JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS animal. It is not known how far such protein contributes to melanin formation in vivo or the continued distribution of melanin to the epidermal tissue. The importance of inherited factors in the activity of the melanocytes has been stressed, but there are other influences, hormonal and nutritional, for example, on this activity. In the succeeding sections the influence of local treatments and of certain nutritional factors will be discussed in illus- tration of the differences and similarities of behaviour of the melanocytes in comparison with the keratin-producing cells. INDEPENDENCE OF PIGMENT FORMATION AND KERATIN PRODUCTION It has already been noted that normal development of hair takes place in the absence of any potential melanocytes and that the melanocytes of the mouse were able to develop and function normally independently of the keratinising mouse tissues. Many observations of human hair illustrate this degree of independence of pigment and keratin formation. In the human of the white races it frequently happens that the scalp hair in childhood is almost white, but becomes darker particularly in the years of puberty when a general increased activity of the skin structures takes place. In later life, the loss of pigmentation can take place while the hair grows vigorously. The earliest biological effect of X-rays to be noted was that of hair loss from a site to which they have been applied. The effect has been investigated using different animals, but the most detailed and careful work has been carried out on mice (Chase, 1949). It was shown for mice growing black hair that a dose of radiation could be given which appeared to destroy the activity of the pigment-producing cells while allowing hair growth to continue in the normal manner. Irradiation of skin where the hair follicles were in a resting condition produced the most marked effect. The new hail s growing in that area were grey. This effect is permanent and so reproducible that it has been suggested as a biological standard for radiation. As the amount of radiation to which the area is exposed is increased, temporary epilation as well as loss of melanocyte activity is observed, but a point is reached when hair ceases to be formed and the loss of hair is permanent. Similar observations have been made with other animals, the hamster, which gives results similar to those obtained with the mouse, the rabbit, the cat and the guinea-pig. As the coarseness of the hair increases, the dose level at which greying occurs approaches that at which loss of hair takes place, and in the human the level at which temporary loss of hair occurs does not lead to loss of pigmentation. In fact, the effect of low levels of X-ray dosage can produce an increase in pigmentation of the skin which is observed, of course, also by exposure to ultra-violet light. Thus, it appears that, in the human, X-radiation at low levels of dosage can stimulate the activity of the melanocytes, but temporary loss of hair occurs before a level 278

THE SKIN AS A COMMUNITY OF STRUCTURES damaging to the melanocytes is reached. In other animals, the melanocyte is inactivated by a level which has no permanent effect on the production of keratin in the fo•m of hair. There are substances which have an effect on certain tissues similar to that produced by •adiation, and some of these radiomimetic substances have been injected into black mice. Among such substances are nitrogen-con- taining substances related to mustard gas, the so-called nitrogen mustards. Local greying of the hair occurs after injection of suitable levels of these substances and the effect persists through successive generations of hair. It is reported that such mice have retained their white patches for as long as they were observed, which was nearly three years (Boyland, 1952). The mechanism by which radiation and nitrogen mustards produce their effects is not known, but it is known that X-rays can produce free hydroxyl radicals by interaction with water. Free hydroxyl radicals are produced in certain chemical reagents, for example, in a solution containing hydrogen peroxide and ferrous sulphate (Fenton's reagent). Permanent greying of the hair similar to that produced by irradiation and by nitrogen mustards has resulted from the injection of Fenton's reagent into coloured mice. It is possible that free radicals are involved in the effects of ultra-violet radiation, X-rays and the radiomimetic substances on the melanocyte. It has been reported recently that the magnetic properties of living tissue indicate the presence of free radicals (Commoner et alia, 1954). This is shown by undenatured protein, but such properties occur in a very marked degree in melanin. Since free radicals can react very readily with each other, these observations may indicate why free radicals such as the hydroxyl radical affect the melanocyte of the mouse so readily. Selective destruction of pigment production has been observed in experiments on the freezing of rat skin (Taylor, 1949). Areas of skin on black rats were cooled in situ by contact with solid carbon dioxide or with a special cooling device. The skin was found to survive contact for five seconds with solid carbon dioxide or slower freezing to -- 10 ø C. Hair of normal shape and texture was produced from the areas treated in this way, but it completely lacked pigmentation. A number of generations of rats with white hair were grown, showing that the loss of ability to form pigment was permanent. Two examples of a selective effect of a drug on pigmentation which have aroused considerable interest have already been reviewed in this journal (Clyman, 1953). In 1936 it was reported that feeding hydroquinone to black rats led to subsequent growth of grey fur and a few years later a derivative of hydroquinone, the mono-benzyl ether, was discovered to be responsible for local loss of skin pigmentation of the arms among Negro workers in a tannery. The skin in those cases appeared to have suffered no other change than the temporary loss of the ability to form melanin. Later 279