FIG. 4.

Replication and cellular proliferation of Fen1^−/− blastocyst outgrowths. Wild-type (top) and Fen1^−/− (bottom) embryos were isolated at E3.5 and cultured for 3 days in cES medium (equivalent to 6 days postcoitum). The genotypes indicated were determined by PCR. Cellular outgrowth was extended for 48 h in the presence of BrdU, and the cells were then immunostained with an antibody specific for BrdU to identify cells that had undergone DNA synthesis during the labeling period. The outgrowths were photographed to monitor their development (left). Nuclei were visualized by DAPI staining (middle). While the vast majority of trophoblast giant cells (TG) easily incorporated BrdU in control outgrowth, no BrdU-positive cells were detected in the Fen1^−/− outgrowth (right, green cells). Magnifications are indicated on the images.