The distribution of factor XIIIa-positive cells in the human fetus and placenta

Immunohistochemical staining for factor XIIIa, a transglutaminase, revealed a variety of positively stained cells in human fetal tissues. Factor XIIIa-positive cells were most numerous in the dermis and connective tissues. Numerous large, stellate cells in placental villi, decidua, and chorionic membranes also expressed factor XIIIa at 7-9 weeks gestational age, before the onset of fetal hematopoiesis. There was heterogeneity in the staining for factor XIIIa in the early and late fetal tissues, in both rounded and in dendritic cells. In preparations of consecutive sections and in double-labelling experiments, some cells expressed both factor XIIIa and certain monocyte markers and were identified in close association with blood vessels and lymphoid organs in the late fetus and in the placental villi at the end of gestation. Other rounded and dendritic cells expressed factor XIIIa but not monocyte markers, and were found in adult and fetal connective tissues at all gestational ages. These results suggest that there are two factor XIIIa-positive cell populations. One population is present at all developmental stages, does not express monocyte markers, and probably differentiates in situ from primitive mesenchyme. The other population appears mainly after the onset of fetal hematopoiesis, coexpresses some monocyte markers, is HLA-DR positive and may be capable of antigen presentation.