Sweat gland

Sweat glands, also referred to as sudoriferous or sudoriparous glands (derived from the Latin term sudor, meaning 'sweat'), are minute tubular structures within the skin responsible for sweat production. These glands are classified as exocrine glands, which are characterized by their ability to produce and secrete substances onto an epithelial surface via a duct. Two primary categories of sweat glands exist, distinguished by their structural characteristics, functional roles, secretory compositions, excretory mechanisms, anatomical locations, and species-specific distributions.

• Eccrine sweat glands are extensively distributed across the human integument, exhibiting variable densities; the highest concentrations are found on the palms and soles, followed by the head, with significantly lower densities observed on the trunk and extremities. The aqueous secretion produced by these glands serves as the principal mechanism for thermoregulation in humans.
• In humans, apocrine sweat glands are predominantly localized to the axillae and perineal regions. These glands do not play a significant role in human thermoregulation; however, they constitute the exclusive effective sweat glands in ungulates, including species such as camels, donkeys, horses, and cattle.

Modified apocrine sweat glands include ceruminous glands, responsible for cerumen (ear wax) production; mammary glands, which produce milk; and ciliary glands, situated in the eyelids.

Structure

Typically, sweat glands comprise a secretory unit responsible for sweat generation and a duct that transports the sweat away. The secretory coil, or base, is deeply embedded within the lower dermis and hypodermis, with the entire gland enveloped by adipose tissue. In both categories of sweat glands, the secretory coils are encircled by contractile myoepithelial cells, which serve to facilitate the expulsion of the secretory product. Both the secretory functions of the glandular cells and the contractions of the myoepithelial cells are regulated by the autonomic nervous system and circulating hormones. The distal, or apical, segment of the duct that terminates at the skin's surface is designated as the acrosyringium.

Each sweat gland is innervated by multiple nerve fibers, which ramify into fascicles containing one or more axons that encircle the individual tubules of the secretory coil. Furthermore, capillaries are intricately interwoven among the sweat tubules.

Distribution

The quantity of active sweat glands exhibits substantial inter-individual variability; however, comparative analyses between distinct anatomical regions (e.g., axillae versus groin) consistently reveal similar directional patterns in gland density. According to estimations by Henry Gray, the palm possesses approximately 370 sweat glands per cm²; the dorsal aspect of the hand, 200 per cm²; the forehead, 175 per cm§4^{5§; the breast, abdomen, and forearm, 155 per cm§67§; and the back and legs, 60–80 per cm§89§.}

On the finger pads, the pores of sweat glands are somewhat irregularly distributed along the epidermal ridges. No pores are present between these ridges, although sweat frequently flows into these interstitial spaces. The substantial thickness of the epidermis on the palms and soles results in the spiral coiling of the sweat glands.

Other Animals

In non-primate mammals, eccrine sweat glands are exclusively located on the palms and soles. Apocrine glands are distributed across the remainder of the body; however, their efficacy in thermoregulation is generally inferior to that observed in humans, with the notable exception of horses. Prosimians exhibit a 1:20 ratio of hair follicles possessing apocrine glands to those without. Unlike humans, who possess eccrine glands primarily on the scalp among hairs, prosimians have eccrine glands interspersed among hairs over most of their body.

The comprehensive distribution of sweat glands demonstrates variability among primate species. For instance, rhesus and patas monkeys possess these glands on the chest, whereas squirrel monkeys exhibit them solely on the palms and soles. Conversely, the stump-tailed macaque, Japanese monkey, and baboon display sweat glands across their entire body.

Domestic animals possess apocrine glands situated at the base of each hair follicle, while eccrine glands are confined to the foot pads and snout. The apocrine glands in these animals, similar to those in humans, generate an odorless, oily, milky secretion. This secretion evolved not for evaporative cooling but rather to coat and adhere to hair, thereby providing a substrate for odor-producing bacteria. Similarly, the eccrine glands on their foot pads, analogous to those on human palms and soles, did not evolve for thermoregulation but instead to augment friction and improve grip.

In canids and felids, specialized apocrine glands, distinct in both structure and function, are found in specific locations, including the eyelids (Moll's glands), ears (ceruminous glands), anal sac, clitoral hood, and circumanal region.

