Expression of the zebrafish VegfA and Kdra genes in the developing embryo. (A) Whole-mount in situ hybridization with probes to vegfAa₁₂₁, vegfAa₁₆₅, or vegfAb. Both vegfAa₁₂₁ and vegfAa₁₆₅ are diffusely expressed at 12 somites, whereas vegfAb is not appreciably expressed at this time point. At 24 hours after fertilization (hpf), both isoforms of vegfAa were broadly expressed, although vegfAa₁₆₅ was more clearly seen in the somites. vegfAa₁₆₅ is more highly expressed in the developing CNS than vegfAb, whereas in the lens vegfAb is more highly expressed, and both are expressed in the developing somites (). At 2 dpf, vegfAa₁₂₁ is expressed in the developing heart vasculature and pectoral fins. The aortic vasculature is also identified in vegfAb in situs, but in contrast, only vegfAb is expressed in the developing pronephros (◀). At 4 dpf, significant expression is restricted to vegfAb in the vasculature surrounding the eye. (B) Wild-type embryos were analyzed by whole-mount in situ hybridization with a probe to either kdrb (top panels) or kdra (bottom panels) at 16 somites (left panels), 2 dpf (middle panels), and 4 dpf (right panels). At 16 somites, expression is limited to the inner cell mass. By 2 dpf, the developing intersomitic vasculature expresses both kdrb and kdra. At 4 dpf, both genes are expressed in the developing subintestinal veins (SIVs) and in the remaining vasculature. See “In situ hybridization and photography” for image acquisition information.