Neurulation, closure initiation points, and neural crest emigration
Neurulation converts the flat neural plate into the hollow neural tube — the primordium of the brain and spinal cord. In mammals, closure initiates at multiple discrete points along the anterior-posterior axis: Closure 1 (hindbrain/cervical junction), Closure 2 (forebrain/midbrain), and Closure 3 (rostral). Between these initiation points, the neural folds elevate, converge, and fuse bidirectionally like a zipper. Failure to close causes neural tube defects: anencephaly (failure at Closure 2/3) and spina bifida (failure at the caudal neuropore). As the tube seals, neural crest cells delaminate from the dorsal neural tube, undergoing epithelial-to-mesenchymal transition and migrating throughout the embryo to form the peripheral nervous system, craniofacial structures, and melanocytes.