Placental site nodules are small, well-circumscribed nodular aggregates of chorionic-type intermediate trophoblastic cells that are embedded in a hyalinized stroma. Placental site nodules have been believed to represent a portion of uninvoluted placental site from a remote gestation. However, the constituent cells in placental site nodules are morphologically more closely related to the IT of the chorion laeve (chorionic-type intermediate tro-phoblast) than to the IT of the placental site (implantation site intermediate trophoblastic cells) (50). In addition, the trophoblastic cells in the placental site nodule exhibit an immunophenotype similar to that of trophoblastic cells in the chorion laeve, but distinct from implantation site IT (Table 1). The cells in a placental site nodule react with the antibody against p63, but only very focally with CD146 and hPL, an immunophenotype that characterizes the intermediate trophoblastic cells in chorionic laeve, but not the implantation site. These findings suggest that placental site nodules are derived from chorionic-type intermediate trophoblast. It remains a mystery, how these cells (from chorion laeve) are retained and survive in the uterus several years after delivery.
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