SummaryBody axis elongation represents a common and fundamental morphogenetic process in development. A key mechanism triggering body axis elongation without additional growth is convergent extension (CE), whereby a tissue undergoes simultaneous narrowing and extension. Both collective cell migration and cell intercalation are thought to drive CE and are used to different degrees in various species as they elongate their body axis. Here, we provide an overview of CE as a general strategy for body axis elongation and discuss conserved and divergent mechanisms underlying CE among different species.
Key words: Convergent extension, Collective cell migration, Cell intercalation, Planar cell polarity
IntroductionConvergent extension (CE) is a key process by which tissues undergo narrowing along one axis and concomitant extension along another axis. The first identified and best-studied example of CE in development is body axis elongation during gastrulation (reviewed by Keller et al., 2000). Generally, CE in body axis elongation is characterized by the collective movement of germ layer progenitor cells towards the dorsal side of the gastrula, where the embryonic body axis will form, accompanied or followed by cell intercalations along their axis of movement. This combination of collective cell movement and cell intercalations triggers narrowing of the body axis along its medial-lateral (ML) axis (i.e. the convergence) and elongation along its anterior-posterior (AP) axis (i.e. the extension). CE thus includes two fundamentally different types of cell movement ( Fig. 1): collective cell migration (see Glossary, Box 1), which commonly describes the coordinated movement of a highly cohesive sheet of cells; and cell intercalations (see Glossary, Box 1), whereby oriented exchanges of neighboring cells alter tissue geometry.In this Primer (see Box, Development: the big picture), we review how cells intermittently use collective migration and cell intercalation in CE-driven body axis elongation during gastrulation in various species. We also discuss the cell-intrinsic and extrinsic factors that determine the contribution of these different movement types to CE. Finally, we highlight the key molecular and genetic pathways involved in CE, but refer the reader to recent reviews of this topic (Gray et al., 2011;Skoglund and Keller, 2010) for further details on these pathways.
CE during zebrafish gastrulation: a case for collective cell migrationAt the onset of gastrulation in zebrafish, mesoderm and endoderm (mesendoderm) progenitor cells internalize at the germ ring margin (see Glossary, Box 1) and subsequently migrate away from the margin towards the animal pole ( Fig. 2A). Progenitors that internalize in the region of the embryonic organizer (the shield) and give rise to the anterior axial mesendoderm (the prechordal plate) migrate as a highly cohesive cluster of cells, typical of collective migration (reviewed by Friedl and Gilmour, 2009). Once all of the prechordal plate progenitor cells have internalized, the prechordal p...