NRP1 expression confers a selective advantage to endothelial cells competing for the tip cell position. (A-D) Immunofluorescence staining of littermate hindbrains of the indicated genotypes; YFP and NRP1 labeling are shown in (A,C), NRP1 only in (A′,C′). Scale bar represents 100 μm. Three-dimensional reconstructions of the boxed areas in (A,C) shown in (B,D,D′). Examples of tissue macrophages, endothelial tip cells, and endothelial stalk cells are indicated with arrows, arrowheads, and curved arrows, respectively; note that some vessels leave the plane of section, and the vessel therefore appears blunt-ended, terminating in a stalk cell. Asterisks in (C′) indicate the position of macrophages lacking NRP1. (E) Percentage of NRP1+ YFP– endothelial cells in the tip versus stalk cell position in Tie2-Cre;Nrp1fl/–;RosaYfp mutant hindbrains.
