Sweat gland

Template:Short description Template:Infobox anatomy Sweat glands, also known as sudoriferous or sudoriparous glands, Template:Etymology,<ref name=oxford>Template:Cite encyclopedia</ref><ref name=saunders03 /> are small tubular structures of the skin that produce sweat. Sweat glands are a type of exocrine gland, which are glands that produce and secrete substances onto an epithelial surface by way of a duct. There are two main types of sweat glands that differ in their structure, function, secretory product, mechanism of excretion, anatomic distribution, and distribution across species:

Eccrine sweat glands are distributed almost all over the human body, in varying densities, with the highest density in palms and soles, then on the head, but much less on the trunk and the extremities. Their water-based secretion represents a primary form of cooling in humans.Template:Sfn
Apocrine sweat glands are mostly limited to the axillae (armpits) and perineal area in humans.Template:Sfn They are not significant for cooling in humans, but are the sole effective sweat glands in hoofed animals, such as the camels, donkeys, horses, and cattle.Template:Sfn<ref name=bullard70 />Template:Sfn

Ceruminous glands (which produce ear wax), mammary glands (which produce milk), and ciliary glands in the eyelids are modified apocrine sweat glands.<ref name="grayanat" /><ref name="Ackerman2005" />

StructureEdit

File:Gray946.png

Body of a sweat gland cut in various directions

Generally, sweat glands consist of a secretory unit that produces sweat, and a duct that carries the sweat away. The secretory coil or base, is set deep in the lower dermis and hypodermis, and the entire gland is surrounded by adipose tissue.<ref name=grayanat /><ref name=caceci />Template:Sfn In both sweat gland types, the secretory coils are surrounded by contractile myoepithelial cells that function to facilitate excretion of secretory product.Template:Sfn Template:Sfn 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. The distal or apical part of the duct that opens to the skin's surface is known as the acrosyringium.<ref name="Bolognia et al">Template:Cite book</ref>

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.<ref name="kennedy94" />

Differences Between Eccrine & Apocrine Sweat Glands
	Eccrine Glands	Apocrine Glands
Overall diameter of secretory coil	500-700 μm	800 μm
Diameter of individual secretory tubule	30-40 μm	80-100 μmTemplate:Sfn
Composition of secretory epithelium	single layer, mixed clear cells & dark cells	single layer columnar cells<ref name="Bolognia et al"/>
Composition of ductal epithelium	two or more layers of cuboidal cells	double layer of cuboidal cellsTemplate:Sfn
Duct opens to	skin surface	hair follicle, sometimes nearby skin surface

DistributionEdit

File:Fingerprt.jpg

Human sweat gland pores on the ridges of a finger pad

The number of active sweat glands varies greatly among different people, though comparisons between different areas (ex. axillae vs. groin) show the same directional changes (certain areas always have more active sweat glands while others always have fewer).Template:Sfn According to Henry Gray's estimates, the palm has around 370 sweat glands per cm²; the back of the hand has 200 per cm²; the forehead has 175 per cm²; the breast, abdomen, and forearm have 155 per cm²; and the back and legs have 60–80 per cm².<ref name=grayanat />

In the finger pads, sweat glands pores are somewhat irregularly spaced on the epidermal ridges. There are no pores between the ridges, though sweat tends to spill into them.Template:Sfn The thick epidermis of the palms and soles causes the sweat glands to become spirally coiled.<ref name=grayanat />

Other animalsEdit

Non-primate mammals have eccrine sweat glands only on the palms and soles. Apocrine glands cover the rest of the body, though they are not as effective as humans' in temperature regulation (with the exception of horses').Template:Sfn Prosimians have a 1:20 ratio of follicles with apocrine glands versus follicles without.Template:Sfn They have eccrine glands between hairs over most of their body (while humans have them between the hairs on their scalp).Template:Sfn

The overall distribution of sweat glands varies among primates: the rhesus and patas monkeys have them on the chest; the squirrel monkey has them only on the palms and soles; and the stump-tailed macaque, Japanese monkey, and baboon have them over the entire body.Template:Sfn

Domestic animalsTemplate:Which have apocrine 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.<ref name="merck" /> Eccrine glands on their foot pads, like those on palms and soles of humans, did not evolve to cool either but rather increase friction and enhance grip.

