Birth dates of trigeminal motoneurons and metamorphic reorganization of the jaw myoneural system in frogs.

Drastic alterations in oral behavior characterize metamorphosis of anuran amphibians. Changes cascade through all components of the jaw apparatus from bone to muscle to nerve. In this investigation, tritiated thymidine autoradiography was used to determine the production schedule of the trigeminal motoneurons in the leopard frog, Rana pipiens. The time of origin of these neurons and their subsequent fate are of special interest given the breakdown of the larval jaw muscles and the de novo generation of adult muscle fibers during metamorphosis. Specifically, we wanted to learn whether trigeminal motoneurons are added, deleted, or reused during metamorphic climax. The entire complement of trigeminal motoneurons was produced over a 4-day span commencing at embryonic stage 13 and terminating at stage 20. Newly formed neurons are added to the primordial trigeminal nucleus in an orderly pattern. Firstborn neurons settle in the ventrorostral region of the nucleus; cells with progressively later birth dates were added in a posterodorsal direction. No additional trigeminal motoneurons are generated during larval maturation or at metamorphosis, thus indicating that the same population of neurons is present throughout the lifespan of the animal. From these observations we suggest that, during metamorphosis, the trigeminal motoneurons that supply the larval muscles switch their allegiance to the newly formed adult jaw muscles. This change of peripheral targets can be viewed as a respecification of the trigeminal motoneurons.