Abnormal development of gestational trophoblast
Several disorders of abnormal trophoblast development are recognized, ranging from abnormal proliferative conditions through to highly malignant tumours. These are grouped under the term trophoblastic diseases». Many of these disorders follow a preceding abnormal gestation and are associated with cytogenetic abnormalities.
Hydatidiform mole is a benign abnormal mass of cystic vesicles derived from the chorionic villi and hydatidiform moles may be partial or complete.
In partial hydatidiform mole the vesicular degeneration of the chorionic villi affects only part of the placenta. Fetal parts and some normal placental villi are present, along with the abnormal trophoblastic tissue. This has a low risk of subsequent development of malignancy. In complete mole no normal placenta is identified and the mole forms a bulky mass, which
can fill the uterine cavity. No fetal parts or normal placental villi are present. This has a low risk of subsequent development of malignancy.
In both cases a characteristic feature of the mole is atypical hyperplasia of the syncytiotrophoblast and cytotrophoblast cells on the surface of the distended villi.
The incidence of both types of mole is about one in two thousand in the UK and USA. However, they occur far more frequently in some parts of Asia, South America and Africa, the incidence in Taiwan, for example, being one in four hundred pregnancies.
A small proportion (probably about 10%) of both partial and complete moles show invasion of the myometrium by the molar component, but this is not evidence of development of malignant neoplasm.
Evacuation of both complete and partial moles from the uterus may not be complete; some residual trophoblastic tissue is frequently left behind, particularly where there is deep invasion of the myometrium. Detection of such cases is by ultrasound imaging, as well as by demonstrating continued elevated levels of HCG secreted by trophoblast. These cases should be regarded as having persistent trophoblastic disease, and require chemotherapy to eradicate the residual trophoblastic tissue.
In some cases of persistent trophoblastic disease, there is a risk of subsequent development of malignant tumour of trophoblast, choriocarcinoma.
Choriocarcinoma is a malignant tumour of trophoblastic tissue. Around 50% of choriocarcinomas develop from a hydatidiform mole, only 20% arising after a normal pregnancy. The time-lag between pregnancy and the development of choriocarcinoma is very variable; it is usually a matter of a few months, but may occasionally take many years. It is rare in the UK and USA (approximately 1 in 50,000 pregnancies) but, like mole, is more common in Asia, South America and Africa.
Morphology. Macroscopically, the tumour forms hemorrhagic masses in the endometrial cavity, with a peripheral rim of viable tumour surrounding extensively necrotic and hemorrhagic debris. Microscopically, it is composed of masses of cytotrophoblastic cells, often covered by a rim of pleomorphic syncytiotrophoblast; there is no evidence of structured villous formation. Trophoblastic tissue has a propensity for invading vessel walls, and blood-borne metastases occur early to many sites, particularly lung and brain. Formerly rapidly fatal, this tumour responds to cytotoxic chemotherapy, and the prognosis with correct treatment is now excellent (particularly if post-treatment monitoring of HCG levels is carried out).
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