Figure 5.

Hypothesis for differences in intrinsic patterning between in vitro and in vivo. The observation that differentiating neural cysts can generate a floor plate in vitro in the absence of a notochord (which acts as the source for Shh‐mediated ventral‐dorsal patterning) may be due to the differences in a threshold level (RT_(i), RT_(ii)) required to initiate an intrinsic patterning event in these cells. A: In vitro, if it were possible to accurately determine the axial orientation of these cysts, the floor plate (Bi, indicated by dark blue shading) would not be positioned in the same place in different cysts, contrasting with the precise positioning of the floor plate in the embryo (Bii). In the case of the embryo, the positioning of the somites, notochord and surrounding tissues serve to restrict the formation of the floor plate to one particular region, effectively raising the threshold for patterning (Aii, Bii).