Physiological and morphological changes in developing peripheral nerves of rat embryos.

Physiological and morphological properties of intercostal nerves were studied in rat embryos of 13-21 days of gestation (birth is at 21-22 days). Rat intercostal nerves emerged from the spinal cord at day 13 of gestation when they were surrounded by a few Schwann cells. The nerve bundles contained growth cones, and large and small diameter unmyelinated profiles. At 13-17 days of gestation, large diameter profiles and irregularly shaped growth cones made up a significant portion of the axon population in intercostal nerves. At day 13 electrical stimulation of the nerve evoked action potentials. Thus, intercostal nerves are excitable at day 13 in utero, prior to the formation of functional nerve-muscle contacts (day 14-15). A functional nerve-muscle junction is, therefore, not a prerequisite for conduction of action potentials. From the onset of excitability Na+ was the major carrier of the action potential inward current, but there was also a small Ca2+ current that lasted until 18 days of gestation. At 17-18 days of gestation, Schwann cell proliferation and extension of Schwann cell cytoplasm into the nerve subdivided it into numerous smaller bundles. Axons located in the periphery of many bundles were wrapped by Schwann cell cytoplasm and were probably the first ones to become myelinated. During the same period extracellularly recorded action potentials consisted of multiple negative peaks, and conduction velocity increased about 2-fold. Myelination began at day 22 and was completed within 3 weeks after birth when the ratio of myelinated-to-unmyelinated axons reached its adult value.