THE TYROSINE-HYDROXYLASE GENE IS EXPRESSED IN ENDODERM AND PANCREAS OF EARLY QUAIL EMBRYOS

The initial expression of the gene encoding tyrosine hydroxylase (TH) was studied in the trunk of quail embryos by in situ hybridization. We detected the presence of quail TH mRNA on embryonic day 3.5 (E3.5) in the sympathetic ganglia and aortic plexus, both neural crest derived structures. In contrast, the TH gene was expressed much earlier in the endodermal layer of E2 embryos, i. e. from the 8-somite stage onwards . TH mRNA was found also in the pancreatic bud, an endoderm-derived st ructure. The TH protein and catecholamines were subsequently looked fo r in these structures. TH immunoreactivity was found in cells of E2 ex planted endoderm, but no catecholamine histofluorescence was observed before or after a few days in culture. TH-positive cells were also det ected in cultures of pancreatic rudiments, explanted from E3 to E6 qua il embryos. We suggest that the TH-positive cells of the endoderm are the progenitors of the catecholaminergic cells of the pancreas and of the enterochromaffin cells of the gut. The hypothesis that the TH-posi tive cells of the endoderm are involved in the expression of the catec holaminergic phenotype by neural crest cells is discussed.