ABSTRACT
In all vertebrates an organizer region is responsible for triggering the differentiation of ectoderm into neural tissue, neural induction. Early nervous system patterning along the anteroposterior axis comes about during gastrulation. The neuraxis of the early embryo comes to be divided into a series of compartments, neuromeres. Hensen's node and the notocord induces the formation, within the ectoderm above, of a neural plate. Formation of the neural tube is called neurulation. Failure of the neural tube to close causes anencephaly and spina bifida, in which the lumbosacral tube fails to close. As the neural tube closes, bone morphogenetic proteins (BMPs) in the epidermal ectoderm induce the formation of a dorsal roof plate, which then produces its own BMPs. The sonic hedgehog and BMPs act as positional signals that are believed to produce their effects on dorsoventral patterning by activating families of Pax genes encoding transcription factors that serve a role similar to Hox genes.
