Fig. 2. Tie2-Cre activity in early embryogenesis is restricted to the vasculature. (A-E) Widespread vascular activity of Tie2-Cre in the progeny of a Tie2-Cre X R26R lacZ reporter cross at E8.25, revealed by X-gal staining (blue color). (A) The highly vascularized yolk sac (YS) shows intense Tie2-Cre activity. The primitive vasculature throughout the embryo proper (arrowheads) also shows Tie2-Cre activity. (B) Close-up photograph of intersomitic sprouts (arrows) from the dorsal aorta (DA) of a similar embryo to (A) shows Tie2-Cre activity in vessels undergoing angiogenic sprouting. (C-E) Sections of littermates of A, showing Tie2-Cre activity in vessels (arrowheads) of the head (C), hindbrain (D) and trunk (E). The endocardial lining of the heart is also positive (E, arrows), as expected (Kisanuki et al., 2000). Sections were counterstained with Hematoxylin.