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Sweat glands (also known as sudoriferous or sudoriparous glands, from Latin sudor, meaning "sweat"), are small tubular structures of the skin that produce sweat. There are two main types of sweat glands:
- Eccrine sweat glands are distributed almost all over the body, though their density varies from region to region. Humans utilize eccrine sweat glands as primary form of cooling.
- Apocrine sweat glands are larger, have a different secretion mechanism, and are mostly limited to the axilla (armpits) and perianal areas in humans. Although apocrine glands contribute little to cooling in humans, they are the only effective sweat glands in hoofed animals such as the camel, donkey, horse, and cattle.
Domestic animalsapocrine glands at the base of each hair follicle but eccrine glands only in foot pads and snout. Their apocrine glands, like those in humans, produce an odorless oily milky secretion evolved not to evaporate and cool but rather coat and stick to hair so odor-causing bacteria can grow on it. Eccrine glands on their foot pads, like those on palms and soles of humans, didn't evolve to cool either but rather increase friction and enhance grip.
Both apocrine and eccrine sweat glands contain myoepithelial cells (from Greek myo– "muscle"), specialized epithelial cells located between the gland cells and the underlying basal lamina. Myoepithelial cell contractions squeeze the gland and discharge the accumulated secretions. The secretory activities of the gland cells and the contractions of myoepithelial cells are controlled by both the autonomic nervous system and by the circulating hormones.
Ceruminous glands, which produce ear wax, and mammary glands, which produce milk, and ciliary glands in the eyelids are considered to be modified sweat glands.
Body of a sweat gland cut in various directions
Sweat glands are simple tubules consisting of a base rolled into a glomerulum and a duct that carries the sweat away. The base, which forms the secretory coil, is set deep in the hypodermis, and the entire gland is surrounded by adipose tissue. In apocrine glands, the secretory tubule is branched and single-layered, whereas it is unbranched, coiled, and double-layered in eccrine glands. All sweat glands' secretory coils are wrapped in long, rod-like contractile myoepithelial cells. In apocrine and eccrine glands, the diameter of the overall coil is around 800 and 500–700 microns respectively. The tubules themselves are also wider in apocrine glands: they have an inner diameter of 80–100 microns, versus the 30–40 micron diameters in eccrine glands. Excretory ducts, which carry sweat away from the secretory coil, are lined by a double layer of cuboidal cells.
Each sweat gland receives several nerve fibers that branch out into bands of one or more axons and encircle the individual tubules of the secretory coil. Capillaries are also interwoven among sweat tubules.
The name apocrine was originally chosen because it was believed that the gland cells used an apocrine method of secretion. Although it is now known that their secretory products are produced through merocrine secretion, the name has not changed. Apocrine sweat glands are coiled tubular glands that produce a viscous, cloudy and potentially odorous secretion. Apocrine sweat glands discharge in the canals of hair follicles. They begin secreting at puberty; the sweat produced may be acted upon by bacteria, causing a noticeable odor. Apocrine gland secretions may also contain pheromones, chemicals that communicate information to other individuals by altering their hormonal balance. Some research has indicated that feminine secretions from apocrine sweat glands can alter the menstrual timing of other women (this is called the McClintock effect), though the research methods used have been criticized. The significance of human pheromones and the role of apocrine sweat gland secretions in humans remains incompletely understood.
Eccrine sweat glands are smaller than apocrine sweat glands, and they do not extend as deep into the dermis. Eccrine sweat glands are coiled tubular glands that discharge their secretions directly onto the surface of the skin. Their density varies greatly according to body regions, the highest density (>250 glands/cm2) being on soles, palms, and scalp.
The clear secretion produced by eccrine sweat glands is termed sweat or sensible perspiration. Sweat is mostly water, but it does contain some electrolytes, since it is derived from blood plasma, although less concentrated. It therefore contains mainly sodium chloride, but also other electrolytes. The presence of sodium chloride gives sweat a salty taste. The total volume of sweat produced depends on the number of functional glands and the size of the surface opening. The degree of secretory activity is regulated by neural and hormonal mechanisms (men produce greater volumes of sweat than women). When all of the eccrine sweat glands are working at maximum, the rate of perspiration for a human being may exceed three liters per hour, and dangerous fluid and electrolyte losses can occur.
Eccrine glands have three primary functions:
- Thermoregulation. Sweat cools the surface of the skin and reduces body temperature.
- Excretion. Eccrine sweat gland secretion can also provide a significant excretory route for water and electrolytes.
- Protection. Eccrine sweat gland secretion aids in preserving the skin's acid mantle, which helps protect the skin from colonisation from bacteria and other pathogenic organisms.
Sweat gland tumors include:
Adenolipomas are lipomas associated with eccrine sweat glands.
As signs in other illnesses
- Acromegaly, a result of excess growth hormone, causes the size of of sweat glands increase, which leads to thicker skin.
- Aquagenic wrinkling of the palms, in which white papules develop on the palms after exposure to water, can sometimes come with abnormal aquaporin 5 in the sweat glands.
- Cystic fibrosis can be diagnosed by a sweat test,as the disease causes the sweat glands ducts to reabsorb less chloride, leading to higher concentrations of chloride in the secreted sweat.
- Ectodermal dysplasia can present a lack of sweat glands.
- Fabry disease, characterized by excess globotriaosylceramide (GL3), causes a decrease in sweat gland function due to GL3 deposits in the eccrine glands.
- GM1 gangliosidoses, characterized by abnormal lipid storage, leads to vacuolization in eccrine sweat gland cells.
- Hunter syndrome can include metachromin granules and mucin in the cytoplasm of the eccrine sweat gland cells.
- Hypothyroidism's low levels of thyroid hormone lead to decreased secretions from sweat glands; the result is dry, coarse skin.
- Kearns–Sayre syndrome, a disease of the mitochondria, involves abnormal mitochondria in eccrine sweat glands.
- Lafora disease is a rare genetic disorder marked by the presence of abnormal polyglucosan deposits. These "Lafora bodies" appear in the ducts of sweat glands, as well as the myoepithelial cells of apocrine glands.
- Lichen striatus, a self-limited eruption of small, slightly scaly papules, includes a lymphoid infiltrate around eccrine sweat glands.
- Metachromatic leukodystrophy, a lysosomal storage disease, leads to the accumulation of lipopigments and lysosomal residual bodies in the epithelial cells of sweat glands.
- Neuronal ceroid lipofuscinosis causes abnormal deposits of lipopigment in sweat gland epithelial cells (among other places).
- Neutral lipid storage disease includes abnormal lipid deposits in cells, including those of the sweat gland.
- Newmann–Pick type C, another lipid storage disease, includes abnormal lipid storage in sweat glands.
- Schindler disease causes cytoplasmic vacuoles that appear to be empty or contain filamentous material to manifest in eccrine sweat gland cells.
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