Fig. 4. (A) Antibody staining of Flk1, Pax2 and Pecam in E12.5 kidney frozen sections (a-c, g-i) and whole mounts of E11.5 kidney organ cultures placed in culture for 48 hours (d-f, j-l). (a,d) Flk1; (b,e) Pax2; (c,f) merge of preceding panels showing non-overlapping expression of Flk1 and Pax2. (g,j) Flk1; (h,k) Pecam; (i,j) merge of preceding two panels showing Pecam-expressing cells are a subset of Flk1-expressing cells. (B) Flk1-blocking antibody effect on branching and nephrogenesis. Cytokeratin green; proximal tubule brush border antigen red. (a,b) From the same embryo. (a) Treatment with rat IgG; (b) treatment with anti-Flk1; (c,d) quantitation of four pairs of organ cultures represented by (a) and (b), (P<0.001 for both terminal branch number and tubules).