Fig. 7. Vegf^lo/lo visceral endoderm cannot support blood island formation in wild-type mesoderm. (A) Schematic representation of the aggregation setup. Tetraploid Vegf^lo/lo embryos were aggregated with Vegf wild-type (wt) GFP+ ES cells resulting in completely Vegf^lo/lo-derived visceral endoderm and wild-type mesoderm. (B-D) Whole-mount photographs of a 9.5 dpc chimaeric embryo and yolk sac derived from tetraploid Vegf^lo/lo/Vegf wild-type GFP+ ES cell aggregation. Embryo proper (B) and vitelline vessels (arrowhead in D) are wild-type GFP+ whereas Vegf^lo/lo-lacZ positive cells constitute the yolk sac visceral endoderm (C). (E) Uniform and intense GFP fluorescence in a 9.5 dpc chimaeric yolk sac derived from tetraploid Vegf^lo/+/Vegf wild-type GFP+ ES cell aggregation indicates proper expansion of mesoderm-derived cell lineages. (G,H) ß-galactosidase staining of yolk sac derived from tetraploid Vegf^lo/lo/wild-type GFP+ ES cell (G) and Vegf^lo/+/wild-type GFP+ ES cell (H) aggregations. Only few blood islands devoid of primitive erythrocytes are seen in association with Vegf^lo/lo visceral endoderm (G), whereas blood island formation proceeds normally when supported by Vegf^lo/+ visceral endoderm (H). (F,I,J) Normal vascular but not haematopoietic development in the embryo proper. Dorsal aortae (arrowheads) show appropriate lumen formation. The number of blood cells in the heart atrium (at) and sinus venosus (sv) is reduced compared with completely wild-type embryos.

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