THE SEBACEOUS GLANDS 4O7 el? , Clõ , C14 and some shorter carbon chains (11,12). The remaining material consists of esterified acids, wax alcohols, squalene, sterols, and a small quantity of paraffin hydrocarbons which some authors believe are of environmental and not of endogenous origin. It seems probable that the long fatty acids, ranging from 5 to 22 carbons with an average length of 16, are synthesized within the sebaceous cell, and that most of these are made into triglycerides. However, the esters are subjected to lipolytic activity by enzymes present in the sebaceous ducts and on the skin surface, and are broken down to diglycerides, mono- glycerides and free fatty acids. Indeed, hydrolysis of triglycerides of exogenous origin has been demonstrated on the skin surface C14 labelled tripalmitin was spread on the back, and 3 hr later labelled free fatty acids were isolated from the surface fat (11). Several functions have been attributed to sebum but they are by no means undisputed. In hairy mammals the sebum may be important in waterproofing the hair and perhaps, as in cattle, it may seasonally reduce the thermal insulation by the coat (13). In man, it has been stated that the lipid film both controls moisture loss from the epidermis and protects the skin from fungal and bacterial infection. Cornified epithelium such as a cutting from a plantar callus becomes hard and brittle if it is allowed to dry out, but remains pliable as long as it contains 10 per cent by weight of water (14). The stratum corneum receives moisture from below and loses it by evaporation at the skin surface, but the major barrier against water loss through the skin is not at the surface but either near the base of the stratum corneum or throughout the entire cornified layer. At relative humidities of 60 per cent or more the moisture content remains high enough to maintain the pliability of the keratin. Under the lower relative humidity of winter weather or in rapidly flowing air the stratum corneum can, however, dry out with consequent chapping of the skin surface. If the cornified epithelium is treated with organic solvents and then extracted with water, the water-holding capacity of the epithelium is greatly decreased (15). This suggests that the water-holding power of the cornified epithelium depends on the presence of lipids, but an alternative explanation might simply be that lipid solvents damage natural barriers. It may be unnecessary to doubt the view dear to cosmetic chemists that under adverse environmental conditions the application of lipid materials helps to keep the stratum corneum pliable and to prevent the chapping of skin, but it is debatable how far naturally-occurring lipids play such a

408 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS role and, if they do, whether sebum contributes to any protective surface film, or whether the lipid is mainly supplied by the epidermal cells them- selves. To complicate the issue further, it is perhaps worth commenting that the hydrolysis of triglycerides on the skin surface must produce a small quantity of glycerol (11). Though this substance has never been demonstrated in sebum, it would have a moisturizing effect by virtue of its hygroscopic properties. It is widely believed that free fatty acids on the skin surface hinder the growth of pathogenic organisms [for review see (16)]. Circumstantial evidence supports the view that sebum, or at least the product of its hydro- lysis, is fungistatic. Fungi causing athlete's foot preferentially colonize areas which are not supplied by sebaceous glands ringworm of the scalp becomes rare after puberty when sebum production increases. It is also evident that free fatty acids have a limited action against certain bacteria, for example Streptococcus pyogenes (16). Both these properties of sebum have been challenged by Kligman (17). Admitting that fatty acids by themselves are antifungal in vitro, he claims that this potential is inacti- vated in the presence of horny material. He also states that the addition of sebum has no effect on streptococci or staphylococci growing on discs of isolated stratum corneum in agar culture. He therefore suggests that the sterilizing power of the skin is due largely to desiccation, a process supported equally by a wide range of inanimate surfaces. A critic of these studies has suggested that they only demonstrate the self-evident proposition that sebum does not prevent the growth of organisms which normally occur on the skin surface. They tell us nothing about the possible role of sebum in prevention of infection by other organisms which might prove patho- genic (18). CONTROL OF SEBACEOUS GLANDS Sebaceous glands are primarily under the control of androgens and this fact has been established by many studies in animals and man. For example, androgens enlarge sebaceous glands in rats (19), rabbits (20), hamsters (21), and mice (22), and also affect homologues such as the pre- putial gland of the rat or the supracaudal gland of the guinea pig (8). The sebaceous glands of man are minute during the prepuberal period but undergo vast enlargement at puberty, when the sebum output of males increases more than fivefold (23). Administration of testosterone increases the sebum output of prepuberal boys (23), but not of adult males where the glands would appear to be already under maximal stimulation by endo- genous androgens (23). In eunuchs, who secrete only about half as much