The
sebaceous glands are microscopic
glandA gland is an organ in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface .- Types :...
s in the
skin-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
that secrete an oily/waxy matter, called
sebum, to lubricate and waterproof the skin and
hairHair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
of mammals. In
humanHumans are the only living species in the Homo genus...
s, they are found in greatest abundance on the
faceThe face is a central sense organ complex, for those animals that have one, normally on the ventral surface of the head, and can, depending on the definition in the human case, include the hair, forehead, eyebrow, eyelashes, eyes, nose, ears, cheeks, mouth, lips, philtrum, temple, teeth, skin, and...
and
scalpThe scalp is the anatomical area bordered by the face anteriorly and the neck to the sides and posteriorly.-Layers:It is usually described as having five layers, which can conveniently be remembered as a mnemonic:...
, though they are distributed throughout all skin sites except the palms and soles.
In the eyelids,
meibomianThe meibomian glands are a special kind of sebaceous gland at the rim of the eyelids inside the tarsal plate, responsible for the supply of meibum, an oily substance that prevents evaporation of the eye's tear film. They prevent tear spillage onto the cheek, trapping tears between the oiled edge...
sebaceous glands secrete a special type of sebum into tears. There are several related medical conditions, including
acneAcne is a general term used for acneiform eruptions. It is usually used as a synonym for acne vulgaris, but may also refer to:*Acne aestivalis*Acne conglobata*Acne cosmetica*Acne fulminans*Acne keloidalis nuchae*Acne mechanica...
,
sebaceous cystA sebaceous cyst is a term that loosely refers to either epidermoid cysts or pilar cysts . Because an epidermoid cyst originates in the epidermis and a pilar cyst originates from hair follicles, by definition, neither type of cyst is strictly a sebaceous cyst...
s,
hyperplasiaSebaceous hyperplasia is a disorder of the sebaceous glands in which they become enlarged.producing yellow, shiny bumps on the face. These bumps are a natural response to the maternal hormones of pregnancy and usually resolve on their own within a few weeks after delivery.Sebaceous glands are...
,
sebaceous adenomaSebaceous adenoma is a cutaneous condition characterized by a slow-growing tumour usually presenting as a pink, flesh-coloured, or yellow papule or nodule.-Significance:...
and sebaceous gland carcinoma (see section below:
Pathology).
Locations and morphology
A branched type of acinar gland, the sebaceous glands exist in humans throughout the skin except in the palms of the hands and
soles of the feetThe sole is the bottom of the foot.In humans the sole of the foot is anatomically referred to as the plantar aspect. The equivalent surface in ungulates is the hoof.- Human sole :...
.
Sebaceous glands can usually be found in hair-covered areas, where they are connected to
hair follicleA hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
s (
see image at top). The glands deposit sebum on the hairs, and bring it to the skin surface along the hair shaft. The structure consisting of hair, hair follicle, arrector pili muscle, and sebaceous gland is known as a
pilosebaceous unit.
Sebaceous glands are also found in non-haired areas (glabrous skin) of
eyelidAn eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...
s,
noseThe visible part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils...
,
penisThe penis is a biological feature of male animals including both vertebrates and invertebrates...
,
labia minoraThe labia minora , also known as the inner labia, inner lips, or nymphae, are two flaps of skin on either side of the human vaginal opening, situated between the labia majora...
and
nippleIn its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...
s. Here, the sebum traverses ducts that terminate in
sweat pores on the surface of the skin.
At the rim of the eyelids,
meibomian glandThe meibomian glands are a special kind of sebaceous gland at the rim of the eyelids inside the tarsal plate, responsible for the supply of meibum, an oily substance that prevents evaporation of the eye's tear film. They prevent tear spillage onto the cheek, trapping tears between the oiled edge...
s are a specialized form of sebaceous gland. They secrete a form of sebum (called meibum) onto the
eyeThe human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
, slowing the evaporation of
tearTear may refer to:*Tears, a type of eye secretion*Tearing, the ripping apart of something by force*Robert Tear, a Welsh singer-Elements in fiction:*Tear, a character Tear Grants in video game Tales of the Abyss...
s.
Sebum
Sebaceous glands secrete the oily, waxy substance called
sebum (
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
, meaning
fatFats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
or
tallowTallow is a rendered form of beef or mutton fat, processed from suet. It is solid at room temperature. Unlike suet, tallow can be stored for extended periods without the need for refrigeration to prevent decomposition, provided it is kept in an airtight container to prevent oxidation.In industry,...
