Tissue interactions during axial structure pattern formation in amphibia

Tissue interactions have traditionally been assigned important roles in establishing the pattern of amphibian axial structure morphogenesis. Those interactions have been postulated to generate the patterns of neural fold morphogenesis, neural tube formation, and somite development. A review of axial structure development together with a brief discussion of the classical viewpoint, is presented. A re-examination of axis formation has recently been carried out with the SEM. Embryos which displayed major defects in notochord development, ranging from diminished length to complete obliteration, were produced by irradiating fertile eggs prior to first cleavage. A comparative SEM analysis of normal and "notochord defective" embryos revealed that, contrary to previous reports, the notochord is apparently a dispensable component of the developing axial structure system. Lastly, TEM examination of the notochord defective embryos allowed some insight into the ultrastructural alterations which occur in the notochord and neural tube cells of irradiated embryos. Additional information about the structure of the notochord, and the cellular mechanics of somitogenesis emerged from those studies.