History

The pores of eccrine sweat glands were initially identified by the Italian physiologist Marcello Malpighi. The sweat glands themselves were subsequently discovered by the Czech physiologist Johannes Purkinjé in 1833. Variations in sweat gland density across different somatic regions were first investigated in 1844 by the German anatomist Karl Krause. French histologist Louis-Antoine Ranvier first categorized sweat glands in 1887 based on their secretory mechanisms, distinguishing between holocrine glands (sebaceous glands) and merocrine glands (sweat glands); the merocrine glands were further subdivided into apocrine and eccrine types in 1917. Apoeccrine glands were identified in 1987.

Categories

Eccrine

Eccrine sweat glands are ubiquitous across the body surface, with notable exceptions including the lips, ear canal, foreskin, glans penis, labia minora, clitoral hood, and clitoris. These glands are approximately ten times smaller than apocrine sweat glands, exhibit a shallower dermal penetration, and discharge their secretions directly onto the epidermal surface. The relative prevalence of eccrine glands diminishes with advancing age.

The transparent fluid secreted by eccrine sweat glands is referred to as sweat or sensible perspiration. Composed primarily of water, sweat also contains various electrolytes, as it originates from blood plasma. The characteristic salty flavor of sweat is attributable to its sodium chloride content.

The overall volume of sweat generated is contingent upon both the quantity of active glands and the dimensions of their surface apertures. Secretory activity is modulated by intricate neural and hormonal pathways; notably, males typically exhibit higher perspiration rates than females. Under conditions of maximal eccrine gland function, human perspiration rates can surpass three liters per hour, potentially leading to critical fluid and electrolyte imbalances.

Eccrine glands fulfill three principal physiological roles:

• Thermoregulation: The evaporation of sweat from the skin surface facilitates evaporative heat loss, thereby inducing cutaneous cooling and a subsequent decrease in core body temperature.
• Excretion: Eccrine sweat gland secretions serve as a substantial excretory pathway for water and various electrolytes.
• Protection: The secretion from eccrine sweat glands contributes to the maintenance of the skin's acid mantle, which is crucial for safeguarding the integument against colonization by bacteria and other pathogenic microorganisms.

Apocrine

Apocrine sweat glands are localized to specific anatomical regions, including the axilla (armpit), areolae (surrounding the nipples), perineum (the area between the anus and genitals), the external auditory canal, and the eyelids. Their secretory component is notably larger than that of eccrine glands, contributing to their overall greater size. Unlike eccrine glands, apocrine glands do not discharge directly onto the skin surface; instead, they release sweat into the pilary canal of hair follicles.

Prior to puberty, apocrine sweat glands remain dormant; however, pubertal hormonal shifts induce glandular enlargement and activation. The secreted substance is denser than eccrine sweat and furnishes a nutrient source for cutaneous bacteria, whose metabolic decomposition of this sweat generates the characteristic pungent odor. Apocrine sweat glands exhibit peak activity during periods of psychological stress and sexual arousal.

In mammals, including humans, apocrine sweat contains compounds resembling pheromones, which are implicated in intraspecies attraction. Research on human sweat has identified distinctions in apocrine secretions and associated bacterial flora between males and females.

Apoeccrine

A subset of human sweat glands defies classification as purely apocrine or eccrine, exhibiting features of both types; these are designated as apoeccrine glands. These glands are intermediate in size, being larger than eccrine glands but smaller than apocrine glands. Their secretory component comprises both a narrow segment, analogous to the secretory coils of eccrine glands, and a broader section, characteristic of apocrine glands.

Apoeccrine glands are located in the axillae and perianal region, featuring ducts that open directly onto the skin surface. It is hypothesized that they develop from eccrine glands during puberty, potentially constituting up to 50% of all axillary glands. These glands exhibit a higher secretory capacity than both eccrine and apocrine glands, significantly contributing to axillary perspiration. While primarily responsive to cholinergic stimulation, apoeccrine glands can also be activated through adrenergic pathways. Similar to eccrine glands, they continuously produce a dilute, aqueous sweat.

Others

Modified apocrine glands encompass specialized sweat glands such as ceruminous glands, mammary glands, ciliary glands of the eyelids, and those located in the nasal vestibulum. Ceruminous glands, situated near the ear canals, generate cerumen (earwax), which combines with oil from sebaceous glands. Mammary glands employ apocrine secretion for milk production.

