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Fig. 1. Internalization of distinct cell types during early stages of
Achaearanea embryogenesis. (A) Early cell types defined in
terms of gene expression and site of appearance. Note that the two cell types
indicated by asterisks are indistinguishable unless site of appearance is
considered. (B) Illustration of a late stage 4 embryo. (C-F) A
flat-mounted late stage 4 germ disc stained for F-actin (C-E, green), At-Fkh
(C-E, purple), At-ß-catenin (F, yellow) and DNA (F, blue). Boxed area in
C is magnified in D-F (D, surface; E,F, inside). The pattern obtained by
immunostaining for At-Fkh was the same as that obtained by in situ
hybridization for its transcript at this stage (compare with
Fig. 3D-F). Arrow in D, the
closing blastopore; arrow in F, At-ß-catenin concentration. cEND cells
are situated just beneath the blastopore. (G) Illustration of a
mid-stage 5 embryo. (H-K') A flat-mounted mid-stage 5 germ disc
stained for At-fkh transcripts (H-K) and DNA (I'-K'). The
upper box in H corresponds to I,I' and the lower one to
J,J',K,K'. (I,I') pEND cells are visible near the rim of the
germ disc (arrows). (J,J',K,K') The focal plane is adjusted to the
surface layer in J and J' and the inner layer in K and K'. cEND
and CM (arrows) cells are visible. (L,L') High
magnification of the equatorial region of a late stage 5 embryo stained for
At-fkh (L, red) and At-twi (L, purple) transcripts and DNA
(L'). The boundary between the germ disc and non-germ disc areas is
seen. Arrowheads indicate pEND cells located below the surface layer, and
arrows indicate probable pMES cell precursors about to internalize. (M)
An illustration of an early stage 6 embryo. (N-Q') A flat-mounted
early stage 6 germ disc stained for At-fkh (N-Q, red) and
At-twi (N-Q, purple) transcripts and DNA (O'-Q'). The
boxed area in N is magnified in O-Q,O'-Q'. The focal plane is
adjusted to the surface layer in O and O', an intermediate level in P
and P' and a one-cell deep level in Q and Q'. Arrows indicate cMES
cell precursors, and arrowheads cEND cells. Asterisks mark the same positions
in O' and P' to show that At-twi-positive cells are
internalizing. (R,S) Illustrations of a late stage 6 embryo
viewed from the lateral (R) or caudal (S) side. At-cad expression is
shown in red. (T-W) A flat-mounted late stage 6 embryo stained for
At-twi (T,U, purple) and At-cad (T,U, red) transcripts and
DNA (V,W). The boxed area in T is magnified in U-W. The focal plane is
adjusted to the surface layer in V and W, and a one-cell-deep level in W.
Arrows indicate likely cMES cell precursors internalizing or about to
internalize. Asterisks mark the same positions in U and V. Small open circles
indicate pMES cells, and arrowheads cEND or pEND cells. a, anterior; bp,
blastopore; cEND, central endoderm cells; cl, forming caudal lobe; CM, cumulus
mesenchymal cells; cMES, central mesoderm cells; d, dorsal; ex, extraembryonic
area; gd, germ disc; p, posterior; pEND, peripheral endoderm cells; pMES,
peripheral mesoderm cells; v, ventral. Scale bars: 20 µm.
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