Migration of immature neurons along tangentially oriented fibers in the subpial part of the fetal mouse medulla oblongata.

Migration of neuronal cells in the subpial part of the medulla oblongata was examined in the fetal mouse by light and electron microscopy. Cells were observed forming a migratory stream in the period between the thirteenth and sixteenth days of gestation, and were associated with tangentially oriented fibers. Many of these tangential fibers were present prior to the onset of the migration, and the fibers were filled with longitudinally arrayed microtubules. The cell-bodies were elongated and arranged along, and often apposed to the fibers. Some relocating neurons extended fibers, i.e. leading processes, in the direction of the migration. Generally, these cells exhibited features of immature neurons; they displayed a high concentration of ribosomal rosettes and contained mitochondria, Golgi apparatus, a few rough endoplasmic reticula, and occasionally, centrioles. Junctional complexes, coated pits and coated vesicles were frequently observed in the region of the migratory stream, and these structures are considered to be related to cell locomotion. The present findings strongly suggest that such tangential fibers, including leading processes of moving neurons, serve as guidance substrates for the relocation of immature neurons in the mouse subpial medullary region.