Spinal motor axons and neural crest cells use different molecular guides for segmental migration through the rostral half-somite

The peripheral nervous system in vertebrates is composed of repeating metam eric units of spinal nerves. During development, factors differentially exp ressed in a rostrocaudal pattern in the somites confine the movement of spi nal motor axons and neural crest cells to the rostral half of the semitic s clerotome. The expression patterns of transmembrane ephrin-B ligands and in teracting EphB receptors suggest that these proteins are likely candidates for coordinating the segmentation of spinal motor axone and neural crest ce lls. In vitro, ephrin-B1 has indeed been shown to repel axons extending fro m the rodent neural tube (Wang & Anderson, 1997), In avians, blocking inter actions between EphB3 expressed by neural crest cells and ephrin-B1 localiz ed to the caudal half of the somite in vivo resulted in loss of the rostroc audal patterning of trunk neural crest migration (Krull et al., 1997). The role of ephrin-B1 in patterning spinal motor axon outgrowth in avian embryo s was investigated. Ephrin-B1 protein was found to be expressed in the caud al half-sclerotome and in the dermomyotome at the appropriate time to inter act with the EphB2 receptor expressed on spinal motor axons. Treatment of a vian embryo explants with soluble ephrin-B1, however, did not perturb the s egmental outgrowth of spinal motor axons through the rostral half-somite, I n contrast, under the same treatment conditions with soluble ephrin-B1, neu ral crest cells migrated aberrantly through both rostral and caudal somite halves. These results indicate that the interaction between ephrin-B1 and E phB2 is not required for patterning spinal motor axon segmentation. Even th ough spinal motor axons traverse the same semitic pathway as neural crest c ells, different molecular guidance mechanisms appear to influence their mov ement. (C) 2000 John Wiley & Sons, Inc.