Dogs and cats have apocrine glands that are specialized in both structure and function located at the eyelids (Moll's glands), ears (ceruminous glands), anal sac, clitoral hood, and circumanal area.<ref name=slatter03 />

HistoryEdit

The pores of eccrine sweat pores were first identified by the Italian physiologist Marcello Malpighi. Sweat glands themselves were first discovered by the Czech physiologist, Johannes Purkinjé in 1833. The differing densities of sweat glands in different body regions was first investigated in 1844 by the German anatomist Karl Krause. Sweat glands were first separated into kinds by the French histologist Louis-Antoine Ranvier, who separated them in 1887 regarding their type of secretion into holocrine glands (sebaceous glands) and the merocrine glands (sweat glands), the latter were then in 1917 divided into apocrine and eccrine sweat glands. In 1987, apoeccrine glands were identified.<ref name="Taylor Machado-Moreira 2013 p.">Template:Cite journal</ref>

TypesEdit

EccrineEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Eccrine sweat glands are everywhere except the lips, ear canal, foreskin, glans penis, labia minora, clitoral hood, and clitoris. They are ten times smaller than apocrine sweat glands, do not extend as deeply into the dermis, and excrete directly onto the surface of the skin.Template:Sfn Template:Sfn<ref name=spearman73>Template:Cite book</ref>Template:Sfn The proportion of eccrine glands decreases with age.Template:Sfn

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. 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 sweat more than women). When all of the eccrine sweat glands are working at maximum capacity, the rate of perspiration for a human being may exceed three liters per hour,<ref>Template:Cite book</ref> and dangerous losses of fluids and electrolytes can occur.

Eccrine glands have three primary functions:

Thermoregulation: sweat (through evaporation and evaporative heat loss) can lead to cooling of the surface of the skin and a reduction of body temperature.Template:Sfn
Excretion: eccrine sweat gland secretion can also provide a significant excretory route for water and electrolytes.Template:Sfn
Protection: eccrine sweat gland secretion aids in preserving the skin's acid mantle, which helps protect the skin from colonization from bacteria and other pathogenic organisms.<ref>Template:Cite book</ref>

ApocrineEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Apocrine sweat glands are found in the armpit, areola (around the nipples), perineum (between the anus and genitals), in the ear, and the eyelids. The secretory portion is larger than that of eccrine glands (making them larger overall). Rather than opening directly onto the surface of the skin, apocrine glands secrete sweat into the pilary canal of the hair follicle.Template:Sfn

Before puberty, the apocrine sweat glands are inactive;<ref name=mosby /> hormonal changes in puberty cause the glands to increase in size and begin functioning.<ref name=braunfalco>Template:Cite book</ref> The substance secreted is thicker than eccrine sweat and provides nutrients for bacteria on the skin: the bacteria's decomposition of sweat is what creates the acrid odor.<ref name=dorlands /> Apocrine sweat glands are most active in times of stress and sexual excitement.<ref name=americanheritage />

In mammals (including humans), apocrine sweat contains pheromone-like compounds to attract other organisms within their species. Study of human sweat has revealed differences between men and women in apocrine secretions and bacteria.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

ApoeccrineEdit

Some human sweat glands cannot be classified as either apocrine or eccrine, having characteristics of both; such glands are termed apoeccrine.Template:Sfn They are larger than eccrine glands, but smaller than apocrine glands.<ref>Template:Cite book</ref> Their secretory portion has a narrow portion similar to secretory coils in eccrine glands as well as a wide section reminiscent of apocrine glands.<ref>Template:Cite book</ref>

Apoeccrine glands, found in the armpits and perianal region, have ducts opening onto the skin surface.<ref>Template:Cite book</ref> They are presumed to have developed in puberty from the eccrine glands,Template:Sfn and can comprise up to 50% of all axillary glands. Apoeccrine glands secrete more sweat than both eccrine and apocrine glands, thus playing a large role in axillary sweating.Template:Sfn Apoeccrine glands are sensitive to cholinergic activity, though they can also be activated via adrenergic stimulation.Template:Sfn Like eccrine glands, they continuously secrete a thin, watery sweat.Template:Sfn

OthersEdit

Specialized sweat glands, including the ceruminous glands, mammary glands, ciliary glands of the eyelids, and sweat glands of the nasal vestibulum, are modified apocrine glands.<ref name=hogin06>Template:Cite book</ref>Template:Sfn Ceruminous glands are near the ear canals, and produce cerumen (earwax) that mixes with the oil secreted from sebaceous glands.Template:Sfn<ref name=hogin06 /> Mammary glands use apocrine secretion to produce milk.<ref name=vanlommel />

SweatEdit

Sweat glands are used to regulate temperature and remove waste by secreting water, sodium salts, and nitrogenous waste (such as urea) onto the skin surface.Template:Sfn Template:Sfn The main electrolytes of sweat are sodium and chloride,<ref>Template:Cite book</ref> though the amount is small enough to make sweat hypotonic at the skin surface.Template:Sfn Eccrine sweat is clear, odorless, and is composed of 98–99% water; it also contains NaCl, fatty acids, lactic acid, citric acid, ascorbic acid, urea, and uric acid. Its pH ranges from 4 to 6.8.<ref name=draelos10 /> On the other hand, the apocrine sweat has a pH of 6 to 7.5; it contains water, proteins, carbohydrate waste material, lipids, and steroids. The sweat is oily, cloudy, viscous, and originally odorless;<ref name=draelos10 /> it gains odor upon decomposition by bacteria. Because both apocrine glands and sebaceous glands open into the hair follicle, apocrine sweat is mixed with sebum.Template:Sfn

MechanismEdit

File:Apocrine.jpg

In apocrine secretion (pictured), portions of the cell are pinched off and later disintegrate.

