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Light and electron microscopic observation was carried out on the duct of eccrine sweat gland surgically obtained from human axilla.1. Between the secretory and coiled portions of the duct, a short transitional portion (ITO and ENJO, 1949) was confirmed to exist.2. The coiled portion comprises double-layered cuboidal cells-surface and basal cells. The former are provided with cuticular border which contains massed tonofilaments presumably serving as an architectural support. The surface of the cuticular border is protruded into the lumen as numerous microvilli which show pictures suggesting microapocrine secretion. Over the whole layer of cuticular border, there occur numerous vesicles containing droplets and particles identical with those found in the duct lumen. This finding implies pinocytotic activity in the cuticular border and favours the hypothesis of reabsorption of certain constituents of sweat, e. g. sodium and chloride, in the duct epithelium.3. Both the surface and basal cells of the coiled portion are characterized by richness in mitochondria, tonofilament bundles, free ribosomes, smooth vesicles and multivesicular bodies, and by a small Golgi complex. Electron lucent glycogen areas are demonstrated only in the cytoplasm of the basal cells.4. The transitional portion is represented by a layer of cuboidal cells interruptedly attached by a layer of flattened basal cells. The cuboidal cells possess a few microvilli and apical cytoplasmic projections but neither cuticular border nor distinct intercellular interdigitations. The cytoplasm of the cuboidal cells is characterized by abundant perinuclear tonofilaments, a large Golgi complex composed of prominent lamellae and numerous vesicles, a large amount of free ribosomes and a small number of mitochondria containing numerous intramitochondrial granules. Elongated dense bodies of unknown nature containing microtubular structures are also recognized in the cytoplasm.5. As the most conspicuous finding of the cuboidal cells of the transitional portion, numerous vesicles occur in the peripheral, especially apical, regions of the cytoplasm which show every morphological feature characteristic of pinocytosis or reverse pinocytosis. This and other observations have lead the authors to the conclusion that the cuboidal cells of the transitional portion also, and even most vigorously, reabsorb certain constituents of the secretion of sweat gland by means of pinocytotic activity. The absorbed substance is believed to be transported, presumably under participation of Golgi complex, to the cell basis to be discharged into pericellular spaces. The flattened basal cells may play a subsidiary role for the transport of substance.6. The secretion from the secretory cells of the human eccrine sweat gland, while flowing first through the short transitional portion and then through the long coiled portion of its duct, is thus believed to be subjected to successive reabsorption by the lining epithelia.

Electron Microscopic Study on the Duct of the Human Eccrine Sweat Gland with Special Remarks on the Transitional Portion Epithelium