Sebaceous gland lipids.

Human sebum contains squalene, wax esters, triglycerides, cholesterol esters, and possibly free cholesterol. The fatty acids of the ester lipids include species with chain branching or with unusual double-bond positions. The alcohol moieties of the wax esters contain unusual chain types similar to those of the fatty acids. Genetic and hormonal factors cause individual differences in sebaceous lipid composition. Genetic factors seem to influence the proportions of the various types of branched-chain fatty acids. Androgenic stimulation of the glands causes an increase in lipid synthesis, and therefore in the ratio of endogenously synthesized lipid to exogenous lipid. Because the endogenously synthesized lipids tend to be different from lipids that are derived exogenously, the overall composition of the secretion changes. Differences in sebum composition are particularly evident when the sebum of prepubertal children is compared with that of young adults. One difference that may be of clinical significance is the different concentrations of linoleate. Higher concentrations of sebum linoleate may protect young children from comedonal acne by preventing an essential fatty acid deficiency from developing in the follicular epithelium.