Wnt Signalling and Regulation of Gastrulation Movements

Abstract

Gastrulation in vertebrate embryos involves a complex set of morphogenetic movements that give rise to the three germ layers: ectoderm, mesoderm and endoderm. The narrowing and lengthening of a group of cells, termed convergent extension, represents one of the main engines producing the driving force for gastrulation and makes an important contribution to the elongation of the anteroposterior axis. The cellular basis of this process is well described in the frog Xenopus (reviewed by Keller et al. 1992, 2000) and in zebrafish (reviewed by Myers et al. 2002). During gastrulation, deep cells in the dorsal marginal zone (DMZ) undergo active mediolateral intercalation and rearrangements between one another. They become bipolar, extending protrusions both medially and laterally. These activities produce traction and cause the asymmetric movements of DMZ cells towards the dorsal midline (Fig. 12.1). Cells undergoing convergent extension will form axial and paraxial mesoderm, as well as neural tissue. The region located just above the dorsal blastoporal lip moves inside the embryo to differentiate into mesoderm, while the region closer to the animal pole remains external to form neural tissue. Involuted cells continue mediolateral intercalation allowing blastopore closure and the elongation of axial and paraxial mesoderm along the anteroposterior axis. Mediolateral intercalation results from coordinated regulation of the protrusing motility behavior of the DMZ cells (Shih and Keller 1992; Wacker et al. 1998). These processes are not established prior to gastrulation but reflect organizing events at gastrula stages (Domingo and Keller 1995). Convergent extension is a cell-autonomous process and can be demonstrated by time-lapse recordings. When two DMZ explants are isolated from early gastrula and ‘sandwiched’ with their inner sides facing each other, they still undergo narrowing and elongation along the anteroposterior axis as they do in the whole embryo (Keller et al. 1992). Ectoderm explants treated with activin, a potent mesoderm-inducing factor that induces dorsal mesoderm, also undergo elongation that mimics convergent extension (Symes and Smith 1987). This provides a simple in vitro assay to analyze cell behaviors and gene function in convergent extension. It is noteworthy that convergent extension is active in the trunk and posterior regions of the embryo in Xenopus, while head mesoderm cells undergo directed migration that also contributes to the elongation of dorsal mesoderm (Winklbauer and Nagel 1991; Davidson et al. 2002).