Ultrastructural studies on the development of human fetal salivary glands.

Parotid and submandibular glands in human fetuses at 16, 18 and 24 weeks of gestation were studied by electron microscopy. The salivary glands of the 16 week fetus were composed of primary ducts and terminal buds. The primary duct was double layered and consisted of inner glycogen-rich ductal cells and outer myoepithelial precursor cells. The terminal bud was a cell cluster which consisted of glycogen-rich ductal precursor cells and myoepithelial precursor cells. In the 18 week fetus, the terminal bud developed into the terminal tubule, and the formation of a lumen was evident. The terminal tubule was composed of ductal cells containing a few secretory granules and immature myoepithelial cells at the basal portion of the tubule. The differentiation of secretory activity was clearly visible in the terminal tubule of the salivary glands in the 24 week fetus. In the parotid gland of the 24 week fetus, one kind of secretory cell was found, but at the same stage, two kinds of secretory cells were distinguishable in the submandibular gland. Some mature myoepithelial cells were distinctly observed in both glands. At this stage, also, immature intercalated ducts and immature striated ducts were present. The relationship between the histogenesis of normal structures and that of tumors is also discussed.