Types of Sweat Glands

1. Eccrine Sweat Glands

Distribution

• Palms of hands (~620/cm²)
• Soles of feet
• Forehead, axillae, and trunk
• Absent from: vermilion border of the lips, nail beds, glans penis, clitoris, and labia minora

Structure

• Secretory coil: located deep in the dermis or upper hypodermis; contains three cell types:
  • Clear cells - on the basal lamina, produce the primary watery sweat; rich in mitochondria with extensive basolateral infoldings
  • Dark cells - line the lumen; filled with eosinophilic secretory granules; release glycoproteins with bactericidal activity (e.g., dermcidin, β-defensin)
  • Myoepithelial cells - on the basal lamina; contract to propel secretion into the duct
• Duct: double layer of cuboidal cells rich in Na⁺/K⁺-ATPase; absorbs Na⁺ from the primary secretion, producing hypotonic sweat; opens independently to the epidermal surface (NOT via a hair follicle)

Secretion

• Primary secretion is isotonic NaCl (similar to blood plasma ultrafiltrate), also contains K⁺, HCO₃⁻, glucose, lactate, urea, uric acid, and antimicrobial peptides
• Ductal reabsorption of Na⁺ and Cl⁻ produces a hypotonic final sweat
• Can produce up to 10 L/day in acclimatized individuals
• Also serves as an auxiliary excretory organ for nitrogenous waste and excess salts

Innervation

• Innervated by sympathetic cholinergic fibers (an exception to the rule - sympathetic system normally uses norepinephrine)
• Acetylcholine (ACh) is the primary stimulus, acting via muscarinic receptors and intracellular Ca²⁺
• Also respond to adrenergic agents (beta-adrenergic, mediated via cAMP)
• Stimulated by: thermal stress (hypothalamic thermoregulatory center), emotional stress (palms, soles, axillae first)
• Botulinum toxin blocks sweating by inhibiting ACh release

Function

Development

2. Apocrine Sweat Glands

Distribution

• Axillae (armpits) - most prominent
• Perineum and anogenital area
• Areolae and nipples of the breast
• External genitalia
• Modified apocrine glands: ceruminous glands (external auditory canal), glands of Moll (eyelash follicles)
• Not functional until puberty; development depends on sex hormones

Structure

• Coiled tubular glands located in subcutaneous fat near the dermis
• Secretory portion: simple epithelium of cuboidal or columnar cells, with a much larger lumen than eccrine glands; surrounded by myoepithelial cells
• Duct: stratified cuboidal epithelium (2-3 layers), NO myoepithelial cells; opens into the hair follicle infundibulum (not directly to skin surface)
• Ducts follow a relatively straight path - unlike the coiled eccrine duct
• No ductal reabsorption occurs (unlike eccrine)

Secretion

• Secretion is milky, cloudy, slightly viscous, protein-rich
• Contains proteins, carbohydrates, ammonia, lipids, and pheromone precursors
• Initially odorless - odor develops when skin bacteria (especially Corynebacterium spp.) break down the secretion
• pH: 6 to 7.5
• Secretion mode: mainly merocrine (exocytosis), but cells show apical blebs suggesting some apocrine mechanism (the name is now considered controversial)
• Secretion mixes with sebum in the hair follicle canal

Innervation

• Sympathetic adrenergic innervation (norepinephrine, epinephrine)
• Beta-adrenergic and purinergic receptors identified
• No muscarinic receptors detected - any cholinergic effect is thought to be humoral
• Stimulated by emotional stress, fear, pain, sexual stimulation - but NOT by heat
• Menstrual cycle influences secretion in females

Function

• Pheromone production (well established in mammals; likely vestigial in humans)
• Male pheromones (androstenol, androstenone) influence the female menstrual cycle
• Female pheromones (copulins) influence male perception and hormonal status
• Axillary hair increases odor by providing a surface for bacterial colonization

3. Apoeccrine Sweat Glands

• Develop from eccrine-like precursors in the axilla during puberty
• Larger than eccrine but smaller than apocrine glands
• Duct opens directly to the skin surface (like eccrine), not into hair follicles
• Respond to both cholinergic and adrenergic stimuli
• Can produce large volumes of secretion
• Clinically significant in axillary hyperhidrosis - apoeccrine glands may account for a major portion of axillary sweat volume

Comparison Table

• Fitzpatrick's Dermatology, Vol. 1-2, pp. 109-114 (Ch. 6: Sweat Glands)
• Junqueira's Basic Histology, 17e, pp. 936-939
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 1362-1373