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Mouse oocytes differentiate through organelle enrichment from sister cyst germ cells

Mouse oocytes receive sister cell factors Mammalian mature oocytes, the largest cells in the body, are capable of programming and reprogramming embryogenesis. Only about 20% of developing mouse germ cells become oocytes. Lei and Spradling show that organelles and the cytoplasm move through intercellular connections between sister germ cells in mouse fetal ovaries to enrich oocyte cytoplasm just before oocytes arrest to form the ovarian reserve (see the Perspective by Pepling). Similar transfers occur during oogenesis in many invertebrate and lower vertebrate species, possibly providing oocytes their special properties for programming embryo development. Science, this issue p. 95; see also p. 35 Sister germ cells provide organelles and cytoplasm to oocytes to augment their developmental potential. [Also see Perspective by Pepling] Oocytes differentiate in diverse species by receiving organelles and cytoplasm from sister germ cells while joined in germline cysts or syncytia. Mouse primordial germ cells form germline cysts, but the role of cysts in oogenesis is unknown. We find that mouse germ cells receive organelles from neighboring cyst cells and build a Balbiani body to become oocytes, whereas nurselike germ cells die. Organelle movement, Balbiani body formation, and oocyte fate determination are selectively blocked by low levels of microtubule-dependent transport inhibitors. Membrane breakdown within the cyst and an apoptosis-like process are associated with organelle transfer into the oocyte, events reminiscent of nurse cell dumping in Drosophila. We propose that cytoplasmic and organelle transport plays an evolutionarily conserved and functionally important role in mammalian oocyte differentiation.