Sebum is a mixture of lipids that the sebaceous glands continuously deposit on the skin's surface. Together with sweat and water, it forms a fine film that softens the epidermis and supplies it with antioxidants, rather like a cream the skin would produce itself.
The comparison has its limits, and it is by spelling them out that we best understand what sebum is.
Where sebum comes from
Sebaceous glands are present on almost the entire surface of the body. Only the palms of the hands and the soles of the feet have none (Smith and Thiboutot, 2008). They are most often connected to a hair follicle, through which sebum reaches the surface.
Their mode of secretion is unusual. The cells of the gland fill with lipids, mature, then break down completely to release their contents. Biologists call this mechanism a “holocrine” secretion (Pappas, 2009). Sebum is therefore made of the very substance of these cells.
Production varies greatly depending on the area. The face, rich in glands, receives far more than the arms (Thiele et al., 1999). It also varies with age and hormonal balance.
What it contains
Human sebum has a composition that exists nowhere else in the body. Two of its constituents, squalene and wax esters, are specific to the sebaceous glands (Picardo et al., 2009; Smith and Thiboutot, 2008).
| Constituent | Approximate share of sebum |
|---|---|
| Triglycerides and free fatty acids | 57.5% |
| Wax esters | 26% |
| Squalene | 12% |
| Cholesterol and its esters | 4.5% |
Average values reported by Picardo et al. (2009). They vary according to the sampling method.
At the skin's surface, part of the triglycerides is split into free fatty acids and glycerol, notably by the bacteria of the skin flora (Pappas, 2009). The sebum covering the epidermis is therefore never quite the same as the one that leaves the gland. It evolves in contact with air, water and the skin flora.
What it does
The role of sebum in humans remains partly debated. Several functions are, however, well documented.
It transports antioxidants. Sebum contains vitamin E, in its alpha and gamma forms. An American research team has shown that it is secreted continuously on the forehead and cheeks. The stratum corneum of the face contains about twenty times more than that of the upper arm (Thiele et al., 1999).
It contributes to hydration. In mice almost devoid of sebaceous glands, the hydration of the stratum corneum drops by more than half. Glycerol, derived from the breakdown of sebum triglycerides, is enough to restore it. A mixture of lipids imitating sebum does not achieve this (Fluhr et al., 2003). This work, carried out in animals, suggests that sebum also acts through the molecules it releases.
It forms the oily phase of the surface film. Mixed with sweat and water, it makes up the so-called “hydrolipid film”, which gives the skin its suppleness and its feel.
Where the comparison ends
A cosmetic cream is applied to the skin. Sebum, by contrast, comes out of it. It is constantly renewed and its composition reflects the activity of the organism.
Above all, sebum does not build the skin barrier on its own. Between the cells of the stratum corneum are other lipids: ceramides, cholesterol and fatty acids. These come from the cells of the epidermis themselves. They form the “cement” that holds water in the skin (Pappas, 2009). On areas rich in glands, sebum dominates the surface. Deeper down, it is the epidermal lipids that ensure watertightness.
The two systems work together. Sebum covers and softens. Epidermal lipids organise the barrier. Both can be removed by washing, then rebuild at their own pace.
In summary
- Sebum is secreted continuously by glands present almost everywhere on the body.
- It consists mainly of triglycerides, wax esters and squalene.
- It brings vitamin E to the surface and contributes to hydration.
- The barrier itself rests on other lipids, produced by the epidermis.
This article is part of the PHYTO 5 Library.
Sources
- Fluhr JW, Mao-Qiang M, Brown BE, Wertz PW, Crumrine D, Sundberg JP, Feingold KR, Elias PM. Glycerol regulates stratum corneum hydration in sebaceous gland deficient (asebia) mice. Journal of Investigative Dermatology. 2003;120(5):728-737. doi:10.1046/j.1523-1747.2003.12134.x
- Pappas A. Epidermal surface lipids. Dermato-Endocrinology. 2009;1(2):72-76. doi:10.4161/derm.1.2.7811
- Picardo M, Ottaviani M, Camera E, Mastrofrancesco A. Sebaceous gland lipids. Dermato-Endocrinology. 2009;1(2):68-71. doi:10.4161/derm.1.2.8472
- Smith KR, Thiboutot DM. Thematic review series: skin lipids. Sebaceous gland lipids: friend or foe? Journal of Lipid Research. 2008;49(2):271-281. doi:10.1194/jlr.R700015-JLR200
- Thiele JJ, Weber SU, Packer L. Sebaceous gland secretion is a major physiologic route of vitamin E delivery to skin. Journal of Investigative Dermatology. 1999;113(6):1006-1010. doi:10.1046/j.1523-1747.1999.00794.x