The aim of the present study is to establish a mouse model of the transplantation of bone marrow cells into the placenta in mid-gestation. The mononuclear fraction of bone marrow cells was isolated by Ficoll gradient centrifugation from the femur bones of C57BL/6 green fluorescent protein (GFP) gene transgenic (Tg) mice. After intraperitoneal injection of pentobarbital sodium, the abdominal cavities of pregnant non-Tg (C57BL/6 or ICR) mice were opened at 9.5 days postcoitum (dpc). The mononuclear fraction of bone marrow cells from Tg mice (3-5 × 105 cells/3 μl) was directly injected into the placental portion of the pregnant uterus, at a depth of approximately 3 mm, using a 31-gauge injector. The placenta was sampled at 14.5 dpc. Confocal laser scanning microscopic analysis of the serial sections of the sampled placenta (150-250 sections/placenta) was carried out to detect GFP-positive cells and to assess immunostaining for cytokeratin, CD34, p57Kip2 and prolactin. Most pregnant mice survived until sampling of the placenta at 14.5-18.5 dpc (88.9% for C57BL6 and 100% for ICR). The survival rate of fetuses from mice in which the placenta was transplanted with GFP-positive bone marrow cells was approximately 50%. A small population (0.154%) of injected bone marrow cells was retained in the placental tissue. Immunohistochemically, cytokeratin, CD34 and p57Kip2 were positively stained in 0.062%, 4.5% and 2.1% of GFP-positive cells, respectively, while prolactin was not positive in any of the cells examined. GFP-positive bone marrow cells were successfully transplanted to the murine placenta. Future investigations of the specific antigens in bone marrow cells retained in the placenta may enable a better understanding of the local regulation of placental development.
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