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The oil on the surface of skin is a complex mixture of sebum, lipids (from the surface skin cells), sweat and environmental material.
Sebum is produced by sebaceous glands. These are found over most of the body, although there are few on the hands or feet and none on the palms and soles. Sebaceous glands on the mid-back, forehead and chin are larger and more numerous than elsewhere (up to 400-900 glands per square centimetre). They are also numerous in the ear canal and around the genitals.
The sebaceous gland consists of lobes connected by ducts, which are lined with cells similar to those on the skin surface.
Most sebaceous glands open out into the hair follicle. Some free sebaceous glands open directly onto the skin surface. These include Meibomian glands on the eyelids, Tysons glands on the foreskin and Fordyce spots on the upper lip or genitals.
Sebum is produced when the sebaceous gland disintegrates. The cells take about a week from formation to discharge. Sebum is a complex and variable mixture of lipids including:
- Glycerides
- Free fatty acids
- Wax esters
- Squalene
- Cholesterol esters
- Cholesterol
Triglycerides produced by sebaceous glands are broken down by bacterial enzymes (lipases) in the sebaceous duct to form free fatty acids.
What does sebum normally do?
Sebum has the following functions:
- It reduces water loss from the skin surface
- It protects the skin from infection by bacteria and fungi
- It contributes to body odour
- It is colonised by the bacteria Proprionibacterium acnes, which may have a role in immune regulation
Sebum production is under the control of sex hormones (androgens). The most active androgens are testosterone, 5-testosterone (DHT) and 5-androstene-317diol. These hormones and others are made by the sex glands (ovary in females, testis in males) and by the adrenal gland. These glands are in turn under the influence of the pituitary gland, located in the brain.
Androgens are made more active by enzymes in the skin and sexual organs. Type 1 5- reductase acts in the skin and Type II 5- reductase acts in the sexual organs. These enzymes convert less active androgens into the active testosterone and 5-testosterone (DHT). These more active androgens stimulate sebaceous gland cells to produce more sebum.
The role of progesterone is unclear. Females produce more sebum in the week before their menstrual period when progesterone levels are higher. But progesterone is known to reduce the activity of the enzyme 5-reductase that one might expect to reduce sebum production.
Sebum varies with age
Sebaceous glands are already active before birth. They are regulated by the mothers hormones and those made by the fetus itself.
The composition of oils on the skin surface varies with age. Excessive sebum production is known as seborrhoea.
- The fetus produces vernix caseosa, a waxy protective layer
- For 3 to 6 months, sebum produced by a newborn baby resembles that of an adult
- Thereafter until the age of 8 it has less wax and squalene and more cholesterol
- Sebum production increases at puberty up to fivefold in men
- Adult males produce slightly more sebum than adult females
- Sebum production declines with age, particularly after menopause in females
The amount of sebum can be altered by disease:
- Pituitary, adrenal, ovarian or testicular disorders may increase or reduce it
- Prolonged starvation reduces the amount of sebum
- Parkinson's disease increases the amount of sebum
The amount can be reduced by certain systemic medications:
- Oestrogens (oral contraceptive pill)
- Antiandrogens such as cyproterone acetate and spironolactone
- Vitamin-A derivatives such as isotretinoin
The amount can be increased by other systemic medications:
- Testosterone given to prepubertal males
- Progesterones with androgenic properties such as medroxyprogesterone, levonorgestrel
- Phenothiazines
The proportion of different components may change with certain medications:
- Co-pyrindiol (Diane-35®, Estelle-35® and Ginet® 84) containing ethinyl (o)estradiol 35mcg and cyproterone 2mg, increases linoleate
- Isotretinoin reduces squalene, wax esters and fatty esters
