An apocrine sweat gland (; from Greek ''apo'' 'away' and ''krinein'' 'to separate') is composed of a coiled secretory portion located at the junction of the dermis and

subcutaneous fat The subcutaneous tissue (), also called the hypodermis, hypoderm (), subcutis, superficial fascia, is the lowermost layer of the integumentary system in vertebrates. The types of cells found in the layer are fibroblasts, adipose cells, and macro ...

, from which a straight portion inserts and secretes into the

infundibular An infundibulum (Latin for ''funnel''; plural, ''infundibula'') is a funnel-shaped cavity or organ. Anatomy * Brain: the pituitary stalk, also known as the ''infundibulum'' and ''infundibular stalk'', is the connection between the hypothalamus and ...

portion of the hair follicle. In humans,

apocrine Apocrine () glands are a type of exocrine gland, which are themselves a type of gland, i.e. a group of cells specialized for the release of secretions. Exocrine glands secrete by one of three means: holocrine, merocrine and apocrine. In apocri ...

sweat glands are found only in certain locations of the body: the

axillae The axilla (also, armpit, underarm or oxter) is the area on the human body directly under the shoulder joint. It includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage, bounded superio ...

(armpits),

areola The human areola (''areola mammae'', or ) is the pigmented area on the breast around the nipple. Areola, more generally, is a small circular area on the Human body, body with a different histology from the surrounding Tissue (biology), tissu ...

and

nipple The nipple is a raised region of tissue on the surface of the breast from which, in females, milk leaves the breast through the lactiferous ducts to feed an infant. The milk can flow through the nipple passively or it can be ejected by smooth m ...

s of the breast, ear canal,

eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eye ...

s, wings of the

nostril A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbi ...

, perineal region, and some parts of the

external genitalia A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...

. Modified apocrine glands include the

ciliary glands Moll's gland, also known as the gland of Moll or ciliary gland, is a modified apocrine sweat gland that is found on the margin of the eyelid. They are next to the base of the eyelashes, and anterior to the meibomian glands within the distal eyeli ...

in the eyelids; the ceruminous glands, which produce ear wax; and the

mammary glands A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in prima ...

, which produce

milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digest solid food. Immune factors and immune-modula ...

. The rest of the body is covered by eccrine sweat glands. Most non-primate mammals, however, have apocrine sweat glands over the greater part of their body. Domestic animals such as dogs and cats have apocrine glands at each hair follicle and even in their urinary system, but eccrine glands only in foot pads and snout. Their apocrine glands, like those in humans, produce an odorless, oily, opaque secretion that gains its characteristic odor upon bacterial decomposition. Eccrine glands on their paws increase friction and prevent them from slipping when fleeing from danger.

Structure

The apocrine gland is made up of a

glomerulus ''Glomerulus'' () is a common term used in anatomy to describe globular structures of entwined vessels, fibers, or neurons. ''Glomerulus'' is the diminutive of the Latin ''glomus'', meaning "ball of yarn". ''Glomerulus'' may refer to: * the filter ...

of secretory tubules and an excretory duct that opens into a hair follicle; on occasion, an excretory duct opens to the skin surface next to the hair. The gland is large and spongy, located in the subcutaneous fat deep in the dermis, and has a larger overall structure and lumen diameter than the eccrine sweat gland. The secretory tubules of apocrine glands are single layered, but unlike the eccrine secretory tubules, contain only a single type of ductal epithelial cell, varying in diameter according to their location, and sometimes branching off into multiple ducts. The tubules are wrapped in

myoepithelial cell Myoepithelial cells (sometimes referred to as myoepithelium) are cells usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the luminal cells. These may be positive for alpha smooth muscle actin a ...

s, which are more developed than in their eccrine gland counterparts.

Sweating

In hoofed animals and marsupials, apocrine glands act as the main

thermoregulator Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...

, secreting watery sweat. For most mammals, however, apocrine sweat glands secrete an oily (and eventually smelly) compound that acts as a

pheromone A 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 like hormones outside the body of the secreting individual, to affect the behavio ...

, territorial marker, and

warning signal Aposematism is the advertising by an animal to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, such as toxicity, venom, foul taste or ...

. Being sensitive to

adrenaline Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands an ...

, apocrine sweat glands are involved in emotional sweating in humans (induced by anxiety, stress, fear, sexual stimulation, and pain). In a five-month-old human fetus, apocrine glands are distributed all over the body; after a few weeks, they exist in only restricted areas, including the armpits and external genitalia. They are inactive until stimulated by hormonal changes in

puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a bo ...

Mechanism

The apocrine gland secretes an oily fluid with proteins and

lipids Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...

that is odorless before microbial activity. It appears on the skin surface mixed with

sebum A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest nu ...

, as sebaceous glands open into the same hair follicle. Unlike eccrine sweat glands, which secrete continuously, the apocrine glands secrete in periodic spurts. Apocrine sweat glands were originally thought to use only apocrine secretion: vesicles pinch off from the secretory cells, then degrade in the secretory lumen, releasing their product. More recent research has also shown that

merocrine Merocrine (or eccrine) is a term used to classify exocrine glands and their secretions in the study of histology. A cell is classified as merocrine if the secretions of that cell are excreted via exocytosis from secretory cells into an epithelia ...

secretion takes place. Myoepithelial cells form a smooth muscle lining around the secretory cells; when the muscles contract, they squeeze the secretory ducts and push out the accumulated fluid into the hair follicle. Sweat and

are mixed in the hair follicle and arrive mixed at the epidermal surface. The apocrine sweat is cloudy, viscous, initially odorless, and at a pH of 6–7.5. It contains water, protein,

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...

waste material, and

NaCl Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...

. The sweat only attains its characteristic odor upon being degraded by bacteria, which releases volatile odor molecules. More bacteria (especially

corynebacteria ''Corynebacterium'' () is a genus of Gram-positive bacteria and most are aerobic. They are bacilli (rod-shaped), and in some phases of life they are, more specifically, club-shaped, which inspired the genus name (''coryneform'' means "club-shape ...

) leads to stronger odor. The presence of axillary hair also makes the odor even more pungent, as secretions, debris,

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...

, and bacteria accumulate on the hairs.

Prevalence

Non-primate mammals usually have apocrine sweat glands over most of their bodies. Horses use them as a thermoregulatory device, as they are regulated by adrenaline and more widely distributed on

equines ''Equus'' , is a genus of mammals in the family (biology), family Equidae, which includes horses, donkeys, and zebras. Within the Equidae, ''Equus'' is the only recognized Extant taxon, extant genus, comprising seven living species. Like Equida ...

than in other groups. Skunks, on the other hand, use the glands to release a secretion that acts as a powerful defense mechanism. The "axillary organs", limited regions with equal numbers of apocrine and eccrine sweat glands, only exist in

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...

gorilla Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or fi ...

s, and chimpanzees. In humans, the apocrine glands in this region are the most developed (with the most complex glomeruli). Men have more apocrine sweat glands than women in all axillary regions. There is currently no evidence that any sweat glands vary substantially by racial group, with most studies claiming to find variation being subject to methodological flaws.

References

Citations

Sources

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External links

Diagram of eccrine and apocrine sweat glands
from Mayo Clinic {{DEFAULTSORT:Apocrine Sweat Gland Skin anatomy Glands Apocrine