Both apocrine and eccrine sweat glands use merocrine secretion, where vesicles in the gland release sweat via exocytosis, leaving the entire cell intact.Template:Sfn<ref name=saunders03 /> It was originally thought that apocrine sweat glands use apocrine secretion due to histological artifacts resembling "blebs" on the cell surface, however, recent electron micrographs indicate that the cells use merocrine secretion.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In both apocrine and eccrine sweat glands, the sweat is originally produced in the gland's coil, where it is isotonic with the blood plasma there.Template:Sfn When the rate of sweating is low, salt is conserved and reabsorbed by the gland's duct; high sweat rates, on the other hand, lead to less salt reabsorption and allow more water to evaporate on the skin (via osmosis) to increase evaporative cooling.Template:Sfn

Secretion of sweat occurs when the myoepithelial cell cells surrounding the secretory glands contract.Template:Sfn Eccrine sweat increases the rate of bacterial growth and volatilizes the odor compounds of apocrine sweat, strengthening the latter's acrid smell.Template:Sfn

Normally, only a certain number of sweat glands are actively producing sweat. When stimuli call for more sweating, more sweat glands are activated, with each then producing more sweat.Template:Sfn<ref name=randall46 />

StimuliEdit

ThermalEdit

Both eccrine and apocrine sweat glands participate in thermoregulatory sweating,Template:Sfn which is directly controlled by the hypothalamus. Thermal sweating is stimulated by a combination of internal body temperature and mean skin temperature.Template:Sfn In eccrine sweat glands, stimulation occurs via activation by acetylcholine, which binds to the gland's muscarinic receptors.Template:Sfn

EmotionalEdit

Emotional sweating is stimulated by stress, anxiety, fear, and pain; it is independent of ambient temperature. Acetylcholine acts on the eccrine glands and adrenaline acts on both eccrine and apocrine glands to produce sweat.Template:Sfn Emotional sweating can occur anywhere, though it is most evident on the palms, soles of the feet, and axillary regions.Template:Sfn Sweating on the palms and soles is thought to have evolved as a fleeing reaction in mammals: it increases friction and prevents slipping when running or climbing in stressful situations.Template:Sfn

GustatoryEdit

Gustatory sweating refers to thermal sweating induced by the ingestion of food. The increase in metabolism caused by ingestion raises body temperature, leading to thermal sweating. Hot and spicy foods also lead to mild gustatory sweating in the face, scalp and neck: capsaicin (the compound that makes spicy food taste "hot"), binds to receptors in the mouth that detect warmth. The increased stimulation of such receptors induces a thermoregulatory response.Template:Sfn

AntiperspirantEdit

Unlike deodorant, which simply reduces axillary odor without affecting body functions, antiperspirant reduces both eccrine and apocrine sweating.Template:Sfn<ref name=draelos10 /> Antiperspirants, which are classified as drugs, cause proteins to precipitate and mechanically block eccrine (and sometimes apocrine) sweat ducts.Template:Sfn The metal salts found in antiperspirants alters the keratin fibrils in the ducts; the ducts then close and form a "horny plug". The main active ingredients in modern antiperspirants are aluminum chloride, aluminum chlorohydrate, aluminum zirconium chlorohydrate, and buffered aluminum sulfate.<ref name=draelos10 />

On apocrine glands, antiperspirants also contain antibacterial agents such as trichlorocarbanilide, hexamethylene tetramine, and zinc ricinoleate.<ref name=britannica-skindis />Template:Sfn The salts are dissolved in ethanol and mixed with essential oils high in eugenol and thymol (such as thyme and clove oils). Antiperspirants may also contain levomethamphetamine.Template:Sfn

PathologyEdit

Some diseases of the sweat glands include:

