Gonadoblastoma. Light micrograph showing a large nest composed of immature large round germ cells with clear cytoplasm (resembling dysgerminoma cells) that are associated with smaller, dark sex cord cells (similar to Sertoli and granulosa cells). There is abundant hyalinised basement membrane material in and around the cell nests. Gonadoblastoma is an uncommon tumour containing cellular nests of dysgerminoma-like primitive germ cells and sex cord cells resembling Sertoli and granulosa cells. It almost always occurs in children or young adults with gonadal dysgenesis. About 80% of patients are phenotypic females; the rest are phenotypic males or have ambiguous genitalia. Pure gonadoblastoma is a benign tumour; however, its clinical course is complicated in 50% of cases by the development of an invasive germ cell tumour (usually dysgerminoma). Haematoxylin and eosin stained section. Magnification: x400 when printed at 10 centimetres wide.