Delayed formation of the floor plate after ablation of the avian notochord.

We have examined the long-term effects of notochord ablation at chick stages 9-10 on formation of the floor plate and motor neurons. Although missing or reduced 2 days postablation, the floor plate and motor neurons were morphologically normal by 4 postoperative days. When isolated whole or ventral, but not lateral, neural plate fragments from stage 9 embryos were cultured for 4 days in collagen gels, floor plate and neural markers were observed. Our results suggest that floor plate and motor neurons can form in a delayed fashion in vivo after notochord ablation and in vitro from isolated neural plates. This suggests that either there is an early induction of floor plate by the chordamesoderm of Hensen's node, or only limited interactions between the neural plate and notochord immediately after neurulation are required for floor plate determination.