Mode of neuronal migration of the pontine stream in fetal mice.

The migration of immature neurons in the pontomedullary subpial region was examined in fetal mice by light and electron microscopy. Immature pontine cells were observed forming a cell strand from the ventral aspect of the fourth ventricle to the pontine flexure during the period between the 14th and 17th day of gestation. These cells were elongated and oriented parallel to the direction of migration, and displayed features of immature neurons: they contained a high concentration of ribosomal rosettes and a few cisternae of rough endoplasmic reticula, as well as Golgi apparatus, mitochondria, microtubules and centrioles. Many of the neurons extended leading processes, and these contained longitudinally-arrayed microtubules. Filopodia extending from the processes were found beneath the pia mater. Relocating cells displayed contact relationships between themselves; in the caudal part of the stream, translocating neurons were apposed to each other and fibers of various diameters, and in the rostral area of the stream, many fibers were noted, and corresponded to leading processes of relocating neurons, to which other cell bodies had close contact. From the arrangement of the immature neurons and their processes, it can be inferred that developing fibers act as guidance substrates for the translocation of embryonic pontine neurons.