Embryogenesis of the Human Pancreatic Islets: A Light and Electron Microscopic Study

The developing pancreases of twenty human embryos and fetuses, 8–23 weeks gestational age, were obtained under conditions optimum for cellular preservation and studied by light and electron microscopy. Primitive pancreatic tubules were the only epithelial structures present at 8 and 8.5 weeks. Alpha cells were first identified at 9 weeks, followed by delta and subsequently beta cells at 10.5 weeks. Endocrine cell progenitors, mixed endocrine cells, or mixed exocrine endocrine cells were not identified. Although potential sampling errors preclude a definitive recitation of the order of islet cell appearance, the alpha and delta cells are numerically the predominant pancreatic endocrine cells during early human embryogenesis. The observation of frequent, well-preserved delta cells and the identification of one undergoing mitotic division support their legitimacy as an independent cell type. Cytoplasmic glycogen was present in primitive tubular cells and differentiated exocrine cells but was excluded from endocrine cells. Other endocrine cells tentatively identified in the developing human pancreas are serotonin, gastrin, epinephrine and norepinephrine cells.