The human sebaceous glands are endocrine secreting steroidogenic
tissues present on all body sites where there is hair. Therefore
they are absent on the palms of the hands and soles of the feet
just like hair follicles. The most important function of the sebaceous
glands, it seems, is the production of the fluid, sebum. Excessive
sebum secretion certainly plays a part in acne development.
The size and density of sebaceous glands is much higher on the
face, neck, chest and back than on other body sites. Acne therefore
occurs mostly at these sites. The number of sebaceous glands remains
roughly the same in life but the size normally increases with
age. The largest of all sebaceous glands are found on the face.
Sebaceous gland development
The genesis of human sebaceous glands is not fully understood.
The sebaceous glands clearly appear in the skin of a human fetus
in the 13th to 16th week of pregnancy. The glands grow out from
the most superficial bulges of the hair follicles as they develop.
When fully grown, the glands remain attached to the hair follicles
by a duct through which sebum flows to the hair follicle canal
and from there to the skin surface. The development of the glands
is closely associated with the differentiation of the hair follicles
and the skin epidermis.
The sebaceous glands are fully functional from their formation,
significantly increasing sebum secretion within a few hours of
birth. Sebum is the first glandular product of the human body
and its production peaks in a week after birth, subsiding afterwards.
The reason for this early spurt in sebaceous gland production
is probably caused by androgens and other hormones passed from
the mother to the embryo across the placenta. The sebum secretion
rates of the mother and the neonatal baby directly correlate.
This correlation is later lost and is independent from breast-feeding.
At this time sebum production per unit of skin surface in new
born babies is about the same as in young adults and the sequence
of transformation of sebaceous glands is the same as in the post
These events confirm the important role of maternal hormones
in the development and activity of the neonatal sebaceous glands.
The androgenic stimulation before birth, it appears, comes through
the placenta from the mother.
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. 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
Sebaceous glands in acne development
It is not clear whether acne is mediated by androgen produced
in the sebaceous glands or exogenously produced androgens. All
sex steroids produced in the skin are derived from DHEAS. DHEAS
is converted to androstenedione in the adrenals as well as in
the sebaceous glands. There are two types of an enzyme – 3
beta HSD - which are involved in the conversion, namely, type
I and II. In the sebaceous glands it is the type I that converts
DHEAS into androstenedione. In the skin another enzyme 17 beta
HSD converts the weak androstenedione into the more potent testosterone.
The still more potent DHT is produced from this testosterone
by the action of a type I isozyme of the enzyme 5 alpha reductase
in the sebaceous gland. The activity of this enzyme is the highest
in body sites which are prone to acne. So there is more acne on
the face because the enzyme 5 alpha reductase is more active in
the sebaceous glands there – in addition to these glands
being larger than those seen in other regions of skin.
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.
Sebaceous glands as steroidogenic tissues
The sebaceous glands can also produce cholesterol from acetate.
This cholesterol is present in the sebum and is used in the cell
membrane to form the epidermal barrier. Its use in hormone synthesis
has been established only recently. An enzyme catalyses the cholesterol
into the steroid pregnenolone, which is converted to DHA by actions
of other enzymes. Various antibodies have been found in the hair
follicles and sebaceous ducts and glands. All this indicates that
skin is a steroidogenic tissue.