Nervous system morphogenesis in vertebrates

Molecular approaches used in the studies of embryonic neural development in the vertebrate embryos have started to unravel pathways involved in neural induction and patterning of the central nervous system. Neural induction begins with inhibition of BMP/GDF type of signaling on the dorsal ectoderm. Factors such as noggin, chordin, follistatin and cerebrus derived from the organizer mediate this inhibition and thus unveil neural fate in the dorsal side. Downstream of this inhibition, the activity of transcription factors such as SoxD and nuerogenin have been implicated with the differentiation of mature neurons. Concomitant and following morphogenetic movements of the neural plate, differentiation of subtypes of neurons begins along the three axes of the neural tube. Antero-posterior differentiation is mediated by secreted factors such as FGF and Wnts as well as retinoic acid. The Hox code also subdivides the neural tube in different A-P gene expression domains and has been suggested to mediate cell identity. Ventral differentiation seems to be mediated by signaling cascades initiated by SHH original from the notochord and later from the floor plate. HNF3 beta, a transcription factor also seems to be involved in ventral differentiation. On the dorsal side both BMPs and Wnts signaling cascades have been implicated in the establishment of dorsal neural fates. Transcription factors such as Pax3 and 7 are also involved in this differentiation which ultimately will produce the roof plate, sensory neurons and dorsal interneurons. Right left assymetry seems to be mediated by lefty (also a member of the TGF beta family), and the transcription factor ptx2. All these influences will ultimately allow the establishment of different fates in the embryonic nervous system.