Indirect peroxidase immunohistochemistry of mature corpus luteum of the cycle (MCL, panel A) and CL of pregnancy (CLP, panels B-F) with monoclonal antibodies to p27, cyclin D1, cyclin E, Ki67, p21, and pRb, as indicated in panels. A) Arrowhead indicates traces of p27 (nuclear envelope) in luteal cells of the mature CL of the cycle; note strong expression in luteal vessels. B) Preservation of the CL during pregnancy is associated with very strong nuclear and moderate cytoplasmic expression of p27 in luteal cells, in addition to the vessels. C) Similar cytoplasmic staining of luteal cells is apparent for cyclin D1 (note unstained vessels), but not for other cell cycle proteins investigated (panels D-F). Note that all nuclei of luteal (white arrows) and stromal cells (white arrowheads) are stained in panels B and F, but only some nuclei of luteal cells are stained in panels C-E (black arrows) and other are unstained (black arrowheads), including stroma. Insert in (D) indicates a lack of Ki67+ dividing cells in the CL of pregnancy (and no background staining); v, luteal vessels; s, stroma.