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Facts about the skin from DermNet New Zealand Trust. Topic index: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Sebum

What is sebum?

Sebum is a complex and variable mixture of lipids including:

Sebum is produced by sebaceous glands when they disintegrate. The cells take about a week from formation to discharge. Triglycerides produced by sebaceous glands are broken down by bacterial enzymes (lipases) in the sebaceous duct to form free fatty acids. The oil on the surface of skin is a complex mixture of sebum, lipids (from the surface skin cells), sweat and environmental material.

Sebaceous glands are found over most of the body, although there are few on the hands and 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.

What does sebum normally do?

Sebum has the following functions:

Hormonal control of sebum

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.

Does sebum vary 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 amount of sebum can be altered by disease:

The amount can be reduced by certain systemic medications:

The amount can be increased by other systemic medications:

The proportion of different components may change with certain medications:

Related information

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Author: Dr Amanda Oakley MBChB FRACP, Hamilton, New Zealand.

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If you have any concerns with your skin or its treatment, see a dermatologist for advice.