10.9 Pathologies of the placenta

Hydatid mole

The hydatid mole pregnancy corresponds to a cystic chorion villus degeneration.

Macroscopically, the mole looks like a heap of transparent bubbles, held together by filaments, and supported by a central core. Hydatid mole

© Institut de pathologie, CHUV, Lausanne

Microscopically, the villus degeneration exhibits no vascularization, a proliferation of trophoblasts (from cytotrophoblasts – Langhans' cells and from syncytiotrophoblasts) and dystrophic alterations of the connective tissue with stroma edema.

Clinically, affected women have spontaneous hemorrhages and signs of "intoxication", such as vomiting, symptoms of kidney failure (edema, proteinuria, high blood pressure) and a light hepatitis (sub-icterus).

2 types of hydatid moles exist:

  • complete hydatid mole
  • partial hydatid mole

The complete hydatid mole is a pregnancy without an embryo. In a normal pregnancy, the embryo comes from the embryoblasts and placenta from the trophoblast. In 0.1 to 0.5% of the cases, the embryo is not present and the ''fetus'' consists only of placental membranes. A hydatid mole develops when spermatozoa enter in ocytes which lack a nucleus.
Since without embryo no circulation is present that takes up and drains away the maternal fluids, the villi of a complete hydatid mole are swollen and look like a bunch of grapes. Normally a complete hydatid mole leads to a miscarriage in early pregnancy. If this does not take place, the "pregnancy " is accompanied by symptoms such as high blood pressure, edema and vaginal bleedings. Like a normal trophoblast a hydatid mole also secretes non-negligible amounts of gonadotropic hormones. Their abnormally high plasma value permits the diagnosis of a mole. In every case, though, the diagnosis must be supported by a cytogenetic analysis of tissue pieces. In the case of a complete mole, the chromosome analysis exhibits a normal diploid karyotype but all chromosomes stem from the paternal organism. Such situations can arise in dispermic fertilization (two sperm cells fertilize an oocyte that has no nucleus) whereupon the two paternal pronuclei fuse in order to form a diploid nucleus.
A diploid nucleus can also arise via the fertilization of an oocyte that has no nucleus by a spermatozoon and its subsequent mitotic division . The karyotype of the moles that arise from dispermic fertilization can be either XX or XY, whereas after monospermic fertilization all karyotypes are XX because YY zygotes are not viable. Examinations of karyotypes have shown that most hydatid moles possess a XX karyotype.


Scheme of a complete hydatid mole

In contrast to the complete hydatid mole partial hydatid moles are triploid, with a weakly expressed embryonic primordium. Embryonic erythroblasts with nuclei can be found. In seldom cases even an abnormal fetus can be found. The swollen chorionic villi are partially present and the classical clinical symptoms (mentioned above) are less obvious. The spontaneous rejection of the partial hydatid mole normally occurs somewhat later (4th to the 6th month).
The karyotype shows that the fetus is triploid with a double paternal set of chromosomes (XXX, XXY, XYY). Research has shown that these moles come from a fertilization of nucleus-containing oocytes with either two sperm cells or a diploid spermatozoon.


Scheme of a partial hydatid mole

The chorion carcinoma

A hydatid mole can also lead to a chorionic carcinoma, a malignant epithelial tumor that stems from trophoblast cells. A chorionic carcinoma can develop, following the interruption of a pregnancy or a miscarriage from the remaining trophoblast tissue. As a rule, proliferation is harmless when it arises from a partial hydatid mole. If the malignant tissue comes from a complete hydatid mole, it forms an invasive mole or a chorionic carcinoma.

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