Ultrastructural differentiation of peripheral nerves and neuromuscular junctions in the hindlimb of the bovine fetus.

The differentiation of sciatic nerve, intramuscular nerves, and neuromuscular junctions was studied by light and electron microscopy in bovine fetuses from 47 days' gestation to neonatal calves 3 days of age. Rudimentary neuromuscular junctions were observed in 2-month-old fetuses, at which time spontaneous movements were observed in the limbs and trunk. By 5 months, the terminal axons occupied a well-developed synaptic gutter with early secondary infoldings and, subsequently, extensive infoldings developed so that at birth the subneural apparatus was well developed. In the peripheral nerve of 47-day-old fetuses, the unmyelinated axons were in bundles, and by 3 months, single axons were separated from the bundles that were surrounded by the Schwann cell on a 1:1 basis in cross sections; myelination of axons had begun. At 5 months, individual axons in the nerves had a thick myelin sheath.