Fox-Fordyce disease: The apocrine sweat glands become inflamed, causing a persistent, itchy rash, usually in the axillae and pubic areas.<ref name=dorland-disease>Template:Cite encyclopedia</ref>
Frey's Syndrome: If the auriculotemporal nerve is damaged (most often as a result of a Parotidectomy), excess sweat can be produced in the rear of the cheek area (just below the ear) in response to stimuli that cause salivation.<ref name="NEJM">Template:Cite journal</ref>
Heatstroke: When the eccrine glands become exhausted and unable to secrete sweat. Heatstroke can lead to fatal hyperpyrexia (extreme rise in body temperature).<ref name=britannica-skindis />
Hidradenitis suppurativa: Occurs when the skin and sweat glands become inflamed with swollen lumps. These are typically painful and break open, releasing fluid or pus. The most commonly affected areas are the underarms, under the breasts, and the groin.
Hyperhidrosis: (also known as polyhidrosis or sudorrhea) is a pathological, excessive sweating that can be either generalized or localized (focal hyperhidrosis); focal hyperhidrosis occurs most often on the palms, soles, face, scalp and axillae. Hyperhidrosis is usually brought on by emotional or thermal stress,Template:Sfn but it can also occur or with little to no stimulus.<ref name=britannica-skindis /> Local (or asymmetrical) hyperhidrosis is said to be caused by problems in the sympathetic nervous system: either lesionsTemplate:Sfn or nerve inflammation.<ref name=britannica-hyper /> Hyperhidrosis can also be caused by trench foot or encephalitis.<ref name=britannica-hyper />
Milaria rubra: Also called prickly heat. Milaria rubra is the rupture of sweat glands and migration of sweat to other tissues. In hot environments, the skin's horny layer can expand due to sweat retention, blocking the ducts of eccrine sweat glands. The glands, still stimulated by high temperatures, continues to secrete. Sweat builds up in the duct, causing enough pressure to rupture the duct where it meets the epidermis. Sweat also escapes the duct to adjacent tissues (a process called milaria).<ref name=britannica-skindis />Template:Sfn Hypohydrosis then follows milaria (postmiliarial hypohydrosis).Template:Sfn
Osmidrosis: Often called bromhidrosis, especially in combination with hyperhidrosis. Osmohidrosis is excessive odor from apocrine sweat glands (which are overactive in the axillae).Template:Sfn Osmidrosis is thought to be caused by changes in the apocrine gland structure rather than changes in the bacteria that acts on sweat.Template:Sfn

TumorsEdit

Sweat gland tumors include:Template:Sfn<ref name="pmid34197659">Template:Cite journal</ref> Template:Div col

Template:Div col end

Adenolipomas are lipomas associated with eccrine sweat glands.Template:Sfn

As signs in other illnessesEdit

Many diseases cause sweat gland dysfunction:

Acromegaly, a result of excess growth hormone, causes the size of sweat glands increase, which leads to thicker skin.Template:Sfn
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.Template:Sfn
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.<ref name=bernstein11>Template:Cite book</ref>
Ectodermal dysplasia can present a lack of sweat glands.<ref name=medline-ectodermal>Template:Cite encyclopedia</ref>
Fabry disease, characterized by excess globotriaosylceramide (GL3), causes a decrease in sweat gland function due to GL3 deposits in the eccrine glands.<ref name=elstein10>Template:Cite book</ref>
GM₁ gangliosidoses, characterized by abnormal lipid storage, leads to vacuolization in eccrine sweat gland cells.<ref name=drut78>Template:Cite journal

</ref>

Hunter syndrome can include metachromin granules and mucin in the cytoplasm of the eccrine sweat gland cells.Template:Sfn
Hypothyroidism's low levels of thyroid hormone lead to decreased secretions from sweat glands; the result is dry, coarse skin.Template:Sfn
Kearns–Sayre syndrome, a disease of the mitochondria, involves abnormal mitochondria in eccrine sweat glands.<ref name=martin84>Template:Cite book</ref>
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.<ref name=rubio92>Template:Cite journal</ref>
Lichen striatus, a self-limited eruption of small, slightly scaly papules, includes a lymphoid infiltrate around eccrine sweat glands.Template:Sfn
Metachromatic leukodystrophy, a lysosomal storage disease, leads to the accumulation of lipopigments and lysosomal residual bodies in the epithelial cells of sweat glands.<ref name=goebel-89>Template:Cite book</ref>
Neuronal ceroid lipofuscinosis causes abnormal deposits of lipopigment in sweat gland epithelial cells (among other places).<ref name=carlen01>Template:Cite journal</ref>
Neutral lipid storage disease includes abnormal lipid deposits in cells, including those of the sweat gland.Template:Sfn
Niemann-Pick disease type C, another lipid storage disease, includes abnormal lipid storage in sweat glands.<ref name=elleder75>Template:Cite journal</ref>
Schindler disease causes cytoplasmic vacuoles that appear to be empty or contain filamentous material to manifest in eccrine sweat gland cells.<ref name=pavelka10>Template:Cite book</ref>
Small fiber peripheral neuropathy can damage the nerves that control the sweat glands. The sweat gland nerve fiber density test can diagnose this condition.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

GalleryEdit

File:Gray940 - sweat gland.png

Transverse view of the skin with sweat glands highlighted

File:Gray893 - sweat glands.png

Transverse view of the eyelid with sweat glands highlighted

NotesEdit

Template:Reflist

ReferencesEdit

External linksEdit

Histology of sweat glands

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