Onset of cell-specific gene expression in the developing mouse pancreas.

A central question in developmental biology has been the initiation of cell-specific gene expression and its temporal relationship to morphogenesis. We have coupled embryo microdissection with the exquisite sensitivity of the polymerase chain reaction to define the onset of cell-specific gene expression during pancreatic organogenesis. Using the precise assignment of gestational age by the number of somites in each embryo, we determined the onset of transcription of major genes of the endocrine and exocrine pancreas during mouse development to within 2-3 hr. Somatostatin mRNA was detected at the 10-somite stage throughout the foregut, consistent with the presence of somatostatin-producing cells throughout the adult gut. Mature mRNA for insulin and glucagon first appears surprisingly early, at the 20-somite stage in the wall of the embryonic foregut and is restricted to only the area of the duodenum from which the pancreas will arise 10-12 hr later. In contrast, exocrine gene transcription begins 24 hr after formation of the pancreatic diverticulum. Thus cell-specific gene expression in the endocrine pancreas begins in a "pre-morphogenetic phase." This early expression of insulin and glucagon could reflect the initiation of an endocrine cell lineage.