Fig. 2. Hand1-null embryos fail to develop an extra-embryonic vasculature. (A,C) A 9.5 dpc wild-type mouse embryo shows a well-developed vasculature within the yolk sac filled with red blood cells. (B,D,E) A 9.5 dpc Hand1^{ß-gal/ß-gal} embryo appears to be devoid of a vasculature containing red blood cells within the yolk sac. Arrowheads indicate red blood cells collecting within the yolk sac. (C-E) ß-Gal staining. (F) Yolk sac section of 9.5 dpc wild-type mouse showing blood vessels between the endodermal and mesodermal layers (arrowheads). HAND1 expression is restricted to the mesodermal cells in the yolk sac (blue staining). (G) Yolk sac section of 9.5 dpc Hand1^{ß-gal/ß-gal} yolk sac showing a disorganized mesodermal layer. a, allantois; em, extra-embryonic membrane; fg, foregut.