Induction of the anterior hemangioblast program in animal caps requires signaling by activin in the absence of FGF. (A) Embryos at the 1-cell stage were injected in the animal pole with 200 to 400 fg activin mRNA either alone or in combination with 400 pg dominant-negative FGF receptor (XFD) mRNA. Embryos were grown to stage 8, and animal caps were excised and cultured in 1 × MBS to stage 18 and then processed for real-time RT-PCR analysis, probing for Scl (i), Runx1 (ii), SpiB (iii), Mpo (iv), Fli1 (v), and Flk1 (vi). Values represent the average of 3 experiments; error bars represent SD. (B) SpiB, Runx1, and Fli1 expression overlap in animal caps in which the anterior hemangioblast program has been induced by injection of activin and XFD mRNA; 400 fg activin plus 400 pg XFD mRNA were injected into the animal pole at the 1-cell stage, caps were excised at stage 8 and cultured to stage 18/19 as judged by sibling embryos. Caps were fixed, embedded, and sectioned in wax. Alternate 10 × 10-μm sections were probed for SpiB (i,ii), Runx1 (iii,iv), and Fli1 (v,vi) by in situ hybridization on sections. Original magnification ×10 for subpanels i, iii, and v. Original magnification ×20 for panels ii, iv, v, and vi (the same sections).