SCL is required for development of hematopoietic stem cells. (A) Wild type (+/+) and SCL mutant (−/−) mouse embryos. Note the red-brown coloration of the normal embryo due to blood within the fetal liver and vessels. This is absent in the mutant which also has a runted appearance. (B) Histological section through the mouse yolk sac with blood islands (arrows) present in the wild-type and lacking in the SCL mutant yolk sac. (C) Histological section through the embryo showing hematopoietic cells in the paired dorsal aortae in the wild type (arrows) and the absence of these cells in the dorsal aortae of the SCL mutant (arrows). (Reprinted with permission from Robb L, Lyons I, Li R, Hartley L, Kontgen F, Harvey RP, Metcalf D, Begley CG: Absence of yolk sac hematopoiesis from mice with a targeted disruption of the scl gene. The Proceedings of the National Academy of Sciences, U.S.A., vol 92, p 7075, 1995. Copyright 1995 National Academy of Sciences, U.S.A.93)