Patterns of uterine cellular proliferation and apoptosis in the implantation site of the rat during pregnancy.

During gestation, the balance between cell proliferation and death is crucial for successful embryo implantation and maintenance of pregnancy. The uterine endometrium responds to blastocyst implantation with extensive proliferation and differentiation of stromal cells into decidual cells, forming the antimesometrial and mesometrial decidua, which regress by apoptosis. In the latter region it is also observed the growth of metrial gland. To elucidate the events underlying this tissue remodelling we investigated the spatial and temporal pattern of expression of the proliferating cell nuclear antigen (PCNA) and localized the apoptotic cells, by the TUNEL assay and by the expression of active caspase-3. We found that PCNA is expressed at high levels during decidualization until day 12 of gestation declining thereafter abruptly. On the contrary, the appearance of apoptotic cells was detected, by the TUNEL and active caspase-3 expression, in the mesometrial decidua on day 12, increasing from days 14 to 16 in the decidua and metrial gland. In the antimesometrial decidua apoptosis was observed from early to day 12 of pregnancy. However, on day 13 only cell debris and neutrophils were observed, indicating also the presence of necrosis. These results suggest that decidual cells undergo, in distinct regions and at different stages of pregnancy, cell death by apoptosis and secondary necrosis.