Regulation of cell polarity, radial intercalation and epiboly in Xenopus: novel roles for integrin and fibronectin

Fibronectin (FN) is reported to be important for early morphogenetic moveme nts in a variety of vertebrate embryos, but the cellular basis for this req uirement is unclear. We have used confocal and digital time-lapse microscop y to analyze cell behaviors in Xenopus gastrulae injected with monoclonal a ntibodies directed against the central cell-binding domain of fibronectin. Among the defects observed is a disruption of fibronectin matrix assembly, resulting in a failure of radial intercalation movements, which are require d for blastocoel roof thinning and epiboly. We identified two phases of FN- dependent cellular rearrangements in the blastocoel roof. The first involve s maintenance of early roof thinning in the animal cap, and the second is r equired for the initiation of radial intercalation movements in the margina l zone. A novel explant system was used to establish that radial intercalat ion in the blastocoel. roof requires integrin- dependent contact of deep ce lls with fibronectin. Deep cell adhesion to fibronectin is sufficient to in itiate intercalation behavior in cell layers some distance from the substra te. Expression of a dominant-negative beta1 integrin construct in embryos r esults in localized depletion of the fibronectin matrix and thickening of t he blastocoel roof. Lack of fibronectin fibrils in vivo is correlated with blastocoel roof thickening and a loss of deep cell polarity. The integrin-d ependent binding of deep cells to fibronectin is sufficient to drive membra ne localization of Dishevelled-GFP, suggesting that a convergence of integr in and Wnt signaling pathways acts to regulate radial intercalation in Xeno pus embryos.