) that is made of fat (
lipidLipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s), wax, and the debris of dead fat-producing cells. In the glands, sebum is produced within specialized
cellThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s and is released as these cells burst; sebaceous glands are thus classified as holocrine glands. Seborrhoea is the name for the condition of greasy skin caused by excess sebum.
Sebum is odorless, but its
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
l breakdown can produce odors. Sebum is the cause of some people's experiencing "oily" hair,
as in hot weather or if not washed for several days.
EarwaxEarwax, also known by the medical term cerumen, is a yellowish waxy substance secreted in the ear canal of humans and other mammals. It protects the skin of the human ear canal, assists in cleaning and lubrication, and also provides some protection from bacteria, fungi, insects and water...
is partly composed of sebum.
Function
All of the sebaceous glands in humans have been demonstrated to show similarity in structure and secrete sebum by a holocrine process. Sebum excreted by the sebaceous gland is primarily composed of tryglicerides, wax esters, and squalene. Wax esters, like squalene, are unique to sebum and not produced anywhere else in the body. Sebum also contains 45% water-insoluble fatty acids known to have broad antimicrobial activity. Additionally, sebaceous gland secretion provides Vitamin E to the upper layers of facial skin. Sebaceous lipids contribute to maintain integrity of skin barrier, and express pro-inflammatory and anti-inflammatory properties. Recent research suggests that sebum may represent a delivery system for antioxidants, antimicrobial lipids, pheromones, and hydration of stratum corneum. During the last gestation trimester, it is known that sebaceous glands produce vernix caseosa which protects the embryonic skin from amniotic water. Sebaceous secretions in conjunction with apocrine glands also play an important thermoregulatory role. In hot conditions, the secretions emulsify and foment formation of and prevent the loss of sweat drops from the skin. In colder conditions, sebum repels rain from skin and hair Increased facial surface sebum secretion is also associated with the development of acne.
Composition
The composition of sebum varies across species. In humans, the lipid content is as follows:
| Percent composition |
Substance
>-
| 25% |
|
>-
| 41%
triglycerideA triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....
s
>-
| 16% |
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s
>-
| 12% |
squaleneSqualene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...
|
Sapienic acidSapienic acid is a fatty acid that is a major component of human sebum. Among hair-bearing animals, sapienic acid is unique to humans . The equivalent fatty acid in mouse sebum is palmitoleic acid...
is a sebum fatty acid that is unique to humans.
Control
The following treatments have been shown to reduce sebum secretion rates:
- Isotretinoin
Isotretinoin, INN, is a medication used mostly for cystic acne. It was first developed for brain, pancreatic and other cancers. It is used to treat harlequin-type ichthyosis, a usually lethal skin disease, and lamellar ichthyosis. Its effects are systemic and nonselective...
- SMT D002
SMT D002 is a medication which has potential for use as an acne treatment. In pill form it has reportedly proved very successful at reducing sebum production during clinical trials carried out by UK-based drug discovery company Summit Corporation PLC...
- Spironolactone
Spironolactone , commonly referred to as simply spiro, is a diuretic and is used as an antiandrogen.It is a synthetic 17-lactone drug that is a renal competitive aldosterone antagonist in a class of pharmaceuticals called...
(suitable for females only)
Changes during development
The sebaceous glands of a human
fetusA fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
in uteroIn utero is a Latin term literally meaning "in the womb". In biology, the phrase describes the state of an embryo or fetus. In legal contexts, the phrase is used to refer to unborn children. Under common law, unborn children are still considered to exist for property transfer purposes.-See also:*...
secrete a substance called
Vernix caseosaVernix caseosa, also known as vernix, is the waxy or cheese-like white substance found coating the skin of newborn human babies.-Composition:...
, a "waxy" or "cheesy" white substance coating the skin of newborns.
The activity of the sebaceous glands increases during
pubertyPuberty is the process of physical changes by which a child's body matures into an adult body capable of reproduction, as initiated by hormonal signals from the brain to the gonads; the ovaries in a girl, the testes in a boy...
because of heightened levels of
androgenAndrogen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...
s. In males, sebaceous glands begin to appear predominantly on the
penisThe penis is a biological feature of male animals including both vertebrates and invertebrates...
