Microscopic description and IHC:

Histological sections demonstrate predominantly enlarged edematous chorionic villi with mild and focal trophoblastic hyperplasia and have irregular scalloped borders. A second population of small immature fibrotic villi is also identified. Autolyzed fetal parts are noted. Cytogenetics studies demonstrated the presence of XXX karyotype.

Discussion:

Partial mole (PHMs) has been estimated to occur in 1:700 pregnancies and typically results from fertilization of one egg by two sperm producing diandric triploidy. The usual presentation of PHM is late first or early second trimester bleeding with no uterine enlargement and missed or incomplete abortion.

Evidence of fetal development is common including stromal blood vessels containing nucleated red blood cells, amnion, chorionic plate and fetal parts.

PHMs typically have two populations of villi (enlarged hydropic villi and small-sized villi that are often fibrotic), geographic scalloped borders of villi, mild trophoblastic hyperplasia and trophoblast inclusions.

In contrast to complete hydatiform mole (CHMs), p57 immunostain shows nuclear staining of cytotrophoblasts and villous stromal cells.

In placental mesenchymal dysplasia, the placentas may show hydropic villi and aneurysmal stromal blood vessels but without trophoblastic hyperplasia or pseudoinclusions.

PHM-related complications are rarely reported including invasive/metastatic mole and development of placental trophoblastic tumor or choriocarcinoma.

References:

Atlas of Gynecologic Surgical Pathology, 3rd ed. Chapter 10.

Genest D. Partial Hydatidiform Mole: Clinicopathological Features, Differential Diagnosis, Ploidy and Molecular Studies, and Gold Standards for Diagnosis. Int J Gynecol Pathol, 2001; 20(4):315-322.

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