A, Immunostaining of cytokeratin 8 (CK8), a marker of epithelial cell lineage, showed that epithelial alignment is collapsed and the trophoblast invades the stroma at implantation sites in Rb1f/f mice, while epithelial alignment is persistent and trophoblast invasion into the stroma is compromised in Rb1d/d mice. Scale bar = 200μm; arrowhead, an embryo. 