Cytotactin expression in somites after dorsal neural tube and neural crest ablation in chicken embryos.

The spatiotemporal expression of the extracellular matrix protein cytotactin/tenascin during somitogenesis suggests that it plays a role in the morphogenetic events that give rise to the pattern of neural crest (NC) development. In the present study, the spatial distribution and molecular forms of cytotactin in somites were examined using in situ hybridization, Western blotting, and immunohistochemistry during normal development and after injury. In situ hybridization showed that prior to NC cell invasion cytotactin mRNA was restricted to the caudal half of the newly formed epithelial somites. As each epithelial somite matured, giving rise to a sclerotome and dermamyotome, the mRNA was first restricted to the dermamyotome and later restricted to the rostral protion of the sclerotome, consistent with the previously reported protein distribution. Immunocytochemical analysis of the distribution of cytotactin and NC cells in embryos with ablations that removed NC cells, or with simple wounds that left NC cells in place, demonstrated that the presence of NC cells is neither necessary nor sufficient for the correct positioning of cytotactin. Immunoblotting analysis showed that cytotactin synthesized by sclerotomes in the absence of NC cells was of similar molecular mass to that produced in their presence. These findings are in accord with the notion that the abnormalities of cytotactin distribution are related to the wounding process. We conclude that, contrary to the suggestion of Stern et al. [Stern, C. D., Norris, W. E., Bronner-Fraser, M., Carlson, G. J., Faissner, A., Keynes, R. J. & Schachner, M. (1989) Development 107, 309-319], there is no causal link between the presence of NC cells and the distribution and molecular mass of sclerotomal cytotactin.