PRENATAL DEVELOPMENT OF RAT PRIMARY AFFERENT-FIBERS .1. PERIPHERAL PROJECTIONS

Development of the peripheral innervation patterns of the L1-S1 lumbos acral ganglia and motor segments in embryonic day 12-17 (E12-17) rat e mbryos was examined using carbocyanine dyes. Individual dorsal root ga nglia (DRGs) and/or isolated ventral horn (VH) segments, or individual peripheral nerves, were isolated in rat embryos fixed at different st ages and filled with one of three carbocyanine dyes; DiI, DiA, and DiO . Individual experimental preparations included labeling of 1) single DRGs; 2) multiple DRGs with alternating dyes, DiO, DiI, and DiA; 3) si ngle isolated VH segments; 4) multiple VH segments with alternating dy es; 5) single VH segments and the corresponding segmental DRGs with di fferent dyes; and 6) two or more individual peripheral nerves labeled with different dyes. Results from these preparations have shown that t he first fibers exited the lumbar ventral horn and DRGs at E12. At E13 major nerve trunks (e.g., femoral and sciatic) were visible as they e xited the plexus region. By E14 afferent fibers were present in the ep idermis of the proximal hindlimb, and the major nerve trunks extended into the leg. Fibers originating from L3 to L5 (DRG and VH) reached th e paw by E14.5-E15, and the epidermis of the most distal toes was inne rvated by E16-E16.5. While afferent fibers and motor axons of the same segmental origin mixed extensively in the spinal nerve, fibers of dif ferent segmental origin combined in the plexus and major nerve trunks with little or no interfascicular mixing. Dermatomes observed at E14 w ere in general spotty and non-contiguous. However, by E16 the dermatom es resembled mature forms with substantial overlap only between adjace nt ones. Thus the adult pattern of spatial relationships between cutan eous afferent fibers in the periphery is established early in developm ent. (C) 1995 Wiley-Liss, Inc.