Salivary and Sweat Glands

Salivary and sweat glands are compound and simple exocrine glands that illustrate the two-step model of secretion shared by many fluid-producing glands: a secretory end-piece (acinus or coil) makes a primary fluid, and the duct then modifies it before it reaches the surface. Salivary glands produce saliva for the mouth, while eccrine sweat glands produce a watery secretion central to thermoregulation.

Definition

Salivary glands are compound exocrine glands of the oral cavity with serous, mucous, or mixed secretory units and a branching duct system; sweat glands are skin exocrine glands, the eccrine type being simple coiled tubular glands producing a watery, thermoregulatory secretion.

Scope

The entry covers the histological organization of the major salivary glands (serous, mucous, and mixed acini, intercalated and striated ducts, myoepithelial cells) and of skin sweat glands (eccrine coiled tubular glands and apocrine sweat glands), and the two-stage secretion-then-ductal-modification mechanism by which both produce a final fluid. It is descriptive histology and physiology, not clinical guidance.

Core questions

• How are the secretory units and ducts of the major salivary glands organized?
• How does the duct modify the primary secretion in salivary and sweat glands?
• How do eccrine and apocrine sweat glands differ histologically?
• What is the role of myoepithelial cells in these glands?

Key concepts

• Serous, mucous, and mixed (seromucous) acini
• Intercalated and striated (ductal) segments
• Two-stage secretion: primary fluid then ductal modification
• Myoepithelial cells
• Eccrine (merocrine) sweat glands and thermoregulation
• Apocrine sweat glands
• Fluid and electrolyte transport in acinar cells

Mechanisms

In salivary acinar cells an isotonic, plasma-like primary fluid is produced by transepithelial movement of chloride and water driven by intracellular calcium signalling; the duct then reabsorbs sodium and chloride and secretes potassium and bicarbonate, yielding the hypotonic final saliva (Melvin et al., 2005; Proctor & Shaalan, 2021). The eccrine sweat gland follows the same logic: its coiled secretory segment makes a primary fluid that the duct desalts before it reaches the skin surface, a system that supports evaporative cooling (Cui & Schlessinger, 2015). Myoepithelial cells around the secretory units assist expulsion of the product.

Clinical relevance

Understanding the normal histology of salivary and sweat glands underlies the interpretation of glandular biopsies and the description of conditions in which secretion is reduced or altered, such as dry mouth or disorders of sweating. The entry is reference background on normal structure and function and does not provide diagnosis or treatment advice for any individual.

Evidence & guidelines

Mechanistic statements are drawn from physiology reviews of salivary acinar secretion (Melvin et al., 2005), of disease-related changes in salivary function (Proctor & Shaalan, 2021), and of eccrine sweat gland development and secretion (Cui & Schlessinger, 2015), supplemented by standard histology texts (Ross & Pawlina, 2020; Mescher, 2018).

History

The two-stage model of salivary secretion, in which an acinar primary fluid is modified by the duct, was established by twentieth-century gland physiology and refined by later molecular work on the ion transporters and calcium signalling involved (Melvin et al., 2005).

Seminal works

• melvin-2005
• cui-schlessinger-2015

Frequently asked questions

Why is saliva hypotonic if it starts as a plasma-like fluid?

The acinar cells produce an isotonic primary secretion, but as it passes along the duct the cells reabsorb more salt than water, so the final saliva that reaches the mouth is hypotonic.

What is the difference between eccrine and apocrine sweat glands?

Eccrine sweat glands are simple coiled tubular glands that open directly onto the skin and produce the watery sweat used for thermoregulation, whereas apocrine sweat glands are larger, open into hair follicles, and produce a more viscous secretion in regions such as the axilla.