, on the shaft and around the rim of the penile head during and after puberty. This is however normal, not to be confused with an
STDSexually transmitted disease , also known as a sexually transmitted infection or venereal disease , is an illness that has a significant probability of transmission between humans by means of human sexual behavior, including vaginal intercourse, oral sex, and anal sex...
.
In females, they appear predominantly in the
labia minoraThe labia minora , also known as the inner labia, inner lips, or nymphae, are two flaps of skin on either side of the human vaginal opening, situated between the labia majora...
.
Pathology
Sebaceous glands are involved in skin problems such as
acneAcne vulgaris is a common human skin disease, characterized by areas of skin with seborrhea , comedones , papules , pustules , Nodules and possibly scarring...
and
keratosis pilarisKeratosis pilaris is a common, autosomal dominant, genetic follicular condition that is manifested by the appearance of rough bumps on the skin...
. In the skin pores, sebum and
keratinKeratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
can create a hyperkeratotic plug called a "microcomedone". The prescription drug
isotretinoinIsotretinoin, INN, is a medication used mostly for cystic acne. It was first developed for brain, pancreatic and other cancers. It is used to treat harlequin-type ichthyosis, a usually lethal skin disease, and lamellar ichthyosis. Its effects are systemic and nonselective...
significantly reduces the amount of sebum produced by the sebaceous glands, and is used to treat acne.
The extreme use (up to 10 times doctor-prescribed amounts) of
anabolic steroidAnabolic steroids, technically known as anabolic-androgen steroids or colloquially simply as "steroids", are drugs that mimic the effects of testosterone and dihydrotestosterone in the body. They increase protein synthesis within cells, which results in the buildup of cellular tissue ,...
s by bodybuilders, for muscle gain can cause acne. The sebaceous gland is stimulated due to some steroids conversion into dihydrotestosterone. This may cause serious acne on the face, neck, chest, back and shoulders.
A blocked sebaceous gland can result in a
sebaceous cystA sebaceous cyst is a term that loosely refers to either epidermoid cysts or pilar cysts . Because an epidermoid cyst originates in the epidermis and a pilar cyst originates from hair follicles, by definition, neither type of cyst is strictly a sebaceous cyst...
.
A condition involving enlarged sebaceous glands is known as
sebaceous hyperplasiaSebaceous hyperplasia is a disorder of the sebaceous glands in which they become enlarged.producing yellow, shiny bumps on the face. These bumps are a natural response to the maternal hormones of pregnancy and usually resolve on their own within a few weeks after delivery.Sebaceous glands are...
.
Sebaceous gland carcinoma is a rare and aggressive form of
cancerCancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
involving the sebaceous glands;
sebaceous adenomaSebaceous adenoma is a cutaneous condition characterized by a slow-growing tumour usually presenting as a pink, flesh-coloured, or yellow papule or nodule.-Significance:...
is a more benign neoplasm of the sebaceous glands.
Sebum can also build up around
body piercingsStretching, in the context of body piercing, is the deliberate expansion of a healed fistula for the purpose of wearing body piercing jewelry. Ear piercings are the most commonly stretched piercings, with nasal septum piercings, tongue piercings and lip piercings/lip plates following close behind...
.
Importance to other animals
Certain species of
Demodex miteDemodex is a genus of tiny parasitic mites that live in or near hair follicles of mammals. About 65 species of Demodex are known; they are among the smallest of arthropods. Two species living on humans have been identified: Demodex folliculorum and Demodex brevis, both frequently referred to as...
s feed on sebum and are commonly found in the sebaceous glands of mammals, including those of humans.
The
preputial glandPreputial glands are exocrine glands that are located in front of the genitals of some mammals and produce pheromones. The preputial glands of female animals are sometimes called clitoral glands....
s of
miceA mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
and
ratRats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...
s are large modified sebaceous glands that produce
pheromoneA pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
s.
See Also
- Sebaceous adenitis
In canines, sebaceous adenitis, or SA, refers to the autoimmune disease found in some breeds of dog and more rarely in cats, rabbits and horses. It is an uncommon, idiopathic skin disease, characterised by an immune response against the dog's sebaceous glands , which can lead to the destruction of...
an autoimmune diseaseAutoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body actually attacks its own cells. The immune system mistakes some part of the body as a pathogen and attacks it. This may be restricted to...
affecting the sebaceous gland, known mainly to occur in canines.