Fig. 1. Gastrulation movements in Drosophila, Xenopus and zebrafish. (A,B) Epithelial bending during mesoderm invagination of Drosophila. (A) Stage 6 scanning electron microscopic (SEM) image (ventral view, anterior to the left), courtesy of FlyBase (http://flybase.bio.indiana.edu/). (B) Schematic of invagination process at stages 5 (left) and 6 (right); transverse sections (TS) at level indicated by the asterisk in A, ventral side down. Red spots, RhoGEF2; black spots, β-catenin. Based on data from Kölsch et al. (Kölsch et al., 2007). (C,D) Germ band extension (GBE) of Drosophila ectoderm, driven by planar cell intercalations, without obvious, transient losses in epithelial integrity. (C) SEM image of Drosophila embryo at late stage of GBE (dorsal view, anterior to the left), courtesy of FlyBase. (D) Schematic of cell rearrangements at lateral side indicated by the asterisk in C. Two pairs of cells are labelled with different colours. (E-G) Bottle cell formation, a variant of epidermal bending, and convergent extension (CE) in Xenopus. (E) Semi-section of Xenopus embryo at stage 10.5 (early gastrula; dorsal to the right, animal pole up); position of the bottle cells is indicated by the asterisk, dorsal midline is indicated by the blue line. (F) Schematic of TS through forming bottle cells (dorsal side to the right, animal pole up). Black spots show the accumulation of β-catenin. Based on data from Lee and Harland (Lee and Harland, 2007). (G) Drawing of mesodermal cells during CE (dorsal views, animal pole up); based on data from Unterseher et al. (Unterseher et al., 2004). At early stages, cells are apolar, with protrusions multipolar (left). Later they become bi-polar and elongated along the mediolateral axis (right; dorsal midline to the right). (H-K) Zebrafish gastrulation. (H) Zebrafish embryo at 80% epiboly stage (midgastrula; lateral view, dorsal side to the right, animal pole up). Positions of cells depicted in I and J are indicated with an asterisk or a blue line, respectively. (I) Schematic of prechordal plate cells migrating towards the animal pole of zebrafish embryo (dorsal view, anterior up). Based on data from Yamashita et al. (Yamashita et al., 2004). Cells at the leading edge form protrusions that preferentially point into the direction of their migration. In following cells, protrusive activity is lower, and cells are in direct contact with each other (Montero et al., 2005). (J) Schematic of individual migrating mesodermal cells during dorsal convergence; based on data from Bakkers et al. and von der Hardt et al. (Bakkers et al., 2004; von der Hardt et al., 2007). Cells are elongated along the mediolateral axis and preferentially project cell protrusions in the dorsal/medial direction of their migration. Migrating cells often form contacts between each other, either via their protrusions (two left cells in J), or, after protrusion retraction, along larger cell surface regions (two right cells in J). (K) Phalloidin staining of the actin network in enveloping layer (EVL) cells during epiboly, when cells flatten out. Despite their tight epithelial organization, EVL cells have multiple basal lamellipodia (arrows). (L,M) Ingression of mesodermal cells through the primitive streak (PS) in chicken embryos. (L) SEM of the ventral surface of the blastoderm of a stage 3c chick embryo [reprinted, with permission, from Lawson and Schoenwolf (Lawson and Schoenwolf, 2001)]; arrowhead points to Hensen's node, arrows indicate primitive groove formed along the PS. (M) Schematic of ingressing cells through a TS of a stage 3c chick embryo PS. PS cells display protrusive activity while delaminating from the epithelial epiblast; magenta colour indicates remnants of basement membrane. e, epithelial epiblast; h, hypoblast; m, mesodermal cells; ps, primitive streak.