Peripheral development of cranial nerves in a cyclostome,Lampetra japonica: morphological distribution of nerve branches and the vertebrate body plan

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The development of peripheral nerves was studied in a Japanese marine lamprey, Lampetra japonica, in whole-mount and sectioned embryos from hatching until the earliest ammocoete. Nerve fibers were immunohistochemically stained with a monoclonal antibody against acetylated tubulin. Branchiomeric nerves first developed in a simple metamerical pattern, each associated with a single pharyngeal arch. Of those, the ophthalmicus profundus, maxillomandibular, and facial nerves later developed a highly modified branching pattern, whereas postotic nerves were less specialized and showed the stereotypical branching pattern of post-trematic nerves. The early distribution of melanocytes in myotome-free space largely overlapped with the morphology of the cranial nerve and ganglion anlage, and resembled the cephalic crest cell distribution pattern in the early chick embryo. It was suggested that the cephalic crest cell distribution, which is also inhibited by myotomes in the lamprey, would be the common basis for branchiomeric nerve patterning. In later development of the lamprey embryo, myotomes 1 through 3, which had originated in the postotic region, grew rostrally into the preotic region, laterally covering all of the branchiomeric nerves. This results in a deep position of the cranial nerves, which is not observed in gnathostomes.