CorpusHuman Anatomy
IntegumentaryGland

Eccrine sweat gland

Glandula sudorifera eccrina

Also known as: merocrine sweat gland, atrichial sweat gland, sweat pore, eccrine gland

Eccrine sweat glands are simple coiled tubular merocrine glands, distributed over almost the whole skin, that secrete hypotonic watery sweat onto the surface for evaporative cooling.

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Overview

Number≈2–4 million
Normal sweat chloride<30–40 mmol/L (CF ≥60)
Max output≈1–2 L/h

Each gland consists of a tightly coiled secretory portion in the deep dermis or upper hypodermis and a duct that ascends through the dermis, enters the epidermis between rete ridges and spirals (the acrosyringium) to open at a sweat pore. The secretory epithelium contains clear cells (which secrete water and electrolytes, rich in glycogen and mitochondria, with intercellular canaliculi), dark cells (which secrete glycoproteins) and an outer layer of myoepithelial cells that expel sweat. The duct is lined by a two-layered (stratified) cuboidal epithelium with more intensely staining cells.

Sweat is formed in two stages. The secretory coil produces an isotonic, plasma-like primary fluid by active Cl⁻ secretion (Na⁺/K⁺/2Cl⁻ co-transport basolaterally, Cl⁻ channels apically) with Na⁺ and water following. As the fluid passes along the duct, Na⁺ (ENaC) and Cl⁻ (CFTR) are reabsorbed, leaving a hypotonic final sweat. At high sweat rates there is less time for reabsorption, so sweat NaCl rises; aldosterone increases ductal Na⁺ reabsorption during heat acclimatisation, conserving salt.

The glands are innervated by sympathetic postganglionic fibres that release acetylcholine acting on muscarinic M3 receptors — a notable exception to the adrenergic rule. Thermal sweating is driven by the hypothalamus; palmar and plantar glands also respond to emotional stress (the basis of the galvanic skin response).

Location

Almost all skin; most numerous on palms, soles, forehead and axillae; absent from lips, nail beds, nipples, glans penis, clitoris and labia minora.

Parts & subdivisions

  1. Secretory coil
    Simple cuboidal-to-columnar epithelium of clear and dark cells with myoepithelial cells; in deep dermis/hypodermis.
  2. Dermal duct
    Narrow, two-layered cuboidal epithelium; reabsorbs Na⁺ and Cl⁻ via ENaC and CFTR.
  3. Intraepidermal duct (acrosyringium)
    Spiral channel through the epidermis, lined by keratinocytes.
  4. Sweat pore
    Surface opening; in thick skin pores lie along the crests of friction ridges.

Anatomy

Innervation

Sympathetic cholinergic postganglionic fibres (ACh → M3 muscarinic receptors); thermal control from the preoptic hypothalamus.

Histology

Secretory coil: pale-staining, larger diameter; duct: smaller, darker, two layers of cuboidal cells, no myoepithelium.

Development

Grow directly from the epidermis as solid cords into the dermis (unlike apocrine glands, which bud from hair follicles); first on palms and soles at ≈week 12–16.

Function

  • Thermoregulatory evaporative cooling (≈2.4 kJ lost per gram evaporated)
  • Excretion of water, NaCl, K⁺, urea, lactate and ammonia
  • Antimicrobial defence (dermcidin, acidic surface)
  • Moistens palms and soles to improve grip
Secretions

Hypotonic sweat (≈99% water) with NaCl (Cl⁻ normally <30–40 mmol/L), K⁺, urea, lactate, bicarbonate and antimicrobial peptides. Rate up to ≈1–2 L/h, higher in heat-acclimatised people.

Clinical significance

Cystic fibrosis sweat test

Defective CFTR prevents ductal Cl⁻ (and therefore Na⁺) reabsorption, so sweat is salty: sweat chloride ≥60 mmol/L is diagnostic, 30–59 intermediate. Infants may taste salty when kissed; affected people are prone to salt depletion in heat.

Anticholinergic toxicity

Muscarinic blockade (atropine, tricyclics, antihistamines) abolishes sweating: "hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter" — risk of hyperthermia.

Miliaria (heat rash)

Obstruction of eccrine ducts by keratin in hot, humid conditions leaks sweat into the epidermis (miliaria crystallina: superficial clear vesicles) or dermis (miliaria rubra: itchy red papules, "prickly heat").

Test yourself

Eccrine sweat glands are stimulated mainly by which neurotransmitter?
  1. Noradrenaline acting on α1-adrenoceptors
  2. Acetylcholine released by sympathetic postganglionic fibres
  3. Acetylcholine released by parasympathetic fibres
  4. Adrenaline from the adrenal medulla

Answer: Acetylcholine released by sympathetic postganglionic fibres

Eccrine glands are innervated by sympathetic postganglionic fibres that are exceptionally cholinergic (ACh on muscarinic M3 receptors). The skin has no parasympathetic supply. This is why anticholinergic poisoning causes hot, dry skin.

More integumentary system questions

Compiled from standard anatomy and physiology references — see the Integumentary System sources. For education only; not medical advice.