Neural induction and patterning by fibroblast growth factor, notochord and somite tissue in Xenopus

Two natural neural inducing sources have been used, the notochord and the somites together with the growth factor bFGF, to investigate the anterior/posterior patterning of neural tissue in an animal cap explant model in Xenopus laevis. Notochord and somite tissue from stages 12.5/13 and 16, respectively, were manually isolated, and combined heterochronically with responding animal cap ectoderm aged to gastrula stages. Somite recombinants were also constructed with animal caps injected with noggin mRNA. The responses of the ectoderm were analyzed by reverse transcription polymerase chain reaction (RT-PCR) detection of marker gene expression, and in some cases by in situ hybridization. The requirement for FGF receptor function was analyzed using the dominant negative FGF receptor (XFD). The experiments showed that bFGF is capable of direct neural induction in caps aged to stage 10.5. It was also shown that notochords are capable of inducing anterior neural tissue in gastrula stage animal cap ectoderm, and this induction is sensitive to XFD in the responding tissue. Injection of noggin mRNA results in the induction of anterior neural differentiation, and it was demonstrated that this induction was insensitive to the expression of XFD in the responding tissue. It was also shown that somite tissue recombined with gastrula stage animal cap ectoderm, can induce both anterior and posterior nervous tissue and can also posteriorize noggin-induced anterior neural tissue when combined with noggin-injected animal cap ectoderm. This response is partially sensitive to XFD expression. The results shed light on the role of competence of animal cap ectoderm and the signals from postgastrulation axial and paraxial mesoderm in the patterning of the amphibian nervous system.