Cell death of uterine natural killer cells in murine placenta during placentation and preterm periods

In the murine uterus granulated metrial gland (GMG) cells appear only during normal pregnancy. GMG cells belong to a member of natural killer (NK) cells and play an important role in fetus survival and placental growth. Our previous study revealed that mouse GMG/uterine NK (uNK) cells in the late pregnancy rapidly disappear from the uterus, due to the degenerative change classified as necrosis. But there are few reports regarding appearance and morphology of uNK cells during late pregnancy. We examined histologically and histochemically how and when uNK cells undergo cell death. The uNK cells in the metrial gland increased in number and reached maximum until day 12 of pregnancy. Sudden disappearance, however, occurred after day 15 and the granules reduced in both number and size. In situ DNA fragmentation detection revealed that DNA fragmented uNK cells increased in number during days 13 to 15 and reached 70.2% at day 15 of pregnancy. From days 13 to 17, uNK cells were positive against anti-perforin antibody. Ultrastructurally, uNK cells at day 15 showed poor organelles and unusual granules in structure. In uNK cells at day 17, condensation of nucleus chromatin, reduction in size and phagocytosis into other uNK cells were observed. These results suggested that uNK cells undergo at least two types of cell death, classified as necrosis and apoptosis, at the different stages of pregnancy, and that perforin is not a mediator for cell death.