Immunohistochemical labeling of early erythroid cells. (A) Section through the 7.5-dpc yolk sac showing staining for runt box polypeptides in the developing blood islands (original magnification × 20). (B) Negative control for (A), ie, an adjacent section stained with anti-runt box antiserum after preincubation with GST/runt polypeptide. Given the lack of stain, a phase contrast image is shown for better definition (original magnification × 20). bi, blood island; en, extraembryonic endoderm. (C) Section through a 11.5-dpc mouse embryo, showing staining for runt box polypeptides in the fetal liver (lv) and in the urogenital ridge (g). The neighboring tissues are negative (original magnification × 20). (D) Same as (C), but preimmune control (original magnification × 20).