Sweat

Sweat glands facilitate thermoregulation and waste elimination by releasing water, sodium salts, and nitrogenous waste products, such as urea, onto the epidermal surface. Sodium and chloride constitute the primary electrolytes in sweat; however, their concentration is sufficiently low to render sweat hypotonic at the skin's surface. Eccrine sweat is characterized by its clarity, lack of odor, and a composition of 98–99% water, alongside NaCl, fatty acids, lactic acid, citric acid, ascorbic acid, urea, and uric acid, with a pH ranging from 4 to 6.8. In contrast, apocrine sweat exhibits a pH between 6 and 7.5, containing water, proteins, carbohydrate waste, lipids, and steroids. This type of sweat is oily, cloudy, viscous, and initially odorless, acquiring its characteristic scent through bacterial decomposition. Given that both apocrine and sebaceous glands discharge into the hair follicle, apocrine sweat invariably mixes with sebum.

Mechanism

Both apocrine and eccrine sweat glands employ merocrine secretion, a process where secretory vesicles within the gland release sweat through exocytosis, preserving the integrity of the cell. Although historical observations of histological artifacts, appearing as "blebs" on the cell surface, initially suggested apocrine secretion for apocrine sweat glands, contemporary electron micrographs confirm their utilization of merocrine secretion. Within both apocrine and eccrine sweat glands, sweat is initially generated in the glandular coil, where its tonicity matches that of the blood plasma. During periods of low sweat production, salts are conserved and reabsorbed by the glandular duct. Conversely, elevated sweat rates result in diminished salt reabsorption, thereby facilitating increased water evaporation from the skin via osmosis, which enhances evaporative cooling.

Sweat secretion is initiated by the contraction of myoepithelial cells encircling the secretory glands. Eccrine sweat promotes bacterial proliferation and volatilizes the odoriferous compounds present in apocrine sweat, thereby intensifying its pungent aroma.

Typically, only a subset of sweat glands is actively engaged in sweat production. In response to stimuli necessitating increased perspiration, additional sweat glands become activated, each subsequently augmenting its sweat output.

Stimuli

Thermal

Both eccrine and apocrine sweat glands contribute to thermoregulatory sweating, a process directly governed by the hypothalamus. Thermal sweating is elicited by the combined influence of internal body temperature and average skin temperature. For eccrine sweat glands, stimulation is mediated by acetylcholine, which binds to the gland's muscarinic receptors.

Emotional

Emotional sweating is triggered by psychological stressors such as stress, anxiety, fear, and pain, operating independently of ambient temperature. Acetylcholine primarily affects eccrine glands, while adrenaline influences both eccrine and apocrine glands to induce sweat production. Although emotional sweating can manifest across the body, it is most pronounced in the palms, soles, and axillary regions. The perspiration observed on the palms and soles is hypothesized to be an evolutionary adaptation in mammals, serving as a flight response by enhancing friction and preventing slippage during strenuous activities like running or climbing in stressful scenarios.

Gustatory

Gustatory sweating denotes thermoregulatory perspiration prompted by food consumption. The metabolic surge resulting from ingestion elevates core body temperature, consequently inducing thermal sweating. Furthermore, piquant and spicy foods can elicit mild gustatory sweating on the face, scalp, and neck; this occurs because capsaicin, the compound responsible for the "hot" sensation in spicy foods, activates oral receptors sensitive to warmth. The heightened stimulation of these receptors subsequently triggers a thermoregulatory response.

Antiperspirant

Unlike deodorant, which merely mitigates axillary odor without influencing physiological functions, antiperspirant actively diminishes both eccrine and apocrine perspiration. Classified as pharmaceutical agents, antiperspirants induce protein precipitation, thereby mechanically obstructing eccrine and, occasionally, apocrine sweat ducts. The metallic salts present in antiperspirants modify the keratin fibrils within these ducts, leading to their occlusion and the formation of a "horny plug". Key active components in contemporary antiperspirant formulations include aluminum chloride, aluminum chlorohydrate, aluminum zirconium chlorohydrate, and buffered aluminum sulfate.

For apocrine glands, antiperspirants additionally incorporate antibacterial compounds like trichlorocarbanilide, hexamethylene tetramine, and zinc ricinoleate. These salts are typically dissolved in ethanol and combined with essential oils rich in eugenol and thymol, such as those derived from thyme and clove. Certain antiperspirant formulations may also include levomethamphetamine.

Pathology

Several pathological conditions affecting the sweat glands are enumerated below:

Fox-Fordyce disease

This condition involves inflammation of the apocrine sweat glands, resulting in a chronic, pruritic rash typically localized to the axillary and pubic regions.

