The mouse oocyte Balbiani body reassembles in growing follicles to continue organelle selection and upon dispersal influences nuclear position

The Balbiani body (Bb), a universal oocyte structure enriched in mitochondria and organelles, forms during zygotene but grows and behaves differently in several animal groups. The mouse Bb forms during pre-follicular oogenesis, but whether it persists during and after the primordial follicle state is unknown, unlike the long lived Bbs in the oocytes of lower vertebrates. Here we report that the mouse Bb, which has been postulated to facilitate mitochondrial selection and organelle quality control, re-assembles in growing mouse follicles. The re-assembled Bb is organized around a single microtubule-organizing center (MTOC), surrounded by mitochondria, Golgi apparatus, endoplasmic reticulum, and endosomes, like the original Bb. Thus, mouse oocytes contain a Bb whose action is paused by quiescence, but otherwise more closely resembles the primordial follicle Bb and the Bb in zebrafish and Xenopus oocytes. The mouse Bb eventually disperses as its MTOC relocates from the perinuclear region to the oocyte cortex, driven by opposing microtubule and actin forces that also reposition the nucleus to the oocyte center. We suggest that the Bb re-assembles and persists during oocyte growth until its function in mitochondrial and organelle quality control is complete.