EPIDERMAL ECTODERM IS REQUIRED FOR FULL ELEVATION AND FOR CONVERGENCEDURING BENDING OF THE AVIAN NEURAL PLATE

Previous studies suggest that bending of the neural plate requires the juxtaposition of neural plate and non-neuroepithelial tissues. The cu rrent study examines the role of one of these tissues, the epidermal e ctoderm, in bending. Chick blastoderms were harvested from fertile egg s incubated for 24 hr and cultured dorsal-side-up on agar-albumen subs trates. In one experiment, a rectangular flap of epidermal ectoderm on one side of each blastoderm was separated from underlying layers and gently reflected onto the area opaca; a fragment of tungsten wire was placed on top of the flap to hold it down and to prevent healing. Embr yos were then allowed to develop in a humidified incubator for 2-18 hr . Asymmetric neurulation was observed between the operated and control sides as early as 2 hr after surgery. The amount of asymmetry was qua ntified in serial transverse sections from embryos collected 8 hr afte r surgery. Elevation of the lateral edge of the neural plate on the op erated side averaged one half to two thirds of that on the control sid e, and convergence of the operated side around the dorsolateral hinge point toward the dorsal midline did not occur. These results demonstra te that epidermal ectoderm is required for full elevation and for conv ergence during bending. In another experiment, lateral epidermal ectod erm was removed, leaving only a medial strip consisting of both the ep idermal component of the future neural fold and flanking future epider mis. This experiment revealed that although epidermal ectoderm is nece ssary for full elevation and for convergence of the neural folds, a me dial strip of epidermal ectoderm is sufficient to drive bending. Colle ctively, these results further support the idea that neurulation is a multifactorial process driven by both intrinsic and extrinsic factors acting in concert. (C) 1997 Wiley-Liss, Inc.