"We compared developmental competences of prepubertal and adult sheep oocytes across in vitro maturation (IVM) in terms of ultrastructural morphology, mitochondrial functionality and ATP content. After IVM, metaphase II (MII) oocytes were processed for transmission electron microscopy (TEM), confocal microscopy (CM) and capillary electrophoresis. Germinal vesicle (GV) oocytes were used as controls. By TEM: in IVM 7h, cortical clusters of mitochondria appeared bigger in adults than in prepubertal oocytes; in IVM 19h, a reduction\/dispersion of mitochondrial clusters was evidenced, especially in prepubertal oocytes; in IVM 24h, big clusters were newly visible only in adult oocytes. CM showed in GV a subcortical distribution of active mitochondria, that became uniform in the cytoplasm of MII, thus indicating cytoplasmic maturation. ATP showed a fluctuation timed with the maturative morphological changes observed. In conclusion, even if in prepubertal oocytes maturation is completed after 24 h of IVM as in the adult, the different mitochondrial distribution and ATP content in prepubertal oocytes, could contribute to explain their limited developmental competence. "
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