General characteristics and morphology
Sweat glands are classified as simple unbranched tubular glands. Their secretory portions are shaped like long tubules tightly coiled into glomeruli and are located deep within the reticular dermis, at the border with the subcutaneous adipose tissue. These structures are present in both the "thick" skin of the palms and soles and the "thin" skin of the rest of the body.
The main functional tasks of sweating are heat dissipation to maintain temperature homeostasis and providing an additional excretory pathway. Normally, the daily volume of secreted fluid is 600–700 ml. The fluid consists of 98% water, with the remaining 2% represented by salts (predominantly NaCl) and metabolic products (lactate, urea, uric acid, ammonia, and certain protein components).
Cellular composition of the secretory portions
The wall of the secretory portion is formed by cells of two functional groups:
- Secretory cells (glandular epithelium). These are tall cells with round 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 products).
- Myoepithelial cells. These are flattened cells located at the periphery, embracing the secretory cells from the outside. Due to their pronounced contractile activity, they facilitate the expulsion of the formed secretion from the lumen of the gland into the excretory duct.
Structure of the excretory ducts
Excretory ducts are long tubules characterized by a spiral course. Because of their tortuous path, histological sections rarely capture them in a single plane entirely, making them appear as separate fragments ("curved worms").
The duct wall is lined by a stratified cuboidal epithelium. Unlike the secretory portions, the ducts have a noticeably smaller diameter and strong basophils (staining significantly darker in preparations).
During the excretion process, the long unbranched duct passes through all layers of the skin: from the reticular dermis through the papillary layer and epidermis, opening onto the surface via a sweat pore. An important feature of the excretory duct cells is their regenerative potential—in case of skin damage, they can serve as a source for epidermal restoration.
Classification of sweat glands
Depending on the mode of secretion and morphological features, glands are divided into two types:
| Sign | Merocrine (eccrine) | Apocrine |
|---|---|---|
| Localization | Almost the entire skin surface | Skin of the forehead, axillae, anogenital region |
| Mechanism of secretion | Exocytosis without cell damage | Detachment of the apical part of the secretory cell |
| Size of the glomerulus | Small (30–35 µm) | Large (150–200 µm) |
| Where they open | Via a sweat pore onto the skin surface | Into hair funnels |
| Composition of secretion | Low organic content (~2%) | High organic content (up to 5%), cytoplasmic proteins |
| Embryogenesis | From epidermal buds growing into the mesenchyme | From hair follicle primordia |
Specialized types of apocrine glands are distinguished separately, for example, ceruminous glands of the external auditory canal, which produce earwax.
To remember the physiology of regulation: "Sweat flows from stress, but acetylcholine is the effector"—this is a rare exception for the sympathetic nervous system.
Class and exam questions
How to distinguish the excretory duct from the secretory portion of a sweat gland on a histological slide?
The ducts have a smaller diameter, are formed by a stratified cuboidal epithelium, and exhibit pronounced basophilia (staining darker). The secretory portions are larger, lighter, and lined by a single layer of tall secretory cells surrounded by myoepitheliocytes.
Why does sweat in the axillae have a specific pungent odor?
Apocrine glands are located in these zones. Their secretion involves the detachment of a portion of the cell and is rich in proteins. The odor itself arises secondarily as a result of the decomposition of organic secretion components by bacteria on the skin surface.
By what feature on a skin histological slide can it be reliably determined that this is "thin" skin?
The main diagnostic marker of "thin" skin is the presence of sebaceous glands and hair follicles in the field of view. In "thick" skin (on the palms and soles), they are completely absent.
What else to study on this topic
- Mechanisms of sympathetic neurohumoral regulation of sweating (the role of acetylcholine)
- Histology and ultrastructure of clear and dark secretory cells
- Embryogenesis of skin appendages: development from epidermal buds and follicle primordia
- Histological structure and holocrine type of secretion of sebaceous glands
- Formation of the acid-base balance (pH) of the skin under the action of microbial lipases
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