Morphological development of the rat fetal pancreas.

Rat fetal pancreata of days 12 to 19 of gestation and those of neonates were examined with light and electron microscopes. The pancreas arose from the primitive epithelium of the foregut as two primordia (dorsal and ventral pancreatic primordia) of epithelial cell nests surrounded with abundant fetal mesenchymal tissue between days 11 and 12. Primitive pancreatic islets, which mainly consisted of the endocrine cells with A cell type secretory granules, also appeared in the primordia at this age. The epithelial cell nests elongated gradually and had a branching cord-like arrangement. Two pancreatic primordia fused together on day 15, and an apparent ductular structure was observed on days 15 to 17. An acinar configuration of exocrine cells with zymogen granules gradually formed between days 17, 18 and 19. After birth, the volume of pancreatic parenchyma markedly increased and that of mesenchymal tissue prominently decreased. Ultrastructurally, the pancreatic primordia in the early stage showed immature epithelial features with the surrounding basement membrane and inconspicuous intercellular junctions. This might imply a plasticity in cellular differentiation of protodifferentiated pancreatic epithelial cells toward a ductular structure of immature exocrine pancreas which appeared later. In addition to the basement membrane, abundant fetal mesenchymal tissue was considered to play a possible role in the early morphogenesis in pancreatic development.