been due) to differences of environment and manner of use are in fact of developmental origin and of ‘cellular genetic’ status. Such differences are conserved through repeated cell-divisions after the transplantation of particular areas of skin to anatomically unnatural environments. The regional anatomy of the melanocyte system is less complex. The melanocytes of the hair bulbs and of the superficial epidermis, responsible for the pigmentation of skin and hair respectively, are artificially interchangeable. It is therefore probable that they represent purely topographical variants of a homogeneous cellular genetic system. Differences of pigmentation between the variously coloured areas of a patched animal are due to differences between the pigmentary activities of individual melanocytes, and these are perpetuated in cellular heredity; they are not due to differences of structure, density, or distribution.