Mouse germline cysts contain a fusome that mediates oocyte development

Mouse female primordial germ cells undergo five synchronous, incomplete mitotic divisions, and send each resulting germline cyst into meiosis to fragment and produce 4-6 oocytes and 24-26 supportive nurse cells. How oocytes are specified, linked appropriately to nurse cells and acquire high quality organelles and cytoplasm remains unclear. We show that mouse cysts develop an asymmetric Golgi, ER and microtubule-associated structure, Visham, similar to the oocyte-determining fusome in Drosophila cysts. Starting as the EMA granule, Visham distributes asymmetrically in cyst cells along with Pard3, and enriches in future oocytes. Transient spindle remnants containing stable acetylated microtubules link early cyst cells; a gap in these microtubules beginning at the 8-cell stage suggests cysts are programmed to fragment into six-cell groups. Visham also acts during meiosis to mediate Pard3-dependent polarity, Xbp1-dependent adaptive UPR, and organelle rejuvenation. In Dazl mutants, cysts still form, but normal Visham behavior, polarity, rejuvenation and oocyte production are blocked.