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Fig. 12. Model of differentiation of allantoic mesoderm. (A) Mesoderm exits the
primitive streak and is deposited into the exocoelom as the allantoic bud
(light pink). We propose that allantoic mesoderm acquires intrinsic factors,
possibly homeobox-encoded proteins, that initiate a series of downstream
differentiative events. Outer allantoic cells exhibit junctional complexes,
possibly based on their outside position, and form nascent mesothelium
(light-blue circles). (B) Older core mesoderm (dark pink) and older
mesothelial cells (dark blue) are displaced to the distal region as new
mesoderm (light pink) emerges from the streak
(Downs and Harmann, 1997 ). By
this time, the series of downstream events initiated in A has progressed, as
evidenced by Flk1-expressing angioblasts in the distal region
(Downs et al., 1998) (dark
pink). (C) FLK1 continues to spread proximally as older cells are displaced to
distal regions because of the addition of new mesoderm through sustained
streak activity (Downs and Bertler,
2000). At a distance of approximately 220 µm away from the
streak, VCAM1 becomes visible in distalmost mesothelial cells (light-green
color). Repression of VCAM1 in the proximal region may be the result of
suppressive factors emanating from the streak. (D) Factors emanating from the
streak continue to suppress VCAM1 to a distance of 220 µm whilst
endothelium spreads proximally to the base of the allantois, at which time the
allantoic vasculature amalgamates with those of the yolk sac and fetus
(Downs et al., 1998) (not
shown). Contribution of new mesoderm to the allantois from the primitive
streak has slowed or ceased altogether
(Downs and Bertler, 2000)
(this study), levels of VCAM1 are at their most robust in the distal allantoic
region (Downs, 2002) (dark
green color), and chorio-allantoic union is nearing its maximal frequency
(Downs and Gardner, 1995;
Downs, 2002) (data not shown).
After union with the chorion, the mesothelial surfaces of the allantois and
chorion appear to break down, and the allantoic vasculature penetrates the
chorionic ectoderm (Downs,
2002) (not shown). (E,F) Set of two schematic diagrams based on a
previous study that described allantoic morphology
(Downs et al., 1998) and
spatiotemporal localization of FLK1 (Downs
et al., 1998). The allantois is subdivided into distal, mid- and
proximal regions. (E) Distal-to-proximal differentiation of allantoic mesoderm
into angioblasts, the precursors of endothelial cells
(Sabin, 1920) and which
express Flk1 (Yamaguchi et al.,
1993). (F) Distal-to-proximal sequence of angioblast conversion
into FLK1-positive endothelium. The horizontal arrows indicate the correlation
between the time of formation of angioblasts for a given region in E with the
time of formation of endothelium in F. It is thought that binding of Vascular
Endothelial Growth Factor (VEGF) to its cognate receptor, FLK1, triggers
epithelialization of allantoic angioblasts, resulting in formation of an
endothelial cell plexus (Iwaguro et al.,
2002). Although VEGF is secreted by mesothelium
(Miquerol et al., 1999;
Downs et al., 2001) and
allantoic cells are responsive to VEGF
(Downs et al., 2001), a
spatiotemporal timecourse of VEGF and its isoforms has not been performed in
the allantois. Abbreviations as in Fig.
2.
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