306 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The phylogenetic history of the human scalp is a fascinating one and we have begun to study it in other primates. It is much too early to make sweeping generalizations, but we have some clues. For example, the scalp of the few South American and African monkeys we have studied, has no eccrine sweat glands in the frontal and parietal regions, although there are many in the brow, and in the temporal and occipital regions. Of the higher apes we have studied only the chimpanzee, whose scalp does have some eccrine glands. The human scalp, being rich in eccrine sweat glands must be a relatively recent phylogenetic differentiation. These odd bits of information do not make the biology of the scalp completely intelligible and they do not make a definition of normality more plausible. They point out, however, that the scalp is a peculiarity of man, and that the process of balding is a norma/feature of ageing. What, then, is normal skin ? What is normal for the scalp is not normal for other regions and no two regions are exactly alike. This is not all what of ageing? The flabby skin of an aged person is as normal as the turgid skin of a youthful one, and norma/By must include the total biological life-history of skin. Skin is a kaleidoscopic organ. With the exception of the sweat glands, which are fairly stable, it has a turbulent rate of turnover. It is ever so sensitive to changes, internal or external, and makes constant adjustments to these changes. This instability is the wisdom of its survival skin stoutly resists attacks, but when it can no longer resist them it makes adjustments to them. There must be numberless regulatory mechanisms that guide the proper growth and differentiation of this system. The epidermis must prolif- erate exactly at the same pace that the surface dead layer is worn off. Some hair follicles must be stimulated to grow and others, perhaps adjacent to growing ones, must be suppressed or inhibited from growing. The eccrine sweat glands must be maintained in constant readiness to function in thermoregulation the apocrine glands must secrete in response to different stimuli, some of them psychic. The regulation of each of these systems is very precise and complicated. To mention just one example, it is not enough for the undifferentiated cells of the epidermis to divide in order to replace the cells lost at the surface the daughter cells must also synthesize specific kinds of complex fibrous proteins and lipids, and the cells must ascend in just a certain way and at a certain rate to the surface. A master unifying control system must guide the proper function of all of the subunits of skin. In spite of the very different functions that the units of the skin per- forms normally, these cells, or building blocks, are potentially all alike. l)octors Walter C. Lobitz, Jr. and Albert M. Kligman, and their colleagues, have made beautiful observations on the totipotentiality of epidermal

PARTICULARITY Oh' HUMAN SKIN 3o7 cells. During wound healing a//of/,he cells around the wound, and not just those of the epidermis are summoned and rushed to the rescue. The cells from the pilary canals, those of the sebaceous glands and those of the outer layer of the sweat ducts glide over the denuded surface and become transformed into epidermal cells, indistinguishable from those derived from epidermal cells. Even the secretory cells of eccrine and apocrine sweat glands, under certain conditions, modulate into totally different cells, ranging from typical epidermal types to mucus secreting cells. Potential sebaceous cells normally show a bimodal pattern of differentia- tion: some synthesize and store lipids, others become keratinized. When disturbances occur, lipid transformation is impeded and most of the cells synthesize keratin. Many other examples could be cited that show that the imprint of specialization is carried lightly by epidermal cells. Under certain conditions the controls that guide the normal path of differentia- tion are probably obliterated, and the epidermal cells, left without control unmask the potential that can be expressed most favorably. There exist profound structural and functional differences between the skin of man and that of most other mammals. In spite of glib statements that the skin of the pig resembles that of man, these are totally different organs. A study of the skin of other primates should give us clues of the evolution of the particular attributes of human skin. For example, whereas most of the sweat glands on the human skin are of the eccrine type, those of other mammals, with rare exception, are apocrine. Even the glands on the body skin of lower primates are apocrine the Pithe- coidea gradually acquire more eccrine glands over their body, and show a concomitant decrease in apocrine glands. In the chimpanzee the number of eccrine glands is greater than that of apocrine glands. A study of the skin of other apes will give us more information about these points. For example, among the other primates, an axillary organ is found only in the higher apes, and attains the greatest development in man, where the glands are numerous and gigantic. The axillary organ, then, is phylogenetically a relatively recent acquisition, and whatever may be its primary biological significance, is a particularity of the skin of man. We are searching for the evolutionary trends that culminated in the establishment of the particular features of human skin. When we have these facts we may have a better insight into the many paradoxes that still confi'ont us in human skin. The academic investigator can afford to study skin with leisure, as he would any other organ of the body. The physician, however, gives di- rection and urgency to these studies. Let the cosmetic chemist capitalize on these findings by compounding substances that make us more attractive, and which are harmless. Armed with fundamental knowledge, he may actually produce cosmetics that may be beneficial to skin.