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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.
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-catenin. ANR5, ankyrin repeats
domain protein 5; EGF, epidermal growth factor; Fz7, Frizzled 7.
