Experimental Analyses of the Shaping of the Neural Plate and Tube

Abstract

Understanding the changing morphology of an embryo presents special challenges. Analyses of neurulation in vertebrate embryos described here required observation from sectioned material and from time-lapse movies, modeling, computer simulation, and experiments. All these approaches were essential, and each approach helped guide the use of the others. Experiments have the special role of letting the embryo decide between our alternative hypotheses. In the newt embryo, induction and patterning events establish in the ectoderm boundaries between epidermis and neural plate, and between neural plate and the notoplate at its midline. Thedifferent domains of cells thus established—epidermis, neural plate and notoplate—develop different adhesive properties such that cell motility behavior along the notoplate boundary and along the spinal cord/epidermis boundary produces forceful intercalation of cells which lengthens the boundaries and distorts (lengthens) the neuroepithelium. Neural plate cells also attempt to crawl beneath the epidermis along their common boundary, raising neural folds and producing a rolling moment directed mediad that is largely responsible for neural tube formation. Both cell motility that leads to columnarization of neural plate cells and contraction of organized subapical microfilament bundles reduce the apical surface area of the neural plate cells and produce an apical tension that aids neural tube formation. Cell relocation reduces the width of the neural plate and increases its length, and the Poisson buckling forces resulting from this elongation of the plate also aid neural tube formation. The newt embryo accomplishes neurulation without growth, but bird and mammal embryos grow during neurulation. Understanding the organization of the products of growth in the amniote neural plate is critical in determining whether growth helps or hinders neurulation.