Eccrine sweat gland

Eccrine sweat glands are simple, coiled tubular exocrine glands derived from embryonic ectoderm that represent the most abundant cutaneous appendages in humans, with approximately 4 million glands distributed across nearly the entire body surface (except the lips, external ear canal, and parts of the external genitalia).

Structure

Each eccrine gland consists of two morphologically and functionally distinct components: a secretory coil located deep in the dermis and a resorptive duct that extends from the dermis through the epidermis to open directly onto the skin surface (the acrosyringium). The secretory coil contains three cell types: clear cells (the primary secretory cells, rich in mitochondria and intracellular canaliculi), dark cells (containing secretory granules), and myoepithelial cells (providing structural support and contractile function). The duct is composed of a luminal and basal layer of cuboidal cells responsible for ion resorption.

Function

The primary role of eccrine glands is thermoregulation. They produce a hypotonic, salt-containing sweat — an individual can secrete up to 4 liters per hour — that cools the body through evaporation from the skin surface. Sweat production follows a two-stage process: an isotonic primary fluid is generated in the secretory coil, then modified by NaCl reabsorption in the duct before reaching the surface.

Beyond thermoregulation, eccrine glands serve several additional functions:

• Electrolyte homeostasis and skin hydration through secretion of lactate, urea, sodium, and potassium

• Innate immune defense via secretion of antimicrobial peptides (dermcidin, cathelicidin/LL-37, lactoferrin), immunoglobulin A, and cytokines (IL-1, IL-31)

• Enhanced grip on palmar and plantar surfaces, where eccrine glands respond to emotional and tactile stimuli rather than thermal stimuli

• Wound healing, with sweat gland progenitor cells in the myoepithelial layer contributing to re-epithelialization

Neural Control

Eccrine glands are innervated by cholinergic sympathetic nerves, which is unique among sympathetic effectors (most are adrenergic). Acetylcholine is the principal neurotransmitter triggering sweat secretion, accounting for over 70% of sweating capacity. Three types of sweating are recognized based on their neuronal control centers: thermoregulatory sweating (hypothalamic, systemic), emotional sweating (cortical, palms and soles), and gustatory sweating (medullary, facial).

Secretory Mechanism

At the molecular level, cholinergic stimulation triggers intracellular calcium release (via IP₃) and store-operated calcium entry (via ORAI1/STIM1 channels), which activates calcium-dependent chloride channels (ANO1/TMEM16A). Chloride secretion drives sodium transport through paracellular tight junctions via a transepithelial voltage gradient, and water follows osmotically through aquaporin-5 channels — consistent with the Na⁺-K⁺-Cl⁻ cotransporter model of sweat secretion.

Clinical Relevance

Dysfunction of eccrine glands can manifest as hyperhidrosis (overactivity, affecting ~2.8% of the US population), anhidrosis (absence of sweating, which can lead to life-threatening hyperthermia), or abnormal sweat electrolyte composition, as classically seen in cystic fibrosis, where elevated sweat chloride serves as a diagnostic marker.