Maturation and recycling of trigeminal motoneurons in anuran larvae

Development of the trigeminal motor system was analyzed in Rana pipiens larvae and adults. The aim of this investigation was to determine the postmetamorphic fate of the primary motoneurons that innervate the larval jaw muscles. Specifically, we wanted to ascertain whether these neurons were deleted in conjunction with their muscular targets during metamorphosis or reused to innervate the adult jaw muscles. Cell counts and horseradish peroxidase tracer were used to distinguish between these two possibilities. The number of trigeminal motoneurons was relatively constant in premetamorphic and prometamorphic larvae. A small reduction in the cellular complement of the motor nucleus occurred during metamorphic climax, but the majority (approximately equal to 90%) of the primary motoneurons were retained from the larval to the adult nervous system. The cell loss may represent motoneurons that innervated specific larval muscles that have no adult successors and thus the entire myoneural unit degenerates. Retrograde tracers indicated that all trigeminal motoneurons extended axons into the jaw muscles of both premetamorphic larvae and adult frogs. These observations provide further support for the recycling of the trigeminal motoneurons.