Fetal cell microchimerism develops through the migration of fetus-derived cells to the maternal organs early after implantation.

Fetus-derived cells are present in the blood and tissues of the maternal body over a long period of time, even after delivery, resulting in fetal cell microchimerism. The exact process by which fetal cells cross the placental barrier to enter the maternal circulation is unclear. The objective of this paper was to determine the time during pregnancy that fetal cells with multilineage potential migrate to the maternal organs. Wild type female mice were crossbred with male transgenic mice, expressing enhanced green fluorescent protein (EGFP). Total hysterectomies were performed at different time points of pregnancy. On day 60 after surgery, mice were injected with either streptozotocin (STZ) to induce insulin-dependent diabetes mellitus, or vehicle. Detection and quantification of fetal cells were then undertaken in a variety of maternal organs via fluorescent microscopy and quantitative PCR amplification of the gfp transgene. In vehicle control mice, fetal cells were detected only in the maternal bone marrow. However on day 30 after STZ injection, fetal cells were detected not only in bone marrow but also in the maternal pancreas, liver and kidney. Histological analysis showed differentiated fetal cells within the pancreatic acinar cells, hepatocytes and tubular epithelial cells. Their morphological appearance was indistinguishable from their maternal counterparts, and their frequency in these organs was constant, regardless of the timing of hysterectomy. These results indicate that most fetal cells with multilineage potential in maternal tissues migrate to the maternal body early after implantation, and thereafter sustain their population over the long term after delivery.