TISSUES EXHIBITING INHIBITORY AND REPULSIVE ACTIVITIES DURING THE INITIAL-STAGES OF NEURITE OUTGROWTH FROM THE DORSAL-ROOT GANGLION IN THE CHICK-EMBRYO

To elucidate the mechanisms underlying the projection of dorsal root g anglion (DRG) axons into the dorsal root entry zone in the dorsolatera l region of the spinal cord, we examined tissue interactions which aff ect neurite outgrowth from DRG, We cultured explants or dissociated ce lls of DRG from embryonic day 4 (E4) chick embryos in combination with E3 spinal cord, notochord, and dermomyotome in three-dimensional coll agen gels. The ventral spinal cord, notochord, and dermomyotome, which are located close to the initial projection pathway of DRG but do not receive direct innervation, strongly inhibited DRG neurite outgrowth and repelled DRG neurites. These inhibitory/repulsive cues appear diff usible in nature, because this activity was observed in the absence of direct contacts between tissue explants and DRG neurites. Furthermore , in heterochronic cultures, E9 DRG lost its responsiveness to inhibit ory/repulsive factors from E3 ventral spinal cord, while retaining res ponsiveness to E3 notochord and dermomyotome, suggesting that the E3 v entral spinal cord may secrete a different inhibitory/repulsive signal than notochord and dermomyotome. Putative inhibitory/repulsive signal s secreted from tissues along the axonal pathway may serve to guide gr owing DRG axons to the dorsal root entry zone. (C) 1998 Academic Press .