PRENATAL DEVELOPMENT OF RAT PRIMARY AFFERENT-FIBERS .2. CENTRAL PROJECTIONS

These studies were designed to determine the pattern of initial affere nt fiber ingrowth into the prenatal spinal gray matter and the establi shment of the topographic organization of the presynaptic neuropil in the dorsal horn. A total of 113 lumbar dorsal root ganglia were labele d with carbocyanine fluorescent dye DiI or DiA in 67 rat embryos and n eonatal pups aged embryonic day 13 to postnatal day 0 (E13-PO). The in itial fiber penetration of the lumbar spinal gray began at E15 and was restricted to the segments of entry. Subsequent growth of fibers into gray matter of adjacent segments began approximately one day later, a nd this delay was continued, about one day for each successive segment . A second wave of ingrowth of putative small-diameter afferents into the substantia gelatinosa began at E19 and also displayed the same ros trocaudal delay. Fiber ingrowth was specific and occupied the somatoto pic area appropriate for the adult, from the earliest stages (E18) in which dorsal horn laminae could be adequately defined. The somatotopic organization of the presynaptic neuropil in laminae III and IV did no t change significantly throughout embryonic development as the amount of overlap between adjacent and non-adjacent ganglion projections rema ined constant throughout embryonic development. In addition, it was fo und that fibers innervating the proximal and distal hindlimb entered t he spinal gray simultaneously at E15 before the innervation of the dis tal toes was established. The results of these studies indicate that t he somatotopic organization of the presynaptic neuropil is established very early in development and requires little refinement to match tha t seen in the adult. The simultaneous penetration of the fibers origin ating from the proximal and distal areas of the limb before innervatio n is complete suggests that this ingrowth may be independent of the es tablishment of specific peripheral connections. (C) 1995 Wiley-Liss, I nc.