PERIPHERAL TARGET-SPECIFIC INFLUENCES ON EMBRYONIC NEURITE GROWTH VIGOR AND PATTERNS

We examined axon-target interactions in cocultures of embryonic rat tr igeminal, dorsal root, nodose, superior cervical ganglia or retina wit h a variety of native or foreign peripheral targets such as the whiske r pad, forepaw and heart explants. Axon growth into these peripheral t arget tissues was analyzed by the use of lipophilic tracer DiI. Embryo nic day 15 dorsal root and trigeminal axons grew into isochronic norma l and foreign cutaneous targets. Both axon populations avoided the sam e age heart tissue, but grew profusely into younger (embryonic day 13) or older (postnatal) heart explants. In contrast, embryonic day 15 su perior cervical or nodose ganglion axons grew heavily into the same ag e heart and forepaw explants and to a lesser extent into the whisker p ad explants. Embryonic day 15 retinal axons grew into all three periph eral targets used in this study. Primary sensory and sympathetic axons , but not retinal axons, formed target-specific patterns in the whiske r pad and forepaw explants. DiI-labeling and immunostaining of primary sensory neurons in coculture revealed that these neurons retain their bipolar characteristics, and express class-specific markers such as p arvalbumin, calcitonin gene-related peptide and TrkA receptors. In the whisker pad explants, axons positive for all three markers were seen to form patterns around the follicles. Our results indicate that devel oping peripheral targets can attract and support axon growth from a va riety of sources. Whereas neurotrophins play a major role in attractin g and supporting survival of subpopulations of sensory neurons, other substrate-bound or locally released molecules must regulate sensory ne urite growth into specific peripheral and central targets. (C) 1998 Wi ley-Liss, Inc.