j. Soc. Cosmetic Chemists 18 413-420 (1967) (•) 1967 Society of Cosmetic Chemists of Great Britain The melanocyte keratinization system and A. JARRETT* Presented at the Symposium on "Colour", organised by the Society of Cosmetic Chemists of Great Britain in Eastbourne, Sussex, on 27t• April 1966. Synopsis---The factors producing skin colour are mentioned together with methods of estimating the redness of the skin due to the haemoglobulin contained in the dermal vascula- ture, and the pigmentation of the skin due to the melanin contained in the epidermal cells. The histochemistry of the melanocyte in respect to its dopa oxidase activity is described together with more recent findings of hydrolyric enzymes in these cells. The possible action of the melanocyte population of the epidermis in determining the type of keratin produced by the epidermal cells is discussed. SKIN COLOUR The colour of the skin is dependent on three main factors-the red of blood contained in the skin vasculature, the black of melanin pigment produced by a group of cells known as melanocytes, and the natural yellowish colour of the epidermis and dermis. The colour contribution due to blood varies according to the state of the capillaries and the degree of oxygenation of the blood. Thus dilated capillaries tend to make the skin red whilst their constriction causes pallor. The degree of oxyge- nation of the blood alters the colour from the bright red of fully oxygenated blood in dilated capillaries to the blue of slowly flowing, deoxygenated blood in dilated blood vessels. It will be readily appreciated that all variations between these two extremes can occur. Brown-black melanin imparts the brown tinge to the skin in varying *University College Hospital Medical School, London, W.C. 1. 413

414 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS degrees, from virtually nothing in the nordic races to almost jet black in certain Negro races. The degree of skin pigmentation is not related to numerical differences in the melanocyte population as a heavily pigmented Negro has the same number of melanocytes per unit area from a given site of skin as a white man. These racial differences in coloration must therefore be due either to greater production of pigment by the melanocytes, or to a decreased rate of destruction, or both. The pigment is produced by specialized dendritic cells, known as melanocytes, situated in the basal layer of the epidermis. It is then passed into the adjacent epidermal cells by the dendritic processes of the melanocytes. This transfer of the melanin granules to epidermal cells has been termed "cytocrine activity" (1). The pigment tends to accumulate in the upper part of the epidermal cells above the nucleus, and thus protects this vital structure from the effects of solar radiation. In white skin there is virtually a complete absence of pigment immedi- ately above the lowest layers of epidermal cells. This sudden reduction cannot be entirely accounted for by a dilution effect due to the division of the melanin-containing epidermal cells with a consequent decrease in the individual cell pigment load. In dark skin, melanin can be readily de- tected in the upper region of the epidermis and sometimes even as high as in the keratin layer. It is thought that in fair skin the pigment becomes rapidly reduced, and this results in a lightening of the colour of the melanin granules. The rapid phase of pigmentation of fair skin following exposure to sunlight is thought to be due to the photodynamic reoxidation of this reduced pigment, whereas the tanning that appears two or three weeks later is considered to be due to increased melanin production effected by actual stimulation of the melanocytes. In man both the skin and the hair are pigmented, and it is probable that the melanocyte population of the hair follicles, and of the intervening epidermis, constitute a single cell population. In animals having a heavy fur covering, the hair may or may not be pigmented but the underlying epidermis is usually devoid of pigment. This is because the overlying hair affords adequate protection from solar radiation and therefore epidermal pigmentation is not required. Pigmentation of the hair does increase the protection afforded by the keratin of the hair fibres, but the main functions of hair colour in animals is camouflage and secondary sexual characteristics. The depth at which the melanin is situated in the skin is important as this causes changes in its apparent colour. Thus pigment within the