SKIN AND HAIR PIGMENTATION 205 from repeated use of hot water bottles. Although the redox potential in living cells must be poised within rather narrow limits just as must the pH, the very great dependence of various steps in the melanin-forming series of reactions on proper redox potentials, makes it likely that physiological variations in this parameter very signi- ficantly regulate melanin formation. The action of huge doses of ascorbic acid in occasionally diminishing the melanosis of Addison's disease •' can be used to illustrate the role of this factor of redox potential. The important role of copper-binding agents in regulating melanin formation in human skin under natural conditions was first proposed by Rothman and co-workers more than a decade ago. • They found that epidermal extracts were able to inhibit the oxidation of tyrosine by the tyrosinase reaction and that this inhibition was largely due to sulfhydryl groups in such extracts. They then made the significant hypothesis that tyrosinase in the skin is normally kept in a partially inhibited state by copper- binding sulfhydryl compounds and that various pigrnentogenic stimuli, such as sunshine, ionizing radiations, and inflammatory skin diseases act by oxidizing or otherwise removing the inhibitory sulfhydryl groups to permit the enzyme to act more freely on its substrate. Brilliant proof that tyrosinase exists in a partially inhibited state in human skin, came with the relatively recent work of Fitzpatrick, Becker, Lerner and Montgomery, TM in which they were able to demonstrate tyrosinase activity in the skin histochemically only after releasing inhibition of this enzyme by repeated ultraviolet irradiations of the skin. Local injections of copper have also been shown to have a releasing action on the inhibited tyrosinase in normal human skin. More recently, Magnin and Rothman have brought forth evidence that in epidermal extracts copper-binding agents other than sulfhydryl groups also help keep tyrosinase partially inhibited. • It appears likely that these additional physiological inhibitors of tyrosinase will prove to be, or at least include, free amino acids. Phenylalanine, for example, has been claimed to have such an inhibiting action on tyrosinase in individuals suffering from phenylpyruvic oligophrenia. TM Various factors which have been found to inhibit the enzymatic process of melanin formation either in vitro or in vivo or in both, can be generally classified into the following categories: (1) Direct enzyme inhibitors or poisons (2) competitive inhibitors (3) induction-period-prolonging agents (4) compounds reacting with intermediates in the melanin-forming chain of reactions (5) reducing agents and (6) inhibitors of largely unknown modes of action. I will not undertake a detailed discussion of these factors as this subject will be covered in another presentation. The obvious variations in the amount of melanin in the skin and hair in both normal and abnormal conditions may be dependent either on varia-

206 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tions in numbers of melanocytes or simply on the physiological activities of these cells in forming melanin. Normal skin of all colors, including Negro skin, vitiliginous skin, and albino skin, all contain approximately the same numbers of dendritic melanocyte-type cells. In albinism there is a genetic lack of the enzyme tyrosinase, while in Negro skin there appears to be a genetically controlled hyperactivity of this enzymatic function. The mechanisms by which genetic factors regulate melanin production so as to account for the many normal variations in color, shade, and patterning of melanin pigmentation throughout the animal world-are largely not understood. The rate at which melanin is lost from the skin must also obviously influence its melanin content. Two pathways of excretion of melanin granules are known. One is upward through the epidermis with ultimate loss from the surface along with the physiological shedding of the horny layer. The second is through the corium into macrophages and the lymphatic system. Loss of melanin into the corium is favored under pathologic conditions where the basal layer of the epidermis becomes disrupted by disease, such as occurs in lichen planus, lupus erythematosus, and incon- tinentia pigmenti. Loss of melanin from the surface is favored under con- ditions of increased desquamation. Normally the rate of melanin loss from the epidermis seems to be very slow in view of the minimum latent period of three weeks required for the leukoderma induced by exposure to mono- benzyl ether of hydroquinone to become manifest. •3 In most abnormal conditions where melanin pigmentation of the skin is increased, the numbers of melanocytes are not increased, in contrast with the situation in lentigenes and various pigmented moles and tumors. Hormonal, nutritional, and neural factors also have important effects on melanin pigmentation. Among hormonal influences, pituitary, thyroid, adrenal, and gonadal hormones have all been shown to influence melanin pigmentation in living animals. The anterior and intermediate lobes of the pituitary produce several polypeptide factors which have melanocyte stimulating properties. These are ACTH from the anterior lobe, and alpha and beta MSH from the intermediate lobe. •4 Alpha MSH accounts for the major portion by far of the melanocyte-stimulating activity present in extracts of whole pituitary. The intrinsic melanocyte stimulating potency of ACTH is only a very small fraction of that of alpha MSH. Both alpha and beta MSH have molecular weights of approximately 4,000 and contain about fifteen different amino acids. The complete amino acid sequences in the two MSH factors have been recently elucidated largely by the brilliant work of Lerner and associates24,25, 26, and Li and associates, 27, •8 and it is interesting to note that very similar or identical arrangements of nine amino acids occur in alpha and beta MSH as well as in ACTH. As little as 10- •0 of a gram of pure alpha MSH will bring about darkening of small