Mechanisms of Neurulation

The process of neurulation results in the formation of the neural tube, the rudiment of the adult central nervous system. Neurulation occurs in two phases in vertebrate embryos, called primary and secondary neurulation. Primary neurulation, the formation of the neural plate and subsequent morphogenetic movements that transform it into a neural tube, forms the entire neural tube in amphibians and reptiles. Secondary neurulation, the formation of an epithelial cord and its subsequent cavitation to form a neural tube, forms the entire neural tube in fishes. Both primary and secondary neurulations occur in birds and mammals. The brain and trunk level of the spinal cord form by primary neurulation, whereas the tail spinal cord forms by secondary neurulation. In humans, primary neurulation is the most significant phase of neurulation from a clinical perspective because this phase, when it occurs abnormally, results in open neural tube defectsserious malformations of the central nervous system (see Chapters 15 and 17, in this volume). In this chapter, we will emphasize primary neurulation because of its clinical importance and because far more is known about primary neurulation that about secondary neurulation.