Differentiation of prospective mouse pancreatic islet cells during development in vitro and during regeneration.

The pancreatic islets of mouse embryos are comprised of four different endocrine cell types and of cells containing a hormone (i.e., glucagon) and a catecholamine enzyme (tyrosine hydroxylase, TH) which appear sequentially during development in vivo. The presence of TH in glucagon cells, however, is transient, since adult pancreatic A cells do not express the enzyme. In this study we sought to determine whether the endocrine precursor cells are primed to differentiate and express catecholamine enzymes during their maturation following a predetermined sequence or whether these processes are regulated by environmental cues. To answer this question, we used immunocytochemical procedures to examine the differentiation of pancreatic rudiments removed from E11 mouse embryos and maintained in culture and of pancreases that regenerated in vitro from E11 pancreatic ducts. We found that although all the endocrine cell types differentiate in the gland in culture, the sequence of their appearance is different from that in vivo, suggesting that the timing of differentiation may be regulated by environmental factors. We also found that, in vitro, the pancreas contains TH-glucagon cells, indicating that the expression of the enzyme by pancreatic A cells is independent of factors present in vivo. Moreover, the fact that the TH-glucagon cells also differentiate during pancreatic regeneration suggests that the expression of the enzyme may be a characteristic stage of endocrine cell precursors during maturation.