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