|Schematic view of hair follicle & sebaceous gland.|
|Cross-section of all skin layers. A hair follicle with associated structures. (Sebaceous glands labeled at center left.)|
|Gray's||subject #234 1069|
The sebaceous glands are microscopic glands in the skin that secrete an oily/waxy matter, called sebum, to lubricate and waterproof the skin and hair of mammals. In humans, they are found in greatest abundance on the face and scalp, though they are distributed throughout all skin sites except the palms and soles. In the eyelids, meibomian sebaceous glands secrete a special type of sebum into tears. There are several related medical conditions, including acne, sebaceous cysts, hyperplasia, sebaceous adenoma 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 feet. Sebaceous glands can usually be found in hair-covered areas, where they are connected to hair follicles (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 eyelids, nose, penis, labia minora and nipples. Here, the sebum traverses ducts that terminate in sweat pores on the surface of the skin. At the rim of the eyelids, meibomian glands are a specialized form of sebaceous gland. They secrete a form of sebum (called meibum) onto the eye, slowing the evaporation of tears.
Sebaceous glands secrete the oily, waxy substance called sebum (Latin, meaning fat or tallow) that is made of triglyceride oils, wax, squalene, and metabolytes of fat-producing cells. In the glands, sebum is produced within specialized cells 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 keeps hair and skin supple. Sebum is odorless, but its bacterial 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. Earwax is partly composed of sebum.
All of the sebaceous glands in humans have been demonstrated to show similarity in structure and secrete sebum by a holocrine process. Sebum secreted by the sebaceous gland is primarily composed of tryglycerides, 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 maintaining the integrity of the 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.
The composition of sebum varies across species. In humans, the lipid content is as follows:
|16%||free fatty acids|
Sapienic acid is a sebum fatty acid that is unique to humans.
The following treatments have been shown to reduce sebum secretion rates:
Changes during development
The activity of the sebaceous glands increases during puberty because of heightened levels of androgens, producing smegma. In males, sebaceous glands begin to appear predominantly on the penis, 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 STD. In females, they appear predominantly in the labia minora.
Sebaceous glands are involved in skin problems such as acne and keratosis pilaris. In the skin pores, sebum and keratin can create a hyperkeratotic plug called a "microcomedone". The prescription drug isotretinoin 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 steroids 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.
It is a common misconception that a blocked sebaceous gland is known as a sebaceous cyst. Cysts that are commonly called sebaceous cysts actually do not involve a sebaceous gland. Instead, they are collection of keratin and dead keratinocytes which develop within the epidermal skin layer. These cysts are called Epidermal Cysts.
A condition involving enlarged sebaceous glands is known as sebaceous hyperplasia.
Importance to other animals
Certain species of Demodex mites feed on sebum and are commonly found in the sebaceous glands of mammals, including those of humans.
- Sebaceous adenitis an autoimmune disease affecting the sebaceous gland, known mainly to occur in canines.
- List of specialized glands within the human integumentary system
- Sebaceous filament
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- BU Histology Learning System: 08801loa - "Integument: scalp"
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