Frey's Syndrome

Damage to the auriculotemporal nerve, frequently occurring post-parotidectomy, can induce excessive perspiration in the post-auricular and posterior cheek region, triggered by stimuli that ordinarily elicit salivation.

Heatstroke

Heatstroke occurs when eccrine glands become depleted and cease sweat secretion, potentially leading to lethal hyperpyrexia, an extreme elevation in core body temperature.

Hidradenitis suppurativa

This condition manifests as inflammation of the skin and sweat glands, characterized by swollen nodules that are typically painful and prone to rupture, discharging fluid or purulent material. Predominantly affected anatomical sites include the axillae, inframammary folds, and inguinal regions.

Hyperhidrosis

Hyperhidrosis, also designated as polyhidrosis or sudorrhea, constitutes a pathological condition characterized by excessive perspiration, which may be generalized or localized (focal hyperhidrosis). Focal hyperhidrosis most frequently affects the palms, soles, face, scalp, and axillae. While commonly precipitated by emotional or thermal stress, hyperhidrosis can also manifest with minimal or no apparent stimulus. Localized or asymmetrical hyperhidrosis is often attributed to dysfunctions within the sympathetic nervous system, such as lesions or nerve inflammation. Furthermore, hyperhidrosis may be associated with conditions like trench foot or encephalitis.

Miliaria rubra

Also known as prickly heat, miliaria rubra involves the rupture of sweat glands and the extravasation of sweat into surrounding tissues. In elevated ambient temperatures, the stratum corneum of the skin can swell due to retained sweat, thereby occluding the ducts of eccrine sweat glands. Despite this obstruction, the glands, continuously stimulated by heat, persist in secreting sweat. This accumulation of sweat within the duct generates sufficient pressure to cause its rupture at the dermo-epidermal junction. Subsequently, sweat leaks from the duct into adjacent tissues, a phenomenon termed miliaria. Miliaria is often succeeded by hypohidrosis, referred to as postmiliarial hypohidrosis.

Osmidrosis

Osmidrosis, frequently termed bromhidrosis, particularly when co-occurring with hyperhidrosis, is characterized by an excessive malodor originating from overactive apocrine sweat glands, predominantly in the axillae. The etiology of osmidrosis is hypothesized to involve structural alterations within the apocrine glands, rather than modifications in the bacterial flora acting upon sweat.

Tumors

Neoplasms originating from sweat glands comprise:

Adenolipomas represent lipomas that exhibit an association with eccrine sweat glands.

Sweat Gland Dysfunction in Systemic Illnesses

Numerous systemic diseases are associated with impaired sweat gland function:

• Acromegaly, stemming from excessive growth hormone secretion, induces an increase in sweat gland size, contributing to cutaneous thickening.
• Aquagenic wrinkling of the palms, characterized by the emergence of white papules on the palmar surfaces following water exposure, may occasionally be linked to anomalous aquaporin 5 expression within the sweat glands.
• Cystic fibrosis is diagnosable via a sweat test, given that the disease impairs chloride reabsorption within the sweat gland ducts, resulting in elevated chloride concentrations in the secreted perspiration.
• Ectodermal dysplasia may manifest with aplasia or hypoplasia of the sweat glands.
• Fabry disease, defined by the accumulation of globotriaosylceramide (GL3), leads to diminished sweat gland function attributable to GL3 deposition within the eccrine glands.
• GM₁ gangliosidoses, a condition marked by aberrant lipid storage, results in vacuolization within eccrine sweat gland cells.
• Hunter syndrome may involve the presence of metachromatic granules and mucin within the cytoplasm of eccrine sweat gland cells.
• Hypothyroidism, characterized by insufficient thyroid hormone levels, results in diminished sweat gland secretions, manifesting as dry, coarse skin.
• Kearns–Sayre syndrome, a mitochondrial disorder, is associated with the presence of anomalous mitochondria within eccrine sweat glands.
• Lafora disease, an uncommon genetic condition, is identified by the accumulation of aberrant polyglucosan deposits, known as "Lafora bodies," which are observed in the ducts of sweat glands and the myoepithelial cells of apocrine glands.
• Lichen striatus, a transient dermatological condition characterized by small, mildly scaly papules, exhibits a lymphoid infiltrate surrounding the eccrine sweat glands.
• Metachromatic leukodystrophy, classified as a lysosomal storage disorder, causes the aggregation of lipopigments and lysosomal residual bodies within the epithelial cells of sweat glands.
• Neuronal ceroid lipofuscinosis results in the anomalous deposition of lipopigment within sweat gland epithelial cells, among other cellular locations.
• Neutral lipid storage disease involves the accumulation of atypical lipid deposits in various cells, including those comprising the sweat gland.
• Niemann-Pick disease type C, an additional lipid storage disorder, is characterized by aberrant lipid accumulation within sweat glands.
• Schindler disease manifests with cytoplasmic vacuoles in eccrine sweat gland cells, which either appear empty or contain filamentous material.
• Small fiber peripheral neuropathy has the potential to impair the innervation regulating sweat glands; this condition can be diagnosed using a sweat gland nerve fiber density test.

