The human placenta is a unique and still very poorly understood organ During gestation, it is responsible for performing most synthetic and transport functions that are necessary to ensure the normal growth and development of the fetus in utero. Subsequently, at birth it is discarded. Current understanding of the role of the placenta in maintaining fetal homeostasis is based primarily on studies using either artificially perfused term placental lobules, placental slices or explants, or isolated plasma membrane fractions (1–3). Studies with placental tissue are complicated by the presence of serial tissue layers in the maternofetal barrier, i e., syncytiotrophoblast, cytotrophoblast, and fetal endothelium, as well as by the presence of other cell types within the placental tissue, such as the Hofbauer cells or placental macrophages.