This is an immunohistologic study of tissue factor (TF) in snap frozen, unfixed, human normal-term placentae. Antibodies to TF were a monoclonal to human brain TF purified on a factor VII-agarose affinity column, and a polyclonal to a synthetic polypeptide representing the carboxyl-terminal nine amino acids of human TF. The results detail the localization and distribution of TF and characterize the cells in which it is found. TF was not observed in trophoblast, trophoblastic basement membranes, or noncellular components of connective tissue. TF was identified in some but not all macrophages, most fibroblast-like cells, and occasionally in perivascular cells and endothelium. The most consistent and intense reactions were obtained with vimentin-positive fibroblast-like cells in loose connective tissue. TF usually was not identified in fetal stem vessel endothelial cells, but TF reactivity was found in some of these cells in chorionic villi with histologic evidence of chronic inflammation. Such areas are uncommonly found in normal-term placentae. The vast majority of TF-reactive cells did not react with antibody to factor VII and were not in contact with blood. The biologic purpose of producing relatively great amounts of TF in areas remote from circulating factor VII is not known.

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