The human sebaceous glands, an endocrine-secreting steroidic tissue, are located in areas of the human body covered by hair. They are not found on the palms and soles. It seems that the most important function of the sebaceous glands is the production of the fluid sebum. Excessive sebum plays a part in acne development.

The sebaceous glands on the face, neck, chest, and back are bigger than on the other parts of the body, accounting for the fact that acne occurs mostly on these areas. With age, the size of the sebaceous glands increases, although its number stays roughly the same. The sebaceous glands found in the face are the largest sebaceous glands on the human skin.

The Development of Sebaceous Gland

Sebaceous glands development in the human body is not fully understood. In the 13th to the 16th week of pregnancy, the sebaceous glands can be clearly seen on a human fetus. As the hair follicles develop, the sebaceous glands grow out from its most superficial bulges. When the hair follicles are fully-grown, the glands remain attached to them by a duct through which sebum flows to the hair follicle canal and out to the skin surface. The cell differentiation of the hair follicles and the skin epidermis is what associates the development of the glands.

From their genesis, the sebaceous glands are fully functional, increasing sebum secretion within a few hours of birth. The first glandular product of the human body is the sebum. Its production peaks in a week after birth, subsiding thereafter. The probable cause of this spike in sebum production after birth is the androgens and other hormones passed from the mother to the embryo across the placenta. In fact, the rate of sebum secretion of the mother and the neonatal baby is directly correlated. This correlation is later lost however; it’s also independent from breast-feeding. The production of sebum per unit of skin in newborn babies at this time is about the same as in young adults. Moreover, the sequence of transformation of sebaceous glands is the same in the postnatal period.

The importance of maternal hormone in the development and activity of the neonatal sebaceous glands is confirmed in the above activity. The androgenic stimulation before birth appears to come from the mother’s placenta.

Sebaceous Glands as an Androgen Synthesizer

The sebaceous glands are important sites where androgens are produced and metabolize. However, most androgens are produced by the gonads and the adrenal glands. The precursor hormone responsible for the local production of androgen in sebaceous glands is dehydroepiandrosterone sulfate (DHEAS). This hormone circulates the blood after it has been released from the adrenal glands. The two hormones, testosterone and dihydrotestosterone (DHT), are the major androgens that interact with the androgen receptor (AR) to influence the sebaceous glands. In fact, DHT is much more potent than testosterone in interacting with the receptors, although the exact mechanism of interaction is unknown.

These receptors regulate sebaceous glands functions. Sebaceous glands are where the highest concentration of ARs is found, which distribution in the human body is consistent with known androgen targets and acne sites. The fewer ARs a person has, the lesser the androgen sensitivity of that person is. ARs in sebaceous glands also regulate cell growth—the fewer ARs there are on cells, the lesser sebum production and the lower the rate of cell proliferation.

Sebaceous Glands and Acne Development

It is unclear whether the androgen produced in the sebaceous glands or the exogenously produced androgens mediates acne development. All skin produced sex steroids are derived from DHEAS, which is converted into androstenedione in the adrenals as well as in the sebaceous glands. The type I and Type II 3 beta HSD enzymes are involved in this conversion. The type I converts the DHEAS into androstenedione in the sebaceous glands. The conversion of the weak androstenedione into the more potent testosterone in the skin is the responsibility of the enzyme 17 beta HSD.

The type I isozyme of the enzyme 5 alpha reductase in the sebaceous glands produced the still more potent DHT from this potent testosterone. This enzyme’s activity is highest in the acne-prone body sites. There is more acne in the face, therefore, because the enzyme 5- alpha-reductase is more active in the sebaceous glands there, in addition to these glands being larger in this area than those seen in the other regions of the skin.

Large amounts of free fatty acids can also be locally synthesized in the sebaceous glands and are involved in antimicrobial activity transporting antioxidants to the skin surface.

The Skin is a Steroigenic Tissue

Cholesterol from acetate can also be produced in the sebaceous glands. The cholesterol present in the sebum is used in the cell membrane to form the epidermal barrier. The use of cholesterol in hormone synthesis has only been recently established. An enzyme catalyses the cholesterol into the steroid pregnolone, that is converted to DHA by the action of other enzymes. The hair follicles and sebaceous ducts and glands were found contain various antibodies, indicating that the skin is a steroigenic tissue.