Growth of Notochord and Formation of Cranial and Mesencephalic Flexures in Chicken Embryo: (flexure formation/proliferative activity/notochord/neural tube/chicken embryo).

The proliferative activity of notochordal cells, neuroepithelial cells and endodermal cells in the chicken embryo is examined during flexure formation along its long axis by using bromodeoxyuridine (BrdU)-immunohistochemistry. White Leghorn chicken embryos, Hamburger and Hamilton stages from 9 to 15 are cumulatively labled with BrdU for 120 min and fixed with Carnoy's fluid. Serial transverse and sagittal sections are immunostained with a monoclonal anti-BrdU antibody. The distribution of labeled and unlabeled cells is determined on various regions of the embryos. Before and during the external flexure development from state 10 to 14, the labeled cells are virtually absent at two regions; The cranial region including the floor of the prosencephalon and pharynx close to the pre-chordal plate and the mesencephalic region comprising the mesencephalic notochord and mid-ventral side of the mesencephalon. At stages 10 and 11, the cranial and mesencephalic flexure become apparent at the respective unlabeled regions. It is concluded that the regional differences in proliferative activity between the notochord, neural tube and endoderm play a causal role in the flexure formation of early morphogenesis in the embryo.