The influence of target tissue age on neurite outgrowth from chick embryo trigeminal motor nucleus explants

In a previous study, explants from the neural tube of 2-day chick embryos, containing the trigeminal motor nucleus, showed specific enhancement of neurite outgrowth in the presence of culture medium conditioned by target musculature of an age at which innervation would normally be taking place in vivo. In the present study, similar explants were grown in the presence of target-conditioned medium from midincubation (10-day) embryos and late-incubation (18-day) embryos. At both of these later developmental stages, the target muscle-conditioned medium failed to produce enhanced neurite outgrowth. Rather, fewer processes were extended from the trigeminal explants in both experimental conditions compared to control cultures. This inhibitory effect was statistically significant in some cases, and did not appear to be related to the amount of target-conditioned medium present. It is concluded that trophic influences on nerve fiber outgrowth by target tissue are stage-specific in this system, and it is speculated that inhibitory influences such as those demonstrated may interact with excitatory influences in the normal patterning of the developing nervous system.