Placental vascular anomaly with diffuse mesenchymal stem villous hyperplasia. A new clinico-pathological entity?

Diffuse mesenchymal hyperplasia of placental stem villi produces a pathological increase of placental volume, gives images of partial hydatidiform mola on ultrasound examination and is associated with elevated levels of alpha feto-protein. On gross examination, the placenta is enlarged, the vessels on the fetal plate show aneurysmal and varicose dilatations, and the stem villi appear as distinct, semitranslucent lobulated structures. At microscopy level, stem villi show excessive proliferation of mesenchymatous tissue with foci of myxoid degeneration and large macrophages containing alcian blue positive material within cytoplasmic vacuoles. Involved areas show blood vessels with abnormally thin walls, which are negative to factor VIII and alpha feto-protein immunohistochemical reactions. Pathological trophoblastic proliferation or stromal trophoblastic inclusions are not part of this condition. Although there is a clear predominance of a female genotype in this cases, the true sex incidence in placental vascular anomalies with diffuse mesenchymal stem villus hyperplasia has yet to be defined.