SEGMENTAL INDEPENDENCE AND AGE-DEPENDENCE OF NEURITE OUTGROWTH FROM EMBRYONIC CHICK SENSORY NEURONS

Targets in limb regions of the chick embryo are further removed from t he dorsal root ganglia that innervate them compared with thoracic gang lion-to-target distances. It has been inferred that axons grow into th e limb regions two to three times faster than into nonlimb regions. We tested whether the differences were due to intrinsic properties of th e neurons located at different segmental levels. Dorsal root ganglia ( DRG) were isolated from the forelimb, trunk, and hind limb regions of stage 25-30 embryos. Neurite outgrowth was measured in dissociated cel l culture and in cultures of DRG explants. Although there was consider able variability in the amount of neurite outgrowth, there were no sub stantive differences in the amount or the rate of outgrowth comparing brachial, thoracic, or lumbosacral neurons. The amount of neurite outg rowth in dissociated cell cultures increased with the stage of develop ment. Overall, our data suggest that DRG neurons express a basal amoun t of outgrowth, which is initially independent of target-derived neuro trophic influences; the magnitude of this intrinsic growth potential i ncreases with stage of development; and the neurons of the DRG are not intrinsically specified to grow neurites at rates that are matched to the distance they are required to grow to make contact with their per ipheral targets in vivo. We present a speculative model based on Poiss on statistics, which attempts to account for the variability in the am ount of neurite outgrowth from dissociated neurons. (C) 1995 John Wile y and Sons, Inc.