A term referring to part of a spectrum of gestational trophoblastic disease, i.e. neoplastic growths of elements of tissue present in the placenta. Abnormal cytogenetics results in hyperplasia of the trophoblastic villi. The compromises that the mother's immune system has to make to permit pregnancy render this otherwise foreign tissue dangerous, though less so than choriocarcinoma.
Hydatidiform moles, particularly complete moles, carry a risk of invasion and/or metastasis (15% in complete mole, 0.5% in partial mole) and more rarely progression to choriocarcinoma (3% in complete mole, rarely seen in partial mole). Nowadays, in the West, most are detected early by obstetric ultrasound scans.
Hydatidiform moles are followed-up with monitoring of serum β-hCG levels to make ensure that there is no residual or metastatic foci following evacuation of products of conception. Avoidance of pregnancy for 6 months is advised to avoid confusion.
The characteristic appearance is of a mass of cysts or bubbles. Hydatidiform moles are classified into partial and complete. Partial moles are non-viable embryos with abnormal trophoblastic growth while complete moles are represent abnormal trophoblastic disease with paternally-derived (androgenetic) genetic material.
Partial moles are generally the result of a triploid conceptuses, i.e. where there are two complements of paternal genetic material and one from the mother (though the reverse maternal/paternal ratios have been described, as have been tetraploid variants). This results in a non-viable embryo along with abnormal trophoblastic proliferation. Miscarriage or evacuation of retained products of conception generally resolve the problem. Rarely, chemotherapy is required for persistent partial moles.
- Fetal parts may be present
- Normal admixed chorionic villi present
- Scalloping of villi (such that the appearances sometimes describes as the coast of Norway)
- Vessels may contain nucleated (fetal) red blood cells
Complete moles are thought to be formed by fertilisation of an ovum that lacks nuclear material due to aberrant meiosis. Fertilisation by a 23X sperm results in duplication of the paternal genetic material to form a 46XX genotype (but with maternal mitochondrial DNA). Occasionally, simultaneous fertilisation by both a 23X and a 23Y sperm results in a heterozygous 46XX or 46XY genotype (and triploid and tetraploid variants are also rarely seen). Some genetic material vital for embryogenesis is thought to be inactivated during spermatogenesis. Therefore, in the absence of maternal nuclear material, an embryo does not form, but trophoblastic tissue proliferates. Like normal placental tissue, hydatidiform moles secrete human chorionic gonadotropin (hCG), resulting in serum levels of up to several thousands units. This may manifest clinically as hyperemesis. Evacuation of the mole may be sufficient to cure the disease, but the properties of trophoblast that allow controlled invasion of the endometrium may become dysregulated and result in local invasion and distant metastasis, though they are not as malignant as cancerous tissue. Chemotherapy may be required.
- Absence of amnion and fetal tissue (fetal red blood cells implies partial mole)
- Changes in the morphology chorionic villi (may vary with gestation):
- Invaginations that may appear as pseudoinclusions (not to mistaken with scalloping of partial mole)
- Karyorrhectic nuclear debris within villi
- Excessive trophoblastic proliferation in a circumferential pattern (as opposed to the normal polar proliferation) and with nuclear pleomorphism.
- Absence of p57kip2 immunohistochemical staining. This is a paternally imprinted gene, i.e. it is epigenetically inactivated. As all nuclear genetic material (mitochondrial DNA excluded) is paternal derived, in a complete mole, p57 staining will be negative.