• Sudomotor

Notes

Eroschenko, Victor P. (2008). "Integumentary System." In DiFiore's Atlas of Histology with Functional Correlations. Lippincott Williams & Wilkins. pp. 212–234. ISBN 9780781770576.

• Eroschenko, Victor P. (2008). "Integumentary System". DiFiore's Atlas of Histology with Functional Correlations. Lippincott Williams & Wilkins. pp. 212–234. ISBN 9780781770576.Folk, G. Edgar Jr., and Semken, A. Jr. (1 September 1991). "The evolution of sweat glands." International Journal of Biometeorology, 35 (3): 180–186. Bibcode:1991IJBm...35..180F. doi:10.1007/BF01049065. ISSN 0020-7128. PMID 1778649. S2CID 28234765.Kasture, P. V., Gokhal, S. B., Parakh, S. R., and Paradkar, A. R. (7 September 2008). Pharmaceutics-II: Second Year Diploma in Pharmacy (10th ed.). Nirali Prakashan. pp. 15.14–15.16. ISBN 9788185790220.Kurosumi, Kazumasa, Shibasaki, Susumu, and Ito, Toshiho (1984). "Cytology of the Secretion in Mammalian Sweat Glands." In Bourne, Geoffrey H., and Danielli, James F. (eds.), Protein Diffusion in Cell Membranes: Some Biological Implications. Orlando, Florida: Academic Press. pp. 253–330. ISBN 9780123644879.James, William D., Berger, Timothy G., and Elston, Dirk M. (2011). Andrews' Diseases of the Skin: Clinical Dermatology (11th ed.). London: Elsevier. ISBN 9781437703146.Krstic, Radivoj V. (18 March 2004). Human Microscopic Anatomy: An Atlas for Students of Medicine and Biology. Springer. pp. 464, 466–469. ISBN 9783540536666.Rubin, Raphael, and Strayer, David Sheldon (29 March 2011). Rubin's Pathology: Clinicopathologic Foundations of Medicine. Lippincott Williams & Wilkins. pp. 1043, 1048. ISBN 9781605479682.Shibasaki, Manabu, Wilson, Thad E., and Crandall, Craig G. (2006). "Neural control and mechanisms of eccrine sweating during heat stress and exercise." Journal of Applied Physiology, 100 (5): 1692–1701. doi:10.1152/japplphysiol.01124.2005. ISSN 8750-7587. PMID 16614366.Sørensen, Vibeke W., and Prasad, Gaya (1973). "On the fine structure of horse sweat glands." Zeitschrift für Anatomie und Entwicklungsgeschichte, 139 (2): 173–183. doi:10.1007/BF00523636. PMID 4352229. S2CID 9847627.Slegers, J. F. G. (1964). "The mechanism of sweat-secretion." Pflügers Archiv für die gesamte Physiologie des Menschen und der Tiere, 279 (3): 265–273. doi:10.1007/BF00362480. ISSN 1432-2013. PMID 14194022. S2CID 9644549.Tsai, Ren-Yu (1 January 2006). "Treatment of Excessive Axillary Sweat Syndrome (Hyperhidrosis, Osmidrosis, Bromhidrosis) with Liposuction." In Shiffman, Melvin A., and Di Giuseppe, Alberto (eds.), Liposuction: Non-Cosmetic Applications. Germany: Springer. pp. 496–497. ISBN 9783540280439.Wilke, K.; Martin, A.; Terstegen, L.; Biel, S. S. (June 2007). "A short history of sweat gland biology". International Journal of Cosmetic Science. 29 (3): 169–179. doi:10.1111/j.1467-2494.2007.00387.x. ISSN 1468-2494. PMID 18489347.
  • Histology of sweat glands