Stereotyped axonal bundle formation and neuromeric patterns in embryos of a cyclostome, Lampetra japonica

Early embryonic development of the nervous system of a lamprey, Lampetra japonica, was studied by using immunohistochemical techniques and by scanning electron microscopy. The earliest appearance of axons was detected at Tahara's stage 21-, when dorsolateral and ventral longitudinal fasciculi were present in the hindbrain and spinal cord regions. The branchiomeric nerve roots began to appear at stage 22; the fibers were joined to the dorsolateral fasciculus proximally and also extended distally into each pharyngeal arch. The anterior neural tube was divided into several neuromeres: the mid-hindbrain sulcus became apparent first, then the portion rostral to this sulcus was subdivided into two portions by the syn-parencephalic boundary. In the hindbrain around stage 23, rhombomeres developed transiently, of which, rhombomere 4 was the most distinctive. Putative crest cells forming the octavofacial nerve root anlage were selectively adhering to rhombomere 4, whereas no crest cells were found on rhombomere 3. The assignment of the crest-derived nerve anlage to rhombomeres is conserved between gnathostomes and L. japonica. The neuromerical scheme of the neural tube of L. japonica is also mostly in accordance with that in gnathostomes, sharing the basic developmental patterning of axon bundles at early developmental stages. The most distinct difference between these two groups is the topographical relationships between the hindbrain neuraxis and pharyngeal arches, as well as the otic placode.