MATERNAL CELLS ARE WIDELY DISTRIBUTED IN MURINE FETUSES IN-UTERO

Passage of maternal cells into conceptuses in utero is recognized but poorly defined in species with hemochorial placentation. Despite the p otential importance for such a phenomenon in vertical disease transmis sion, only limited data address the frequency of maternal to fetal cel l trafficking or the developmental stage of its initiation. A murine m odel system, involving transfer of LacZ-, scid/scid, or wild type (+/) blastocysts to pseudo-pregnant, LacZ+ transgenic ROSA26 females prov ided both flow cytometric and in situ information. In 100% of the late -gestation pregnancies studied, nucleated LacZ+ maternal cells crossed to conceptuses, In 90% of scid/scid fetuses, nucleated maternal cells were present in at least one lymphoid organ and often in more than on e organ. Thymus was the most frequent site for maternal cell detection while the highest proportions of maternal cells were found in liver. Maternal cells were also visualized in fetal lung, heart, and bone mar row. Maternal cell trafficking into scid/scid fetuses commenced about midgestation, coincident with maturation of a placental circulation. I n late-gestation +/+ fetuses, maternal cells were found extensively th roughout bone marrow but not in other organs. The presence of maternal cells within primary lymphoid organs of fetuses may influence the rep ertoire of the developing fetal immune system and may be an underappre ciated mechanism for vertical disease transmission.