Onset and distribution of factor XIII-containing cells in the mesenchyme of chorionic villi during early phase of human placentation.

To learn more about the distribution and possible function of factor XIII (FXIII)-containing cells of human placenta, paraffin embedded and frozen sections of placenta samples from the first trimester of pregnancies--terminated by legal abortions--were studied by single and double labelling immunomorphological techniques. It was observed that at the fifth gestational week in the chorionic mesenchyme, FXIII-containing small mononuclear, round shaped cells start to appear. The relative amount of the FXIII-containing cells rapidly increased up to the seventh gestational week, reaching nearly 30 per cent of all mesenchymal cells. Simultaneously these cells differentiated into large stellate cells having numerous vacuoles in their cytoplasm. These cells were characterized in double labelling experiments and proved to be macrophages (CD 14+, KiM7+, labelled with antimacrophage monoclonal antibody). In the fifth-seventh weeks of gestation, these cells were homogenously scattered in the immature mesenchymal connective tissue, but from the eight gestational week they tended to accumulate in the peripheral part of chorionic villi while the central mesenchyme showed intense fibrotic changes. The abundance and characteristic distribution of the FXIII-positive macrophages in the chorionic mesenchyme during the first trimester of pregnancy suggest that these cells may have an active role in the formation of connective tissue in the early phase of placentation.