General Characteristics and Morphology
Sweat glands are classified as simple unbranched tubular glands. Their secretory segments form long tubules tightly coiled into glomeruli (balls) and are located deep within the reticular dermis at the border with the subcutaneous adipose tissue. These structures are present in both thick skin (palms and soles) and thin skin (the rest of the body).
The primary functional objectives of sweating are heat dissipation to maintain temperature homeostasis and an accessory pathway for excretion. Under normal conditions, the daily volume of secreted fluid is 600–700 ml. The fluid consists of 98% water, with the remaining 2% comprising electrolytes (primarily NaCl) and metabolic waste products (lactate, urea, uric acid, ammonia, and certain protein components).
Cellular Composition of Secretory Segments
The wall of the secretory segment is formed by two functional groups of cells:
- Secretory cells (glandular epithelium). These are tall cells with rounded nuclei forming a single layer. They differentiate into two types:
- Clear cells: specialized in the active transport of water and ions.
- Dark cells: synthesize and secrete organic macromolecules (metabolic waste products).
- Myoepithelial cells. These have a flattened shape and are located peripherally, enclosing the secretory cells from the outside. Through their pronounced contractile activity, they assist in expelling the formed secretion from the glandular lumen into the excretory duct.
Structure of Excretory Ducts
Excretory ducts are long tubules characterized by a spiral course. Due to their tortuous path, histological sections rarely capture a duct in its entirety, making them appear as isolated cross-sectional fragments («coiled worms»).
Dust walls are lined by a stratified cuboidal epithelium. Unlike the secretory segments, the ducts have a noticeably smaller diameter and strong basophilia (staining significantly darker on histological preparations).
During the excretion of sweat, the long, unbranched duct traverses all layers of the skin: from the reticular dermis through the papillary dermis and epidermis, opening onto the surface via a sweat pore. An important feature of the excretory duct cells is their regenerative potential—following skin injury, they can serve as a source for epidermal repair.
Classification of Sweat Glands
Depending on the mechanism of secretion and morphological features, sweat glands are divided into two types:
| Feature | Merocrine (Eccrine) | Apocrine |
|---|---|---|
| Localization | Almost all skin surfaces | Forehead, axillae, anogenital region |
| Mechanism of Secretion | Exocytosis without cellular damage | Pinching off of the apical portion of the secretory cell |
| Glomerulus Size | Small (30–35 µm) | Large (150–200 µm) |
| Duct Opening | Sweat pore on the skin surface | Hair follicles (infundibulum) |
| Secretion Composition | Low organic content (~2%) | High organic content (up to 5%), cytoplasmic proteins |
| Embryogenesis | From epidermal buds growing down into mesenchyme | From hair follicle germs |
Specialized types of apocrine glands include ceruminous glands of the external auditory meatus, which produce earwax (cerumen).