Pathfinding and growth termination of primary trigeminal sensory afferents in the embryonic rat hindbrain

Axons of the trigeminal ganglion convey sensory information from mechanoreceptors, thermoreceptors, and nociceptors in the face and nasal mucosa, then terminate on several groups of neurons including the principal sensory nucleus and the nuclei of the spinal trigeminal tract. To understand guidance mechanisms during the development of trigeminal sensory axons (TA) in the embryonic brain, we first investigated the growth pattern of TA in relation to organization in the hindbrain using flat whole-mount preparation from rat. We found that the primary TA from the trigeminal ganglion entered the brainstem and grew longitudinally within the hindbrain. Whereas descending axons ran just medial to the primary vestibular axons to innervate the spinal nucleus, ascending axons stayed near the entry point. In flat whole-mount culture, the TA extended both ascending and descending branches as they do in vivo. Rostral hindbrain was found to be a less permissive substrate for the TA compared to caudal hindbrain. In addition, the nonpermissive property of the ventral hindbrain substrate restricted the invasion of TA along the entire length of the hindbrain. Thus, cooperation of absolute and relative permissiveness of the substrate plays important roles in the guidance of TA to their targets.