Lipid  is utilized by living cells in two basic  ways. Firstly, lipid is essential for membrane formation and therefore is necessary for the structural integrity of the cell. This structural function is largely subserved by phospholipids and sphingolipids.  Secondly,  lipid is utilized as an energy source. Triglycerides and, to a lesser extent, wax esters are metabolized to this end. It is unlikely that sebaceous lipid is utilized much as an energy source.

 

  Skin surface lipid in man is formed by contributions from both the epidermis and the sebaceous glands. This human lipid mantle is unique in several ways.  Some of the lipids, such as squalene, are not generally found in lipid-synthesizing tissues elsewhere in the body. Skin surface lipid  in man has a composition which is quite distinct from that of any other animal. More than half of human sebum  is composed of triglycerides and free fatty acids, whereas  the seba of sheep, rabbits and rodents contain less than 10 per cent free fatty acids and almost no triglycerides. Human skin surface lipid is also unique in its tremendous biochemical complexity.

 

  In a recent study of the sebum fatty acid composition of a group of  patients with  severe acne, there were surprisingly  large individual  differences in composition.1 However, subjects retained their sebum fatty acid composition within narrow limits over the two-month period of the study. It is thus unlikely that fluctuations occur as a result of changes in diet or metabolism. However, very large differences between subjects are seen which, in view of the slight individual variance, are statistically highly significant.

 

  These individual variations in sebum fatty acid compositions would seem also be be sufficiently different to be the basis for olfactory recognition of individuals, at least by animals that have an acute sense of smell. It is well recognized that trained dogs not only can identify an individual but can follow his scent over open ground many hours after his passage.2 It seems possible that what is sensed is the fatty acid composition of the lipids adhering to exfoliated corneocytes, which are showered continuously upon the environment.3

 

  Thus, it is likely that the skin surface lipid composition may be as specific to one human  being as his fingerprint. It is not unlikely that this lipid chemical fingerprint may assume future importance in the realms of forensic medicine. The well-equipped criminal of tomorrow may not only have to wear gloves, but also carry a bottle of lipid solvent with him!

 

  In thirteen identical twin pairs, intrapair differences in the proportions of iso-even fatty acids in sebaceous wax esters were found to be very small, while interpair differences were as large as in the non-twin population.4 Intrapair  differences  in iso-even fatty acids  were also quite small in four of eight  fraternal twin pairs. Heritability estimates suggested  that proportions of iso-even fatty acids in human sebum are controlled, at least in part, by genotype.

 

  Because of the extreme complexity of human surface lipid, it  is not intended to review in great  depth  the enormous volume of data which now exist in this field in man. For those who wish to read a first-class account of skin  lipid  (including methodology), the reviews of Nicolaides5 and Downing and Strauss6 and the many papers published by the Iowa group  (Downing, Stewart, Strauss et al) can be thoroughly recommended.

 

  Many investigators have analysed the skin  surface lipid in acne subjects in an attempt to gain some insight into the pathogenesis of this condition. One overwhelming problem,  which arose from such an approach in the earlier studies, is  the fact that lipid is usually sampled after secretion from normal, healthy sebaceous glands as well as from diseased pilosebaceous units.