Prospective forebrain-midbrain from axolotl neurulae can be reprogrammed to differentiate as Mauthner cell-containing...

In premetamorphic amphibians, the Mauthner cells (M-cells), a single pair of large neurons, are present in the medulla at ear level. M-cells are easily identified morphologically. Lability of the major axes of the CNS in the axolotl (Ambystoma mexicanum) through midneural plate stages suggests that regionalization of the CNS does not occur prior to that time. Thus, prospective forebrain-midbrain from early midneurulae was unilaterally substituted for prospective hindbrain in hosts of the same stages. Light microscopic examination of feeding larvae showed that the implanted tissue developed as hindbrain and, in addition, produced an M-cell. Proof that the graft itself differentiated as medulla was obtained through implantation of [3H]-thymidine labeled tissue into unlabeled host embryos and through implantation of pigmented tissue into albinos. The competence of prospective forebrain-midbrain taken from early midneurulae to produce medullae as well as M-cells indicates that specification of spatial pattern in the developing CNS has not yet occurred and that very precise regulating factors from the surrounding host tissues can override the original fate of the graft to bring it into accord with the system as a whole. The data also provide the first unequivocal demonstration of the generation of an identified central neuron from foreign tissue in a vertebrate.