Effect of P replacement on endometrial histology and MMP ICC. The photomicrographs are arranged as a grid with each treatment in columns and histological endpoints in each row. a-h: GMA sections showing endometrial histology (a-d; Magnification 100× original magnification and e-h; functionalis zone, Original Magnification 25×). i-t: ICC for MMPs. Strong staining for MMP1, MMP2, and MMP-3 was observed at 48 and 72 h in control animals. Replacement of P during, but not after, the critical period blocked menstrual breakdown (compare b to d and f to h). Replacement of P at 24 hours blocked expression of all three MMPs (j, n, r), but P replacement at 48 hours blocked MMP-1 and MMP-3 but not MMP-2 (l,p,t).