Sebaceous Glands
Salient features
· Sebaceous glands
are unilobular or multilobular structures that consist of acini connected to a
common excretory duct and are usually associated with a hair follicle
·
Sebaceous glands produce sebum via
holocrine secretion and have multiple biological functions, including hormonal
(e.g. androgen synthesis) and immune modulation
·
Except for ectopic or free sebaceous
glands of the vermilion lips, orogenital mucosa, areolae, and eyelids, these
glands are associated with hair follicles
·
Sebum consists of free fatty acids,
wax and sterol esters, triglycerides, and squalene
·
Sebum production is a sensitive
indicator of androgenic activity; it increases at the time of puberty and
decreases in later adulthood, particularly in postmenopausal women
·
Sebaceous follicles are rich in
microorganisms including Malassezia spp., Staphylococcus epidermidis, and Propionibacterium spp.
Introduction
Except for the free
sebaceous glands found on the vermilion lips and oral mucosa (Fordyce spots or
granules), eyelids (meibomian glands), areolae (Montgomery tubercles), and
labia minora and prepuce (Tyson glands), sebaceous glands are associated with
hair follicles. Sebaceous glands produce sebum, which is released into the
infundibular portion of the hair follicle. Sebum production increases at the
time of puberty and is a critical factor in the pathogenesis of acne vulgaris.
Sebaceous glands also play important roles in cutaneous endocrine function
(including the hypothalamic–pituitary–adrenal-like axis of the skin as well as
androgen metabolism) and innate immunity, producing neuropeptides, androgens,
cytokines, and antimicrobial peptides that also contribute to acne pathogenesis
Structure
The
distribution of sebaceous glands within the skin is variable. However, they are
particularly well developed on the scalp, face, upper back, and chest. There
are three different types of pilosebaceous units: vellus, sebaceous, and
terminal.
A
sebaceous follicle consists of four parts: the keratinized follicular
infundibulum, the hair, the cauliflower-like convoluted sebaceous gland, and
the sebaceous duct which connects the gland with the infundibulum. The
infundibulum is divided into two parts. The distal portion, or
acroinfundibulum, is very similar to the adjacent epidermis. It displays
keratinization with a granular layer, and corneocytes (squames) are shed into
the lumen. The lower part, or infrainfundibulum, is quite different. It shows a
distinct pattern of tricholemmal keratinization and no granular layer.
The
normal flora (resident microbiota) of sebaceous follicles is complex, including
both bacteria (e.g. S. epidermidis, Propionibacterium spp. such as P. acnes) and fungi (e.g. Malassezia spp.). These follicles also
harbor Demodex mites, which increase in number in older
adults and are rarely seen in prepubertal children.
Development
In
the human fetus, sebaceous glands develop in the 13th to 16th week of gestation
from bulges (epithelial placodes) on the developing hair follicles. The bulge
region of the follicle contains the epidermal stem cells that generate multiple
cell lineages, including epidermal and follicular keratinocytes, as well as
sebaceous glands.
Sebaceous glands are present at
birth, and sebum production is relatively high at this time. It soon declines
and remains low until puberty, at which time it again increases. Androgens, in
particular 5α-dihydrotestosterone (DHT), appear
to be the major factor that controls the development of the glands and the
production of sebum. The level of sebum production at the end of puberty
remains constant through mid-adulthood. Sebum production declines in women
after menopause and in men during the sixth to seventh decade of life.
Three different
types of pilosebaceous units. A Vellus follicle with a small sebaceous gland
and short thin hair. B Sebaceous follicle with a large multilobular sebaceous
gland and mid-sized hair. C Terminal follicle with a fairly large sebaceous
gland and thicker hair.
Function
Sebum is
a light yellow viscous fluid. It is composed of triglycerides, free fatty
acids, squalene, wax and sterol esters, and free sterols. The admixture of
ceramides, mostly derived from epidermal lipids, occurs in the acroinfundibulum
before the entire sebum mixture reaches the skin surface. Secretion of sebum is
holocrine, that is, the sebocytes disintegrate and thereby release their sebum
as they migrate towards the central gland and sebaceous duct. Sebocyte turnover
time is approximately 14 days, and the flow of sebum is relatively continuous.
The amount of sebum produced varies among individuals and races, but the
average rate in adults is approximately 1 mg/10 cm2 every
3 hours.
The cells within sebaceous glands contain
androgen receptors that bind DHT, which is then translocated to the nucleus.
They also express functional receptors for neuropeptides such as
corticotropin-releasing hormone (CRH), melanocortins, and substance P. The
latter receptors contribute to proliferation and differentiation of cells in
the sebaceous gland (sebocytes) and modulate their production of lipids,
androgens, and cytokines in response to stimuli such as emotional stress.
Sebaceous lipids promote skin barrier function and possess both pro- and
anti-inflammatory properties.
Pathophysiology
Sebum production that is less than
0.5 mg/10 cm2 every 3 hours is associated with sebostasis or
dry skin, and 1.5–4.0 mg/10 cm2 every 3
hours is considered excessive and results in the clinical condition known as
seborrhea. Sebaceous follicles are most commonly found on the face, behind the
ears, and on the upper portions of the chest and back – the same distribution
as acne vulgaris. Clinically, these areas tend to be oilier than the rest of
the body. In general, acne patients have larger sebaceous glands and produce
more sebum than do patients with normal skin. The function of sebaceous
follicles is influenced by genetic factors and levels of circulating hormones.
