Placenta as a selective barrier to cellular traffic.

Transplacental passage of cells from mother to fetus during murine pregnancy was examined by using glucose phosphate isomerase (GPI) or fluorescein tagging as markers. Female mice of strains (BALB/cCr X C3H/HeJ)/F1 or (A/J X C57BL/6J)F1 (both Gpi-1a/b) were mated to the corresponding Gpi-la males (BALB/cCr or A/J, respectively). Cells of the liver, blood, and/or spleen in the offspring were typed at days 15, 16, 17, or 18 of gestation, the day of delivery, or 1 day postpartum. Only two of 172 Gpi-1a/a mice obtained from these matings showed evidence of maternal cell trafficking. Sensitivity of the assay was 1% Gpi-1a/a population. Fluorescein-labeled BALB/cCr peripheral red blood cells (RBC) or white blood cells (WBC) were injected i.v. into syngeneically mated BALB/cCr mothers on day 18. After 24 hr, the blood or liver of the neonates was formalin-fixed and examined in the fluorescence-activated cell sorter (FACS). Some RBC crossed the placenta, but WBC were usually not detected in fetal liver in significant numbers. This technique was very sensitive, and we estimated that no more than 225 WBC could enter the fetus via this route. Thus, we conclude that passage of significant numbers of maternal WBC into the fetus is rare, and perhaps this passage is related to a placental abnormality.