Ultrastructure of human eccrine sweat glands with special reference to the transitional portion.

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Abstract
Thin sections of human eccrine sweat glands were observed with the transmission electron microscope. The fine structures of the secretory coil, transitional portion and coiled duct were compared. The secretory coil consists of dark, clear and myoepithelial cells. Dark cells contain irregularly shaped secretory granules which appear dark after glutaraldehyde-osmium double fixation. If Os is applied first to the specimen, the electron density of the secretory granules is lowered, and they often become vacuoles. These granules originate as an accumulation of secretory substance in the innermost cisterna of the Golgi stack which may correspond to the GERL [Golgi-associated endoplasmic reticulum from which lysosome is formed]. This secretory substance contains mucopolysaccharides shown by methanamine-silver stain. Exocytosis of the granules was demonstrated. Clear cells abut the intercellular canaliculi but contain no secretory granules. Well developed smooth ER [endoplasmic reticulum] is spread over the entire cytoplasm, except for the surface of the cell. The Golgi apparatus is situated near the smooth ER, and sometimes the elements of the Golgi apparauts and smooth ER are intermingled. Glycogen particles are heavily loaded in the cytoplasm of clear cells. The transitional portion, which has not been closely examined, contains epithelial cells which form a simple cuboidal epithelium and resembles the clear cells of the secretory coil. These cells contain a triangular mass of filaments attached to the zona adherens which are forerunners of the terminal web (periluminal filamentous zone) of the coiled duct. The intercellular interdigitation is weak and basal infoldings are absent in the transitional portion. This portion has myoepthelial cells, which are numerous at the proximal end but sparse in the distal half. The coiled duct consists of 2 layers of epithelial cells, i.e., luminal and peripheral cells, but the myoepithelium is absent. Numerous mitochondria are heaped in both cells reflecting active reabsorption of Na+ in the coiled duct. The transitional portion contains structures intermediate between the secretory coil and the coiled duct, and its function may be neither secretion nor absorption; the well developed myoepithelium at the initial segment of this portion may act as a sphincter of the duct system regulating the flow of sweat in the gland.