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.