Developmental Morphology of Branchiomeric Nerves in a Cat Shark, Scyliorhinus torazame, with Special Reference to Rhombomeres, Cephalic Mesoderm, and Distribution Patterns of Cephalic Crest Cells

Peripheral nerve development was studied in the cat shark, Scyliorhinus torazame, using whole-mount and sectioned embryos. Nerve fibers were immunohistochemically stained using a monoclonal antibody against acetylated tubulin and, in early embryos, cephalic crest cells were observed by scanning electron microscopy. The initial distribution patterns of crest cells were identical to the typical vertebrate embryonic pattern, in that three crest cell populations were associated with even-numbered rhombomeres, prefiguring the pattern of the branchiomeric nerve roots. In older pharyngula, however, the trigeminal and postotic branchiomeric nerve roots were found to have shifted caudally along the neuraxis: the trigeminal nerve root finally arose from r3, while the glossopharyngeal nerve root arose from the presumptive region r7 of the hindbrain. The shift apparently takes place between the root fibers and the dorsolateral fasciculus. From observing the topographical relationships between the peripheral nerves and other epithelial structures (for example, the otocyst and the mesodermal head cavities - the anlagen of extrinsic ocular muscles), it was assumed that the shift was the result of an epigenetic effect caused by allometric growth of the otocyst, the mandibular cavity, and the spiracle epithelia anchoring the mandibular branch of the trigeminal nerve. It was concluded that the deviated morphological pattern of elasmobranch cranial nerves is a secondary phenomenon caused by the well-developed head cavities. In those animals whose head cavities are degenerated, the original pattern of the cranial nerve-rhombomere assignment, which is intact in lamprey embryos, is retained.