A transient RNP compartment boosts translation and growth of mammalian oocytes

Mammalian oocytes undergo an extended growth phase, during which transcription gradually declines. To sustain meiotic divisions and early embryonic development, growing oocytes must accumulate maternal stores and tightly regulate translation. Using immunofluorescence, mass spectrometry, electron microscopy, RNA sequencing, and live ex vivo ovarian tissue-slice imaging, we identified a transient ribonucleoprotein-organelle compartment in growing mouse oocytes, which we named the Zollo body. Although it resembles the Balbiani body observed in nongrowing oocytes of many vertebrate species, the Zollo body arises during growth and shows nascent translation and phospho-mTOR enrichment. Pharmacological dissolution of the Zollo body arrests follicle and oocyte growth in cultured ovarian tissue slices. Thus, the Zollo body is a transient, translation-associated compartment required for normal follicle and oocyte growth.