Role of nonrandomly oriented cell division in shaping and bending of the neural plate.

Neural plate shaping and bending are crucial to the process of neural tube formation. Shaping requires intrinsic forces generated by alterations in neuroepithelial cell behavior, whereas bending requires similar intrinsic forces and extrinsic forces generated by alterations in cell behaviors outside the neural plate. This study evaluates the role of nonrandomly oriented cell division in neural plate shaping and epidermal ectoderm expansion during bending by examining mitotic spindle orientation in the neuroepithelium and epidermal ectoderm throughout neurulation. Neuroepithelial mitotic spindles are oriented preferentially in the rostrocaudal plane, suggesting a role for nonrandomly oriented (i.e., rostrocaudal) neuroepithelial cell division in longitudinal lengthening of the neural plate during shaping. Epidermal ectoderm mitotic spindles are oriented preferentially in both rostrocaudal and mediolateral planes, suggesting a role for nonrandomly oriented cell division in epidermal ectoderm elongation and expansion. In neural plate and epidermal ectoderm isolates separated prior to shaping and bending, mitotic spindles continued to be oriented preferentially in the rostrocaudal plane; however, a preferential mediolateral mitotic spindle orientation could not be demonstrated in the epidermal ectoderm isolates. We conclude that the nonrandom rostrocaudal orientation of cell division in the neuroepithelium and epidermal ectoderm is an autonomous process, occurring in the absence of forces from adjacent tissues, whereas the nonrandom mediolateral orientation of cell division in the epidermal ectoderm is dependent upon interactions with the neural plate.