Mesodermal defects and cranial neural crest apoptosis in α5 integrin-null embryos

α5β1 integrin is a cell surface receptor that mediates cellextracellular matrix adhesions by interacting with fibronectin. α5 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 effects of the α5-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 mesoderm of α5-null embryos compared to wildtype controls. These results suggest that changes in overall cell death or cell proliferation rates are unlikely to be responsible for the mesodermal deficits seen in the α5-null embryos. No increases in cell death were seen in α5-null embryonic yolk sac, amnion and allantois compared with wild-type, indicating that the mutant phenotype is not due to changes in apoptosis rates in these extraembryonic tissues. Increased numbers of dying cells were, however, seen in migrating cranial neural crest cells of the hyoid arch and in endodermal cells surrounding the omphalomesenteric artery in α5-null embryos, indicating that these subpopulations of cells are dependent on α5 integrin function for their survival. Mesodermal markers mox-1, Notch-1, Brachyury (T) and Sonic hedgehog (Shh) were expressed in the mutant embryos in a regionally appropriate fashion. Both T and Shh, however, showed discontinuous expression in the notochords of α5-null embryos due to (1) degeneration of the notochordal tissue structure, and (2) non-maintenance of gene expression. Consistent with the disorganization of notochordal signals in the α5-null embryos, reduced Pax-1 expression and misexpression of Pax-3 were observed. Anteriorly expressed HoxB genes were expressed normally in the α5-null embryos. However, expression of the posteriormost HoxB gene, Hoxb-9, was reduced in α5null embryos. These results suggest that α5β1-fibronectin interactions are not essential for the initial commitment of mesodermal cells, but are crucial for maintenance of mesodermal derivatives during postgastrulation stages and also for the survival of some neural crest cells.