Random clonal analysis of early YS blood and endothelial precursors. (A-D) An example flat-mounted E8.5 YS with the blood island region outlined and the simultaneous detection of LacZ⁺ and EYFP⁺ cell clusters. Boxed areas are enlarged in (B-D). Scale bars, 50 µm. (E-H) High magnification from flat-mounts showing details of representative LacZ⁺ (E) and EYFP⁺ (F) B-clusters, and of LacZ⁺ (G) and EYFP⁺ (H) E-clusters. Scale bars, 10 µm. (I-J) Frequencies observed of B-, E-, and BE-clusters in E8.5 YS after 4OHT induction at E5.5 (I) or E6.5 (J). (K) Single-cluster size distribution (dots) and average size (bars) of B-, E-, and BE-clusters in E8.5 YS after 4OHT induction at E5.5 and E6.5. Images in (A-F) were obtained with a Nikon Eclipse 90i microscope using a Nikon DXM 1200c digital camera and 4X/0.2 NA objective (A) or 20X/0.75 NA (B-D), or 40X/0.95 NA objective and Nomarski contrast (E-H). (See also supplemental Figure 2 and supplemental Table 1.) a and arrowhead, anterior; di, distal; p and arrowhead, posterior; pr, proximal; l, left; r, right.