Choriocarcinomas consist of two cell populations:

1. Cytotrophoblasts.
   • Pale/clear cytoplasm.
2. Syncytiotrophoblasts.
   • Hyperchromatic cytoplasm.
   • Typically multinucleated.

Traditionally, the syncytiotrophoblasts are said to produce the beta-hCG;^[1]however, it has been determined that cytotrophoblast also produce some.^[2]

The syncytiotrophoblasts are often arranged around the outside of cytotrophoblast cell clusters, reminicent of the arrangement in the placenta. On placental villi, the syncytiotrophoblasts are superficial to and, early in pregnancy, cover the cytotrophoblast.

Choriocarcinoma is classified as a germ cell tumour. It can arise in the testis or ovary and from a hydatidiform mole. It may be part of a mixed germ cell tumour.

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