Sebum is a hallmark characteristic of oily skin, which typifies or is usually associ- ated with acne-prone skin. The goal of this chapter is to present recent findings regarding the macromolecular structure of sebum and the influence of variations in composition of sebum on that macromolecular structure. It is further intended to discuss how these findings might relate to the pathogenesis of acne. In order to introduce a basis for this research, a brief review of the current knowledge of the composition of sebum is in order, although some repetition of information in other chapters on this subject is likely. After this review, the results of the thermal analysis of model sebum using differential scanning calorimetry (DSC) are presented along with an attempt to provide an explanation of the phase beha- vior, as it relates to sebum composition and to variations in the composition that might occur as part of a disease state such as acne. Further studies of the influence of vehicles on the phase behavior are presented, along with a discussion of how an imbalance in typical phase behavior may be relevant to the pathogenesis of acne.
Toward that end, our findings show that a model of sebum is a mixture of solid and liquid phases. We speculate that the liquid phase helps dissolve or “soften” the solid phase and that these two phases exist in a delicate balance. We further speculate that if this balance is altered, the consequences could be blockage of the sebaceous duct and of the infundibulum if the liquid part of sebum is too low and can no longer dissolve the solid part. This could result from action of Propioni- bacterium acnes bacteria on the sebum or from metabolic alterations in sebum com- position in situ.
ORIGIN OF SEBUM
Sebum is the oily secretion of the sebaceous gland (Fig. 1). This “oil gland” is located juxtaposed to the hair follicle. The large ducts of the hair follicles are filled with this white pasty sebaceous material derived from the sebaceous gland. Sebaceous glands are generally found all over the body, except on the palms, soles, and dorsum of the feet (1). Most of the glands are associated with hair follicles and hence are termed pilosebaceous glands. In humans, they are concentrated on the scalp, forehead, and face, where there may be as many as 400 to 900 glands/ cm2 and up to 1600 glands/cm2 on the nose (2). Sebaceous follicles are particularly
abundant in the face, ear canal, v-shaped parts of the face, and chest, and on the sides of the upper arm. These skin areas are hence relatively greasy. The sebaceous gland, though present during childhood, is very small. At puberty, the gland becomes enlarged (3). This enlargement and increased activity of the sebaceous gland at puberty is primarily under androgen control (4).
The chemical composition and physical properties of sebum have been studied by several researchers and are discussed next.
Composition of Sebum
Knowledge of sebum lipid components helps us to consider how a penetrating mol- ecule will react upon entering into the follicular canal and how this structure may theoretically be related to acnegenesis. The main lipids in human sebum are trigly- cerides, wax esters, and squalene with smaller proportions of cholesterol and cholesterol esters (4). Most importantly, free fatty acids (FFA) are often found in sebum and are formed from the triglycerides through the action of P. acnes. Bacterial lipases convert triglycerides to mono- and diglycerides as well as FFA in the sebum, many of which are unique to the sebaceous glands (5).
The average composition of human skin surface lipid was given by Downing et al. (6) (Table 1) and of follicular casts by Nordstrom et al. (7). Note the variability in the amounts of triglycerides and FFA; this is attributed to the variable activity of the bacterial lipase, as discussed earlier. The activity of this enzyme seems to be an individual variable unique to each person’s skin. The composition is compared with that of other animals in a summary table (Table 2). The wax and diol esters
appear to be the predominant class of lipid in animals other than humans (8). Stewart et al. (9) have shown that wax esters are lipids that have their origin exclu- sively in the sebum that is derived from sebaceous gland’s synthetic machinery via de nova synthesis. In fact, the same authors have proposed that the quantity of endogenous lipids synthesized de nova per cell relative to the sebocytes’ original endowment of exogenously derived lipids is a major influence on the composition of secreted sebum.
The broad composition of the fatty acid components of various lipid classes found in sebum are given in Table 3 (10). Thirty percent to fourty three percent of the fatty acids are unsaturated. Around 85% are straight chains as opposed to branched and about 75% are even carbon chain lengths. The monounsaturated fatty acids of human sebum are characterized by the unusual placing of a double bond at position D6; this differs from other animals where the D9 monounsaturates are predominant (4). In fact, recent studies (11) have identified a D6-desaturase found in differentiating the sebocytes from the suprabasal layer of the sebaceous gland; this enzyme forms the double bonds at the D6 position. This suggests that the gland tightly regulates the formation of the D6 fatty acids at the appropriate time during lipogenesis. These acids are found in greatest abundance in sebum from adults. Human sebum is unique in that it contains dienoic acids with double bonds at positions D5 and D8 and at positions D7 and D10. These constitute
2% to 3% of the total fatty acid content. The major form was identified as 18:2D5, 8 and, because of its presence in sebum, was named sebaleic acid (4). Branched chain acids are present, which are predominantly iso- and anteiso-isomers, but there are also smaller amounts of branched chain acids, in which the methyl branch occurs at other positions in the chain. Acids having two or three methyl branches may also be present. Up to half of the fatty acids of sebum are monounsaturated with double bonds in the unusual D6 position. The monounsaturated fatty acids include iso- and anteiso-branched species, but no internally branched or multibranched species. The fatty alcohols of the wax esters contain chain branching
and unsaturation, similar to the fatty acids (12). The fatty acid composition of wax esters has been shown to vary with age in humans (4).
