Regulations in the induction of the organized neural system in amphibian embryos.

Some of the recent data on the induction of the neural system in amphibian embryos are reviewed, utilizing a model, according to which two basic events regulate in this system: (1) ectodermal dorsalization, which occurs all over the induced region of the ectoderm and is responsible for the neural and mesectodermal pathways and (2) caudalization, which occurs only on the posterior level of dorsalized ectoderm and is responsible for the posterior mode of induced differentiation, functioning as a gradient with the apex at the posterior end of the embryo. Dorsalization of ectoderm can be caused by treatment with Con A or TPA, both of which are potential mitogens. Not only after the treatment with TPA, but also during normal dorsalization, the activation of protein kinase C occurs in responding cells. The possibility is suggested that an early step of mitogenic transmembrane signal transduction induced by a growth factor regulates dorsalization in intact embryos. Ectodermal dorsalization is responsible for the appearance of neuronal and glial cell lineages, and independent of the ECM network formed on the internal surface of the responding ectoderm during gastrulation. In caudalization, a series of experiments suggests that the regulatory role is played by the transcript of the mesodermal posterior homeobox gene, Xhox 3. The expression of this gene in time and location closely coincides with the pattern of convergent extension, one type of morphogenetic movement, which is expressed in a posterior-anterior gradient. This directed cell motility is responsible for the formation of the body axis of vertebrates, and was shown to be involved in caudalization by earlier induction experiments in urodele embryos.(ABSTRACT TRUNCATED AT 250 WORDS)