The neural folds of the cranial neural plate in mammals are elevated following an earlier phase in which the cranial mesenchyme undergoes marked expansion. In the midbrain region, in particular, this phase of cranial neurulation produces biconvex neural folds that bulge outward. Next, the edges of the neural folds ‘flip around’ and approach the dorsal midline, with a biconcave morphology, until the tips of the neural folds meet in the midline.
Even when the neural folds are still not merged to generate the neural tube, an initial anterior-posterior patterning already exist, with distinct morphology and gene expression in the prospencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain) regions.
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