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OBJECTIVE To study the impact of aneuploid granulosa and stromal cells on folliculogenesis of small ovarian follicles from mosaic Turner syndrome patients using a murine xenograft model. DESIGN Laboratory study SUBJECTS: Ovarian cortical tissue was obtained by laparoscopic surgery from 18 mosaic TS patients (aged 5-19 years) and 13 controls (aged 5-18 years). INTERVENTION Part of each tissue fragment was used to karyotype ovarian cells in non-grafted tissue by fluorescence in situ hybridization. The remaining tissue was xenografted to severe combined immunodeficient mice for 5 months. Grafted tissue was analyzed for aneuploidy, and follicle density and morphology were determined. Expression of proliferating cell nuclear antigen and anti-Müllerian hormone were investigated by immunohistochemistry. MAIN OUTCOME MEASURES The impact of aneuploid granulosa and stromal cells on folliculogenesis. Fluorescence in situ hybridization of ovarian tissue before grafting was performed to determine the level of aneuploidy in stromal cells, and oocytes and granulosa of small follicles. After xenografting the level of aneuploidy of the newly formed layers of granulosa cells was again determined in secondary and antral follicles. RESULTS Follicle density in ovarian tissue from Turner syndrome patients was significantly lower than in controls before grafting. Fluorescence in situ hybridization analysis confirmed that 101/104 oocytes from non-grafted tissue of Turner syndrome patients showed normal X chromosome content, while granulosa and stromal cells were mainly 45,X. Fragments from 12 Turner syndrome patients contained follicles at all stages after xenografting, including secondary and antral follicles. Follicle density in Turner syndrome patients and controls decreased significantly after grafting. Moreover, a shift from high to low proportions of 45,X granulosa cells was observed during folliculogenesis. Expression of PCNA in follicles from TS patients increased significantly during grafting. Secretion of AMH was impaired before grafting in peri-/postpubertal TS girls, but recovered after grafting. CONCLUSION Our study showed that small follicles from mosaic Turner syndrome patients undergo folliculogenesis, despite the presence of aneuploid granulosa and stromal cells. Ovarian tissue cryopreservation could therefore be a valid option to preserve fertility in young mosaic Turner syndrome patients if sufficient numbers of follicles are present, thus preferably before the age of 12.