Exaggerated Placental Site

The EPS is a benign nonneoplastic lesion characterized by an increased number of implantation site intermediate trophoblastic cells that extensively infiltrate the endometrium and underlying myometrium. The EPS can occur in a normal pregnancy, an abortion from the first trimester or a molar pregnancy, especially, a complete mole (24). The trophoblastic cells in an EPS display an identical morphological and immunophenotypical profile to the implantation site intermediate trophoblastic cells in the normal placental site. For example, they are strongly positive for CD146, HLA-G, and hPL, moderately positive for EGF-R and E-cadherin, and negative for p63, HNK-1, and p-hCG (except in the multinucleated intermediate trophoblastic cells). These findings indicate that the differentiation of IT is unaltered in an EPS, and support the view that an EPS is a normal variation of an implantation site (5,25). Despite the extensive infiltration of IT in an EPS, the Ki-67 indices of IT are near zero, suggesting that the increased number of IT in EPS is probably not the result of de novo proliferation of IT in the implantation site (24,25). The precise mechanism underlying the increased number of IT in EPS remains unclear, but may be because of rapid cell-cycle progression of IT in the trophoblastic columns or the suppression of apoptosis of IT in the implantation site.

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