QUANTITATIVE-ANALYSES OF CELL BEHAVIORS UNDERLYING NOTOCHORD FORMATION AND EXTENSION IN MOUSE EMBRYOS

Background: Formation and extension of the notochord (i.e., notogenesi s) is one of the earliest and most obvious events of axis development in vertebrate embryos. In birds and mammals, prospective notochord cel ls arise from Hensen's node and come to lie beneath the midline of the neural plate. Throughout the period of neurulation, the notochord ret ains its close spatial relationship with the developing neural tube an d undergoes rapid extension in concert with the overlying neuroepithel ium. Methods: In the present study, we examined notochord development quantitatively in mouse embryos. C57BL/6 mouse embryos were collected at 8, 8.5, 9, 9.5, and 10 days of gestation. They were then embedded i n paraffin and sectioned transversely. Serial sections from 21 embryos were stained with Schiff's reagent according to the Feulgen-Rossenbec k procedure and used for quantitative analyses of notochord extension. Results: Quantitative analyses revealed that extension of the notocho rd involves cell division within the notochord proper and cell rearran gement within the notochordal plate (the immediate precursor of the no tochord). In addition, extension of the notochord involves cell accret ion, that is, the addition of cells to the notochord's caudal end, a p rocess that involves considerable cell rearrangement at the notochorda l plate-node interface. Conclusions: Extension of the mouse notochord occurs similarly to that described previously for birds (Sausedo and S choenwolf, 1993 Anat. Rec. 237:58-70). That is, in both birds (i.e., q uail and chick) and mouse embryos, notochord extension involves cell d ivision, cell rearrangement, and cell accretion. Thus higher vertebrat es utilize similar morphogenetic movements to effect notogenesis. (C) 1994 Wiley-Liss, Inc.