The innervation of the hind limb of Eleutherodactylus martinicensis: further comparison of cell and fiber numbers during development

In montages of electron micrographs of the sciatic nerve of Eleutherodactylus martinicensis, the numbers of fibers of all classes have been counted, from the 8·5 day embryo, through the early juvenile to the adult. These counts have been compared with the total numbers of cells in the lumbar ventral horn plus those in the lumbar spinal ganglia 8, 9 and 10. In the embryo, both sets of counts rise to a peak on the 13th day, and fall early in the 14th day. In the embryo, the cell count is 600–1000 more than the fiber count, while in the 6-day juvenile onwards, the fiber count is the greater. These differences are held to arise from the play of two independent factors, namely production of axons in the embryo by only a minority of cells in ganglia and ventral horns, and secondly, the extent to which axons branch between spinal roots and sciatic nerve at all stages. In the embryo, numbers of cells and fibers maintain a parallel course up and down the 13-day peak, indicating that many cells which are lost by degeneration had already sent axons into the nerve. Myelinated fibers first appear in the limb nerves at 8·5 days, when limb motility is first seen. The course of formation of the earliest myelin in the sciatic nerve resembles that of fibers in the central nervous system. In spinal roots there are both myelinated and unmyelinated fibers, the proportion of the latter in ventral roots being the greater.