DEVELOPMENT OF THE LONGITUDINAL PROJECTION PATTERNS OF LUMBAR PRIMARYSENSORY AFFERENTS IN THE CHICKEN-EMBRYO

The literature on the anatomical organization of primary sensory affer ents, though extensive, contains relatively little information about t he longitudinal extent of the central collateral projections. Our unde rstanding of intersegmental sensorimotor integration in the spinal cor d and of the developmental mechanisms that establish its underlying ci rcuitry could be significantly enhanced by a more complete description of these projections. To address this issue from a developmental pers pective, we labeled the central projections of lumbar primary afferent s in fixed preparations of the chicken embryo with the lipophilic trac er DiI. At late embryonic stages, the afferent projections had the fol lowing characteristics: Primary afferents originating from a single lu mbar dorsal root ganglion bifurcated to project longitudinally in the dorsal funiculus or Lissauer's tract. Dorsal funiculus axons extended up to seven segments caudally and to at least ten segments rostrally, whereas axons in Lissauer's tract extended up to seven segments in eac h direction. Collaterals branched off the longitudinal axons over a ra nge of about seven segments in each direction. Within this range, coll aterals to specific terminal fields exhibited more restricted ranges. The development of these longitudinal patterns during earlier embryoni c stages was followed from the time the afferents first reached the ne ural tube on day 4 of embryogenesis. The longitudinal axons lengthened as a single bundle up to day 10, with medial axons consistently longe r than lateral axons. After day 10, the longitudinal axons were segreg ated into the dorsal funiculus and Lissauer's tract. Collaterals sprou ted after about 2 days of longitudinal axon growth, by which time the axons had extended several segments in each direction. The segmental r ange over which collaterals were present reached a maximum of 20 segme nts at day 10. Collaterals to the different terminal areas differed in their segmental ranges already by this time. After day 10, the total segmental range of collaterals decreased to the stable level of about seven segments in each direction, which is characteristic of late-stag e embryos. (C) 1995 Wiley-Liss, Inc.