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|Sebaceous Gland Biology|
The human sebaceous glands are hormone-producing tissue present on all body sites where there is hair. The size and density of sebaceous glands are therefore higher on the face, neck, chest and back, and absent on the palms of the hands and soles of the feet. The number of sebaceous glands remains roughly the same in life but the size increases with age, the largest of all being found on the face.
The most important function of the sebaceous glands is the production of sebum, which helps keep hair and skin soft. Sebaceous glands can also synthesize large amounts of free fatty acids locally and are involved in anti-microbial activity transporting antioxidants to the skin surface. However excessive sebum secretion plays a part in acne development. Acne therefore occurs mostly in areas with the highest density of sebaceous glands, i.e. the face, neck, chest and back.
Sebaceous gland development
The development of sebaceous glands is not clearly understood. They begin to appear in the fetus in the 13th-16th week of intrauterine life, and grow out from the most superficial bulge of hair follicles. They remain attached to the hair follicle via a duct through which sebum flows to the hair follicle root and from there to the skin surface.
The sebaceous glands are fully functional from their formation, being the first glandular product of the human body. Its production peaks in a week after birth and subsides thereafter. The reason for this early spurt in production of sebum is believed to be secondary to androgens and other hormones circulating from the mother to the child across the placenta. The sebum secretion rates of the mother and the neonatal baby directly correlate. During this time the sebum production per unit of skin surface in the baby is about the same as in young adults. This correlation between mother and child and the increased production of sebum is later lost and is independent from breast-feeding, after about a week of life.
These events confirm the important role of maternal hormones in the development and activity of the neonatal sebaceous glands.
Sebaceous gland as androgen synthesizers
The sebaceous glands are important sites for producing and metabolizing androgens, which are well known for their stimulation of sebum secretion. Most androgens are produced by the gonads and the adrenal glands. Androgens are also produced in the sebaceous glands locally from the precursor hormone dehydroepiandrosterone sulfate (DHEAS), which circulates in the blood after being released from the adrenal glands.
All sex steroids produced in the skin are derived from DHEAS which is converted to androstenedione via an enzyme called the 3-beta HSD. There are two types of 3-beta HSD, type I and II. It is the type I 3-beta HSD that converts DHEAS into androstenedione in sebacesous glands. Another enzyme 17-beta HSD converts the weak anddrostenedione into the more potent testosterone, which is finally converted to the still more potent DHT by the action of type I 5-alpha reductase in the sebaceous gland.
The sebaceous glands also produce cholesterol from acetate, which is subsequently secreted into sebum. This cholesterol is used in the cell membrane formation and can also be converted via an enzymatic process to prenenolone, which is a precursor to DHA. Thus sebaceous glands are steroidogenic in function.
Sebaceous glands in acne development
Five-alpha reductase, the enzyme that converts testosterone to DHT is found to have its highest activity in body sites that are prone to acne. Hence there is more acne on the face because the enzyme 5 alpha reductase is more active in the sebaceous glands there, in addition to the fact that these glands are larger than those seen in other regions of skin.
It is not clear whether acne is mediated by androgen produced in the sebaceous glands or exogenously produced androgens. Testosterone and dihydrotestosterone, or DHT, are the major androgens that interact with the androgen receptors (AR) to influence the sebaceous glands. DHT is much more potent than testosterone in its interaction with the receptors. However, the exact mechanism of the interaction is unknown.
Androgens regulate sebaceous gland functions through these receptors. The highest density of ARs is found in the sebaceous glands and its distribution in the human body is consistent with known androgen targets and acne sites. People with lower numbers of ARs on their cells are less androgen sensitive. The ARs in the sebaceous glands also regulate cell growth in sebaceous glands. So, fewer ARs on cells means less cell production of sebum and lower rates of cell proliferation, with lesser acne production.