A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter,sebum, which lubricates the hair and skin.
They occur in the greatest number on the face and scalp, but also on all parts of the skin except the palms of the hands and soles of the feet.
In the eyelids, meibomian glands, also called tarsal glands, are a type of sebaceous gland that secrete a special type of sebum into tears.
Areolar glands that surround the nipple are specialized sebaceous glands for lubricating the nipple.
Fordyce spots are visible, sebaceous glands found usually on the lips, gums and inner cheeks, and genitals.
Medical conditions involving sebum include acne, hyperplasia, and sebaceous adenoma, usually attributable to overactive sebaceous glands, which produce excess sebum.
There are two types of sebaceous gland,; those connected to hair follicles and those that exist independently.
Sebaceous glands are found in hair-covered areas.
Sebaceous glands in such areas are connected to hair follicles.
One or more sebaceous glands may surround each hair follicle.
The sebaceous glands are surrounded by arrector pili muscles, forming a pilosebaceous unit.
The sebaceous glands have an acinar structure, in which multiple glands branch off a central duct.
Sebaceous glands deposit sebum on the hairs and bring it to the skin surface along the hair shaft.
The pilosebaceous unit consists of hair, hair follicle, arrector pili muscles, and sebaceous gland.
The pilosebaceous unit is an epidermal invagination.
Sebaceous glands are also found in hairless areas including: skin of the eyelids, nose, penis, labia minora, the inner mucosal membrane of the cheek, and nipples.
Sebaceous glands on the lip and mucosa of the cheek, and on the genitalia, are known as Fordyce spots.
Sebaceous glands on the eyelids are known as meibomian glands.
Sebaceous glands of the breast are called Montgomery’s glands.
The fetus secretes vernix caseosa, a waxy, translucent white substance coating the skin of newborns.
After birth, sebaceous gland activity decreases until there is almost no activity during ages 2–6 years, and then increases to a peak of activity during puberty, due to heightened levels of androgens.
Compared to keratinocytes that make up the hair follicle, sebaceous glands are large cells with many large vesicles that contain the sebum.
Sebaceous gland cells express Na+ and Cl− ion channels.
They secrete sebum, a oily, waxy substance made of triglycerides, wax esters, squalene, and metabolites of fat-producing cells.
The sebum lubricates the skin
and hair.
Sebaceous secretions along with apocrine glands have a thermoregulatory role.
Sebaceous cyst secretions emulsify the sweat produced by the eccrine glands producing a sheet of sweat that is not readily lost in drops of sweat.
Sebum production in heat delays dehydration.
In colder situations sebum becomes more lipid, coating the hair and skin, and rain is effectively repelled.
Sebum is produced in cells within the sebaceous gland that rupture and disintegrate as they release the sebum and the cell remnants are secreted together with the sebum.
The sebaceous cells are constantly replaced by mitosis at the base of the duct.
Sebum, secreted by the
Sebaceous gland secretion, is primarily composed of triglycerides (41%), wax esters (26%), squalene (12%), and free fatty acids (16%).
Sapienic acid is a sebum fatty acid implicated in the development of acne.
While sebum is odorless, its breakdown by bacteria can produce strong odors.
Sex steroids affect the rate of sebum secretion.
Androgens such as testosterone stimulate sebum secretion, and estrogens have been shown to inhibit secretion.
Sebaceous glands serve to protect the body against microorganisms., by secreting acids.
These secreted acids form the acid mantle, a film on the surface of the skin that acts as a barrier to microbes that might penetrate the skin.
The pH of the skin is between 4.5 and 6.2, an acidity that helps to neutralize the alkaline nature of contaminants.
Sebaceous lipids help maintain the integrity of the skin barrier and supply vitamin E to the skin.
The sebaceous glands of the fetus produce vernix caseosa, a waxy white substance that coats the skin to protect it from amniotic fluid.
Sebaceous areolar glands surround the nipple in the female breast.
These Montgomery glands secrete an oily fluid that lubricates the nipple.
These Montgomery glands also secrete compounds that serve as an olfactory stimulus for the newborn.
During pregnancy and lactation these sebaceous glands become enlarged.
Meibomian glands, in the eyelids, secrete sebum named meibum onto the eye, that slows the evaporation of tears.
Meibum creates an airtight seal when the eyes are closed, and its lipid quality also prevents the eyelids from sticking together.
Meibomian glands are also known as tarsal glands, Zeis glands and palpebral glands.
Meibomian glands attach directly to the follicles of the eyelashes, within the tarsal plates of the eyelids.
((Fordyce spots)), or Fordyce granules, are ectopic sebaceous glands.
Earwax is partly composed of sebum.
Sebum secretions are viscous and have a high lipid content, which provides good lubrication.
In the skin pores, sebum and keratin can create a hyperkeratotic plug called a comedo.
Acne is thought to relate to an increased production of sebum due to hormonal factors.
I increased sebum production can lead to a blockage of the sebaceous gland duct, causing a comedo.
A comedo is commonly called a blackhead or a whitehead.
A comedo which can lead to infection, particularly by the bacteria Cutibacterium acnes.
Inflammation of the comedomes
can then change into the characteristic acne lesions.
Comedones occur on the areas with more sebaceous glands, particularly the face, shoulders, upper chest and back.
Comedones may be black or white in color, depending on whether the entire pilosebaceous unit, or just the sebaceous duct, is blocked.
Treatments for acne include, reducing sugars in the diet, antibiotics, benzoyl peroxide, retinoids and hormonal treatments.
Retinoids reduce the amount of sebum produced by the sebaceous glands.
Seborrhoea, overactive sebaceous glands cause of oily skin or hair.
With sebaceous cell hyperplasia, excessive proliferation of the cells within the glands, and visible macroscopically as small papules on the skin, particularly on the forehead, nose and cheeks.
Seborrhoeic dermatitis, is a chronic, usually mild, dermatitis caused by changes in the sebaceous glands.
Seborrhoeic dermatitis in newborns occurs as cradle cap.
A skin condition characterized by psoriasis with an overlapping seborrheic dermatitis is known as sebopsoriasis.
Sebaceous adenoma, is a benign slow-growing tumor, which may rarely be a precursor to a cancer syndrome known as Muir–Torre syndrome.
A sebaceous carcinoma, an uncommon and aggressive cutaneous tumor.
((Sebaceous cyst)) refers to both an epidermoid cyst and a pilar cyst, though neither of these contain sebum, only keratin and do not originate in the sebaceous gland and so are not true sebaceous cysts.
Nevus sebaceous, is a hairless region or plaque on the scalp or skin, caused by an overgrowth of sebaceous glands.