Development of peripheral hindlimb and central spinal cord innervation by subpopulations of dorsal root ganglion cells in the embryonic rat.

The development of rat peripheral hindlimb and lumbar dorsal horn innervation by different subpopulations of dorsal root ganglion cells was investigated from embryonic day (E)13 to birth by using immunostaining. Antibodies to protein gene product (PGP) 9.5, growth associated protein (GAP) 43, and peripherin were used as pan-neuronal markers, RT97 for A fibres; calcitonin gene-related peptide (CGRP), trkA, and the lectin IB4 for small A and C fibres. Size frequency analysis showed that RT97 is a selective marker for large A cells at E18. Although both A and C fibres enter the hindlimb at E13-14, A fibres are the first to innervate the skin and dominate over small fibre innervation until later fetal life. CGRP expression in sensory axons appears at E19 and in all regions simultaneously, suggesting expression in existing fibres. All sensory terminals grow transiently to the skin surface before retracting subepidermally at late embryonic stages. The development of peripheral and central innervation by the same subpopulations of sensory neurons was compared. The entry of A fibre terminals into the lumbar dorsal horn at E14 coincided with hindlimb skin innervation. In contrast, C fibres were not detected in the dorsal horn until E18, 4 days after peripheral innervation. CGRP expression appears in both spinal cord and hindlimb targets at E19. IB4 binding in the central terminals began at E18 but was never observed in embryonic peripheral axons. These results demonstrate that in fetal skin, A fibre innervation dominates over C fibres. In addition, alathough peripheral and central innervation by A fibres coincide, this is not true for C fibres, suggesting that central target factors may control C fibre terminal growth within the dorsal horn.