Fig. 3. Control of endothelial cell proliferation. Embryos and yolk sacs were dissected at E8.25-8.5, and tissues were immunostained with antibodies against phosphohistone 3. Compared with wild type (A), there was consistently more phosphohistone 3 staining in Raldh2^-/- tissues (B). The mitotic index was significantly higher in Raldh2^-/- tissues compared with wild type at E8.5 (6.3±1.5% versus 2.7±1.0%, respectively). Immunostaining of yolk sac cross-sections with phosphohistone 3, and co-staining with DAPI, revealed no difference in cellular mitosis in the endodermal layer of yolk sac, but a significantly higher mitotic index within the mesodermal layer of Raldh2^-/- yolk sacs, compared with wild type (C). Coimmunostaining for phosphohistone 3 and VE-cadherin indicated that the proliferating mesodermal cells in Raldh2^-/- yolk sacs were predominantly endothelial cells (E); proliferating endothelial cells in wild-type yolk sac were infrequently observed (F).