MESODERMAL DEFECTS AND CRANIAL NEURAL CREST APOPTOSIS IN ALPHA-5 INTEGRIN-NULL EMBRYOS

alpha 5 beta 1 integrin is a cell surface receptor that mediates cell- extracellular matrix adhesions by interacting with fibronectin. alpha 5 subunit-deficient mice die early in gestation and display mesodermal defects; most notably, embryos have a truncated posterior and fail to produce posterior somites, In this study, we report on the in vivo ef fects of the alpha 5-null mutation on cell proliferation and survival, and on mesodermal development, We found no significant differences in the numbers of apoptotic cells or in cell proliferation in the mesode rm of alpha 5-null embryos compared to wild-type controls, These resul ts suggest that changes in overall cell death or cell proliferation ra tes are unlikely to be responsible for the mesodermal deficits seen in the alpha 5-null embryos, No increases in cell death were seen in alp ha 5-null embryonic yolk sac, amnion and allantois compared with wild- type, indicating that the mutant phenotype is not due to changes in ap optosis rates in these extraembryonic tissues, Increased numbers of dy ing cells were, however, seen in migrating cranial neural crest cells of the hyoid arch and in endodermal cells surrounding the omphalomesen teric artery in alpha 5-null embryos, indicating that these subpopulat ions of cells are dependent on alpha 5 integrin function for their sur vival, Mesodermal markers mox-1, Notch-1, Brachyury (T) and Sonic hedg ehog (Shh) were expressed in the mutant embryos in a regionally approp riate fashion, Both T and Shh, however, showed discontinuous expressio n in the notochords of alpha 5-null embryos due to (1) degeneration of the notochordal tissue structure, and (2) non-maintenance of gene exp ression, Consistent with the disorganization of notochordal signals in the alpha 5-null embryos, reduced Pax-1 expression and misexpression of Pax-3 were observed, Anteriorly expressed HoxB genes were expressed normally in the alpha 5-null embryos. However, expression of the post eriormost HoxB gene, Hoxb-9, was reduced in alpha 5-null embryos, Thes e results suggest that alpha 5 beta 1-fibronectin interactions are not essential for the initial commitment of mesodermal cells, but are cru cial for maintenance of mesodermal derivatives during postgastrulation stages and also for the survival of some neural crest cells.