Maternal and fetal cellular relationships in the human placental basal plate.

Maternal and fetal cellular relationships in the normal human term placental basal plate were investigated by single and double immunohistochemical labelling techniques. Extravillous fetal trophoblast in the basal plate was uniformly reactive with markers of low-molecular-weight cytokeratins. The predominant maternal leucocyte population in the basal plate consisted of leucocyte-common-antigen-positive, class II MHC-positive macrophages, which exhibited acid phosphatase activity. Double-labelling methods highlighted the close association of these macrophages with extravillous trophoblast: they often extended processes around the fetal cells and were also observed within islands of cytotrophoblast. Other leucocytes were uncommon, although aggregates of T cells were apparent in some tissues.