STEREOTYPED AXONAL BUNDLE FORMATION AND NEUROMERIC PATTERNS IN EMBRYOS OF A CYCLOSTOME, LAMPETRA-JAPONICA

Early embryonic development of the nervous system of a lamprey, Lampet ra japonica, was studied by using immunohistochemical techniques and b y scanning electron microscopy. The earliest appearance of axons was d etected at Tahara's stage 21(-), when dorsolateral and ventral longitu dinal 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 extend ed distally into each pharyngeal arch. The anterior neural tube was di vided into several neuromeres: the mid-hindbrain sulcus became apparen t first, then the portion rostral to this sulcus was subdivided into t wo portions by the syn-parencephalic boundary. In the hindbrain around stage 23, rhombomeres developed transiently, of which, rhombomere 4 w as 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. jap onica is also mostly in accordance with that in gnathostomes, sharing the basic developmental patterning of axon bundles at early developmen tal stages. The most distinct difference between these two groups is t he topographical relationships between the hindbrain neuraxis and phar yngeal arches, as well as the otic placode. (C) 1998 Wiley-Liss, Inc.