Microsurgical analyses of avian neurulation: separation of medial and lateral tissues.

Neurulation, formation of the neural tube, is a complex process involving shaping and bending of the neural plate and closure of the neural groove. We have used avian embryos as model systems to study this process. In the present investigation, blastoderms were cut parasagittally through their entire thickness, either unilaterally or bilaterally, at two mediolateral locations: 1) at the juncture between prospective neural plate and prospective surface epithelium, and 2) at the juncture between the midline strip of prospective neural plate and more lateral prospective neural plate. In the first experiment, shaping of the neural plate seemed normal, but elevation and convergence of the neural folds and closure of the neural groove were inhibited (except at the forebrain level). This result demonstrates that extrinsic forces generated by lateral tissues are required for neural plate bending and neural groove closure. In the second experiment, neuroepithelial cells within the isolated, midline strip became wedge shaped. This result indicates that neuroepithelial cell "wedging" is an active event occurring independently of forces generated by elevation of the neural folds. Additional studies are required to define the natures of neurulation forces and the mechanisms by which they are generated.