Differentiation of the brain stem structures in the salamander, Hynobius nebulosus

Differentiation of the internal structure of the brain stem was analyzed in the salamander with special reference to neurons distributed in the marginal layer. It was found that the salamander brain stem was at first composed exclusively of the mantle layer. The marginal layer later differentiated peripherally. In these developmental stages, the mantle and marginal layers were clearly differentiated: the former was made up exclusively of the somata, while the latter was composed mainly of nerve fibers. As the development proceeded, these organization patterns were modified: a few cells migrated into the marginal layer. Cells migrating into the marginal layer formed various nuclei and layers such as the raphe nuclei, reticular formation and superficial cellular layers of the optic tectum. In later development stages, fibers in the marginal layer were myelinated, and neurons in the marginal layer were observed to become embedded among numerous myelinated fibers. Cytologically, the majority of neurons in early developmental stages were unipolar, extending a process peripherally into the marginal layer. In later developmental stages, neurons in a deep zone of the mantle layer remained unipolar, whereas those in the marginal layer and in the superficial zone of the mantle layer differentiated into multipolar cells. Thus, (1) the marginal layer differentiated peripherally as a cell free region; (2) cells in the mantle layer later migrated into the marginal layer, changing into multipolar neurons; (3) cells in the marginal layer formed reticular formation as well as various nuclei and layers in the peripheral white matter; and (4) as development proceeded, fibers in the marginal layer became myelinated.