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Hydatidiform moles are grape-like swellings of placental villi. They come in two varieties: 1.) Complete hydatidiform moles (CHM); and, 2.) Partial hydatidiform moles (PHM).
1.) CHM are most commonly the result of the fertilization of an “empty” egg by an X-bearing spermatozoon which then reduplicates. Therefore, most moles are 46, XX (possible 46,YY conceptuses vanish). Rarely CHMs are formed by double fertilization of an “empty” egg. Their prognosis is similar. All moles have had at one time in early gestation, an embryo1. This is so because trophoblast does not know how to produce connective tissue which is invariably present in moles. Presumably, because of genetic homozygosity of lethal genes, the embryos die early. Many moles, but not all, have trophoblastic hyperplasia and choriocarcinoma often derives from them. They are much commoner in Japanese populations (~1 in 300 pregnancies; Europeans ~1 in 2,000).
2.) PHM are usually the result of double fertilizations (of a normal egg – then called androgenic) or single fertilization of an egg that contains two genomes (perhaps incorporating a polar boy – then called digynic). The androgenic PHMs are more molar and have usually much less of an embryonic development; the digynic PHMs may be much less molar conformation of the placenta and often have reasonably good fetal development. Often the fetus has characteristic fusions of 3rd and 4th fingers or toes, aside from other anomalies. They rarely go to term. The easiest way to differentiate between the two conditions is to undertake ploidy determination (by flow sorting of cells); others have advocated using p57 KIP2 histochemistry, but this is not so good. Few triploid specimens result in choriocarcinoma. It also needs to be pointed out that many spontaneous abortuses in humans have focal hydatid changes of the placental villi and may be difficult to differentiate from PHMs.
CHMs have not been observed in any nonhuman primate; choriocarcinoma has been reported in a rhesus monkey from its cervix. One PHM has been reported (with fetus) in a pregnant chimpanzee3. Causes for such differences are unknown.
Discrimination of complete hydatidiform mole from its mimics by immunohistochemistry of the paternally imprinted gene product p57KIP2., , Am J Surg Pathol, 10/2001, Volume 25, Issue 10, p.1225-30, (2001)
Complete hydatidiform mole with a coexisting embryo, , Hum. Pathol. , Volume 27, p.731-734, (1996)
Partial hydatidiform mole in a pregnant chimpanzee (Pan troglodytes), , Zoo Biology, Volume 12, p.299–305, (1993)