Drastic alterations in oral behavior characterize metamorphosis of anu ran amphibians. Changes cascade through all components of the jaw appa ratus from bone to muscle to nerve. In this investigation, tritiated thymi dine autoradiography was used to determine the production schedule of the trigeminal motoneurons in the leopard frog, Rana pipiens. The time of origir 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 adul muscle fibers during metamorphosis. Specifically, we wanted toalearr whether trigeminal motoneurons are added, deleted, or reused during meta morphic climax.
The entire complement of trigeminal motoneurons was produced over 4-day span commencing at embryonic stage 13 and terminating at stage 20 Newly formed neurons are added to the primordial trigeminal nucleus in ar orderly pattern. Firstborn neurons settle in the ventrorostral region of the nucleus; cells with progressively later birth dates were added in a postero dorsal direction. No additional trigeminal motoneurons are generated dur ing 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 trigemina 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.