Specific responsiveness of chick trigeminal motor nucleus explants to target‐conditioned media

Explants of the neural tube from stage 11 chick embryos containing the metencephalic trigeminal (V) motor nucleus were cultured in standard control medium, in medium conditioned by appropriate target musculature (the mandibular process of the first visceral arch that gives rise to jaw musculature innervated by motor V) or in medium conditioned by inappropriate target musculature (rostral limb bud tissue). The appropriate and inappropriate muscle tissues were of the same developmental stage (stage 22) and were in similar states of differentiation. At this point in vivo , both are just beginning to be innervated. The neuritic outgrowth from the explants was quantified after 6 days in vitro . While explants from all three groups appeared healthy and exhibited some neuritic outgrowth, the density and complexity of this growth was significantly greater in the group cultured with the appropriate (jaw) muscle‐conditioned medium. Growth in this group significantly surpassed that of both the control and the inappropriate (limb) muscle‐conditioned medium group. The inappropriate muscle‐conditioned medium group did not differ from the control group. These results demonstrate a specific responsiveness of the trigeminal motor nucleus population to its appropriate target tissue. Since relatively small amounts of the muscle‐conditioned medium were used with each explant, it is suggested that there is a high degree of sensitivity of this population to factors present in their target at the time innervation would normally be occurring. It is hypothesized that such selective responsiveness may play a role in guiding or sustaining growth during normal neurogenesis.