Remodeling of bovine endometrium throughout the estrous cycle

The mammalian endometrium changes morphologically and functionally throughout the estrous cycle. In some species, endometrial cells also undergo periodic proliferation and degeneration. However, the remodeling of bovine endometrium throughout the estrous cycle remains unclear. In the present study, we examined how the remodeling of bovine endometrium varied through the estrous cycle by measuring the relative rates of cell proliferation and apoptosis. Cells positive for both KI-67 (a proliferation marker) and cleaved caspase-3 (CCP3: an apoptotic cell marker) were immunohistochemically evaluated throughout the estrous cycle in the luminal and glandular epithelia, and the stroma of bovine endometrium. Percentages of KI-67-positive cells tended to be higher at the early luteal and follicular stages than at the mid and late luteal stages in all cell types. Similarly, percentages of CCP3-positive cells were higher at the early luteal stage than at the mid and late luteal stages in the luminal epithelium and stroma. Furthermore, CCP3 expression levels by Western blot analysis agreed with these immunohistological observations. On the other hand, DNA fragmentation was detected in the bovine endometrium without significant differences during the estrous cycle by the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) method. Together, these results show that cell proliferation and apoptosis undergo cyclic patterns in the bovine endometrium, and suggest that the bovine endometrium is remodeled in each estrous cycle.