Time course of dorsal root axon regeneration into transplants of fetal spinal cord: an electron microscopic study.

Intraspinal transplants of fetal CNS tissue permit or enhance the regeneration of cut central axons of adult dorsal root ganglion (DRG) neurons. Some of these regenerated axons establish synapses with transplant neurons. The aims of the present study were to determine when regenerated DRG axons begin to form synapses with transplanted embryonic spinal cord neurons and whether these synapses are permanent. We also examined the development of transplant neuropil in areas innervated by the regenerated axons. Whole pieces of Embryonic Day 14 spinal cord were introduced into hemisection cavities made at the level of the lumbar enlargement, and the cut L4 or L5 dorsal root was juxtaposed to the transplant. Regenerated DRG axons immunoreactive for calcitonin gene-related peptide (CGRP) were labeled by immunohistochemical methods and examined by electron microscopy from 1 week to 1 year after surgery. CGRP-immunoreactive axon terminals made synaptic contacts with dendrites and perikarya of transplant neurons by 1 week after axotomy. The morphology of the synapses was immature. Large growth cone-like structures were also present at 1 week but not at 2 weeks or later. At 2 weeks, regenerated unmyelinated axons formed terminals similar to those found in animals surviving for 48 weeks. Axoaxonic synapses in which the pre- and postsynaptic elements were immunolabeled for CGRP and regenerated CGRP-labeled myelinated axons were observed at 4 weeks and later. The area of distribution of CGRP staining increased until 12 weeks and the synaptic density of regenerated CGRP-labeled terminals increased for 24 weeks. The results indicate that the synaptic terminals of regenerated primary afferent axons are permanently retained within fetal spinal cord transplants. Transplants may therefore contribute to the permanent restoration of interrupted neural circuits.