Origin and development of striated muscles in the tail of axolotl embryos
Final Report Abstract
In the Mexican axolotl (Ambystoma mexicanum) classical labelling and grafting studies have shown that the posterior neural plate of the neurula is not neural but mesodermal. It gives rise to somites of the tail and posterior trunk but also to neuronal tissue. To reinvestigate this issue with recent methods we implanted GFP positive posterior plates from transgenic donors into white hosts and found that they gave rise to neural tissue (spinal cord) in addition to presomitic mesoderm (psm). Reason for the presence of either domain is that future mesodermal or neural tissue cannot be recognized externally at the time of plate grafting. Using in situ hybridization on neurulae with a mesodermal (bra) and a neuronal (sox2) marker we labelled both domains and replaced classical fate maps based on vital dye staining. Detailed histology on sections and SEM on epidermis-deprived specimens showed for the first time in amphibians that in the neurula and early tailbuds the posterior plate did not continue involution through a residual blastopore. It remained on the surface, stretched posteriorly and made then an anterior turn. It formed presomitic mesoderm on the left and right side of the body that became neatlessly added to the existing psm which had involuted earlier during gastrulation. In addition to these investigations on the morphogenesis and fate of the posterior neural plate we studied whether striated tail muscle and median and tail fin mesenchyme originate only from the posterior neural plate or could also derive from the posterior neural fold (neural crest). We found that both tissues are solely derived from mesoderm and not from neural fold/crest. In the context of recent work in zebrafish, our experiments suggest that trunk neural crest cells in the last common ancestor of tetrapods and ray-finned fish lacked the ability to form ectomesenchyme and its derivatives.
