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Fig. 2. sax-3 mutants display defects in cell movements similar to those
seen in vab-1 embryos. (A) Cartoon of wild-type C. elegans
embryogenesis highlighting some cell movement events during embryogenesis. The
gastrulation cleft is closed by lateral neuroblast movements by 200 minutes,
and enclosure of the ventral epidermis (red cells) is observed by 325 minutes
at 22°C. Lateral (seam cells) are coloured yellow and the dorsal epidermis
is coloured orange. Arrows show the direction of cell movements. (B-G)
Embryogenesis of N2 (wild type) and various single- and double-mutant
combinations. Images are single frames taken from 4D DIC movies of individual
embryos. All embryos are shown as ventral views; anterior is to the left. The
times are given in minutes relative to the first cleavage, and are shown at
the bottom left of each panel. Scale bar: 10 µm. (B) In wild-type
(wt) development the ventral gastrulation cleft is small (<15
µm, dotted line). After the cleft is closed, the epidermal cells migrate
around the embryo to meet at the ventral midline. The leading cells are marked
(arrows). (C) sax-3(ky123) embryos have broader, deeper and more
persistent gastrulation clefts because the neuroblasts either fail to move or
migrate abnormally. Later when the epidermal cells start to migrate the
gastrulation cleft is still open and it is common to see cells `floating'
(asterisk) around in the still open gastrulation cleft. The open gastrulation
cleft and wandering cells may interfere with the epidermal leading cells
(arrows) and posterior pocket cells enclosing the embryo. The embryo usually
ruptures at the ventral side during the elongation process. (D)
vab-1(dx31) is shown for comparison. (E) Double-mutant vab-1(e2);
sax-3(ky123) embryos have cell movement defects that are similar to the
phenotypes of either vab-1 or sax-3 single mutants. The
gastrulation cleft is usually larger and less defined, as the cells are
disorganized (Class I, strongest phenotype). (F) Double-mutant
vab-1(dx31); slt-1(eh15) embryos have broader, deeper and more
persistent gastrulation clefts. Epidermal cells are highly disorganized and do
not migrate to the ventral side, and the embryos usually arrest before ventral
enclosure; a phenotype not seen in either of the single mutants. (G) Double
mutants efn-4(bx80); sax-3(ky123) are completely inviable and display
cell movement defects that are similar to those of the vab-1 ptp-3
and vab-1(dx31); efn-4(bx80) double mutants. The question mark
denotes that the embryo was so disorganized that we could not confidently
identify whether this cell was the leading cell. ptp-3(op147);
sax-3(ky123) also exhibited similar defects (not shown).
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