Several other authors have also examined the fatty acid and fatty alcohol components of the various lipid classes found in sebum. The major portion of the fatty acids in sebum are chain lengths of C16 carbons. However, after this, it depends on whether one is looking at saturated or unsaturated fatty acids (7,9,12,13). For example, for saturated fatty acids, the C14:0 exhibited the next higher fraction followed by C15:0, then C18:0. For unsaturated fatty acids, C16 and then C18 carbon chains predominate. The C16:1 chain is characterized by the large amounts of C16:1D6 fatty acids shown in follicular casts (7). The recent discovery of the D6 fatty acid desaturase in differentiating sebocytes was mentioned earlier. The straight chain fatty acids that are synthesized mainly by the sebaceous glands are C14:0, C14:1, C16:0, C16:1, and C18:2 D5,8, because these increased with increasing ratios of wax esters (of sebaceous origin) to cholesterolþcholesterol esters (both of exogenous origin) in sebum samples from different age subjects. Fatty acids that circulate in the blood, namely C18:0, C18:1, and C18:2 D9,12, tended to decrease with increasing ratios of wax esters to cholesterol þ cholesterol esters, suggesting their exogenous origin in sebum. Stewart et al. (9) found that the fatty acids of triglycerides tended to follow the same compositions as the FFA except that they tended to have a higher level of saturated fatty acids. Although Nordstrom et al. (7) found that in follicular casts, the triglycerides lacked the C16:1D6 and there were significantly higher rela- tive amounts of saturated fatty acids in these triglycerides.
Regarding the remaining lipid classes, Stewart et al. (9) reported that wax esters contained lower amounts of C18:0, C18:1, and linoleate than other lipid classes, suggesting that very few exogenous lipids get into the wax esters. Wax esters also contain more C14:1 and C16:1 and less C14:0 and C16:0 than other lipid classes. These findings suggest that there is some mechanism for differential distribution of fatty acids among the various lipid classes. Regarding the polyunsa- turated fatty acids present in amounts less than 2% to 3%, linoleic acid (C18:2 D9,12) decreases and sebaleic acid (C18:2 D5,8) increases as the ratio of wax esters to cholesterol þ cholesterol esters increases, leading again to the conclusion that lino- leic acid is an exogenous lipid.
A feature of human sebum is its high squalene to the cholesterol ratio. The amount of cholesterol is very small compared to the amount of squalene. In fact, the sebaceous gland has an incomplete enzyme system and is unable to convert squalene to cholesterol (4). This suggests that the source of cholesterol in isolated sebum is of epidermal origin rather than sebaceous.
Copious amounts of phospholipids are required to support the cellular and subcellular membranes of the expanding sebaceous cells during their differen- tiation. In the final stages of differentiation, when the membranes are degraded, the phospholipids also disappear, and it is assumed that the fatty acids become esterified with whatever hydroxyl lipids that are available. These mechanisms explain why sebum does not contain phospholipids, despite the holocrine character of the sebaceous gland function (14).
Physical Properties of Sebum
The physical properties of sebum have been investigated a number of times. Braun- Falco et al. (15) claimed that a lipid obtained from the scalp using ether extraction is a liquid at body temperature. The fact that this lipid is obtained from the scalp, where usually no comedones are found, was ignored. Other workers (16 – 18) have also investigated the physical properties of sebum and its role in acnegenesis, without the concern that the sebum obtained was from the areas of the skin where acne does not normally occur. Hence, if any relationship does exist between the physical properties of sebum and acne, it would be erroneous to use these studies. Still other papers (19) used ether to extract it from the foreheads and this was then analyzed. Here again, only the surface lipid was analyzed and not the lipid from the inner acrofundibulum. Here, a major assumption was made that sebum is a liquid and hence flows out, and this was analyzed. It is reasonable to postulate that since sebum is a mixture of different substances, it will exist in more than one phase. Some phases may be liquids whereas others may be solids at body temperature.
Some of the physical properties have been enumerated. The physical proper- ties of sebum are summarized in Table 4.
Although Miescher and Schoenberg (20) isolated forehead sebum in normal volun- teers and have assigned the freezing point of 338C to 358C to sebum, Butcher and Coonin (19) also isolated forehead sebum from normal volunteers and have ascribed a value of 158C to 178C. From these studies, it was not clear if sebum is a liquid or a solid at body temperature. Since sebum is a mixture of different
lipidic substances, it is assumed that it will not have a single freezing point, but rather a range of temperatures in which transitions from a liquid to a solid takes place. Butcher and Coonin (19) claim that the sebum sample started to freeze up to 308C, but the whole sample solidified at 158C to 178C and stopped flowing.