The ultrastructure of human embryo skin. II. The formation of intradermal portion of the eccrine sweat duct and of the secretory segment during the first half of embryonic life.

The formation of the intradermal eccrine sweat duct lumen began in the 15-week-old embryo with a dissolution of the desmosomal attachment plaques, followed by the separation of apposing inner cells and subsequent formation of microvilli at the luminal surfaces. The luminal cells possessed numerous microvilli and crypts and a periluminal band of tonofilaments. In the secretory portion of older embryos (22 weeks old), columnar secretory cells extended from the basement membrane to the luminal border. The presence in these secretory cells of microvilli, Golgi elements, mitochondria, small secretory vesicles, dense secretory granules and abundant endoplasmic reticulum, implies that preparation for secretory functioning begins in embryonic life. The pyramidal myoepithelial cells differentiated from the basal cells seen in younger embryo, and rested upon a basement membrane. In the oldest embryo examined (22 weeks) there was as yet neither intercellular canaliculi nor sufficient difference in the amount of glycogen to enable one to distinguish between dark and light secretory cells. Isolated cilia of 9 + 0 type were occasionally found in the luminal cells of the duct and the secretory segment.