Basic Helix-Loop-Helix Proneural Genes and Neurogenesis in Xenopus Embryos

During vertebrate development, gastrulation establishes the three germ layers, ectoderm, mesoderm and endoderm, which characterize the triploblastic species. The ectoderm forms the outer layer and gives rise to the epidermis, the central nervous system (CNS), the peripheral nervous system (PNS), the placodes (nasal, lens, otic, and lateral line), and various glandular tissues. In Xenopus, the neurectoderm appears during gastrulation primarily as a consequence of the inhibition of bone morphogenetic proteins (BMPs) which act as epidermalizing agents. Inhibition occurs via the secretion of inhibitory signals from the organizer, that bind and antagonize the activity of BMPs, and also includes transcriptional repression of BMP gene expression (Munoz-Sanjuan and Brivanlou 2002). During neural development, the next important step is to define when and where neural precursors can exit the mitotic cell cycle. These differentiating progenitor cells produce either neurons or glia, the two major building blocks of the nervous system. This process takes place simultaneously with the progressive regionalization of the neural plate to give rise to postmitotic cells with distinct identities at different positions within the neurectoderm.