Notch1 activation is reduced in Dll1 mutants. (A) Histologic section of an E15.5 embryo indicating the level along the anterior-posterior body axis used for the analysis of the fetal vessel phenotype. (B) Antibody staining for cleaved Notch (a-f), full-length Notch1 (g-l), and DLL4 (m-r). Green: α-smooth muscle actin, red indicates antigen. Notch activation is abolished in Dll1 mutant embryos at E15.5 and E18.5 (c-f), whereas Notch1 and DLL4 are still expressed at these embryonic stages (g-r). At E13.5, mutant vessels show expression patterns of the analyzed markers similar to wild type (a,b,g,h,m,n). Although DLL4 is normally expressed it does not seem to be able to activate Notch1. a′ to r′ show magnification of the boxed areas indicated in subpanels a through r.