Regeneration of Bovine Endometrium Throughout the Estrous Cycle.

Abstract

The endometrium undergoes periodic proliferation and degeneration in primates. If pregnancy does not occur, cells of the endometrium degenerate and slough off with blood during menstruation. The cells then proliferate and prepare the endometrium for implantation. On the other hand, the regulation of endometrial regeneration throughout the estrous cycle in non-primate animals including cow is unclear. In the present study, we examined 1) the relative rate of cell proliferation by immunohistochemical localization of a proliferation marker (Ki-67), 2) the relative rate of apoptosis by TUNEL (DNA fragmentation) assay and immunohistochemical localization of cleaved caspase-3 (CCP3) in bovine endometrium, and 3) CCP3 protein levels in bovine endometrium throughout the estrous cycle by western blot analysis. The rates of Ki-67-positive cells in the luminal and glandular epithelia were higher at the early luteal (Days 2-3: Day 0=estrus) and follicular (Days 19-21) stages than at the mid (Days 8-12) and late (Days 15-17) luteal stages, whereas in the stroma layer the rate of Ki-67 was higher at the follicular stage than at the mid and late luteal stages (P<0.05). DNA fragmentation was detected in the luminal, glandular epithelial and stroma throughout the estrous cycle, without significant differences during the estrous cycle. CCP3 staining in the luminal epithelium and stroma was stronger at the early luteal stage than at the mid and late luteal stages (P<0.05), but did not change in the glandular epithelium. Furthermore, the 17 kDa CCP3 expression level by western blot analysis was higher at the follicular stage than at the mid and late luteal stages (P<0.05), and the expression of 19 kDa CCP3 tends to be higher at the early and follicular stages than at the mid and late luteal stages. Together, these results show cell proliferation and apoptosis undergo cyclic patterns in bovine endometrium, and suggest that the endometrium in cow, like that in primates, is regenerated in each estrous cycle.