All in one - integrating cell polarity, meiosis, mitosis and mechanical forces in early oocyte differentiation in vertebrates.

While the differentiation of oocytes is key for embryonic development, and its investigation is crucial for advancing our understanding of human reproduction and fertility, many fundamental questions in oogenesis have been long standing. However, recent technical advances have led to several breakthroughs mainly in mice and zebrafish. Here I review these recent findings, including regulation and organization of the germline cyst, the mechanistics of chromosomal pairing, establishment of cell polarity, and formation of a universal mRNA-protein (mRNP) granule called the Balbiani body. I discuss common themes in oogenesis from frogs, fish and mouse and compare them to findings from C. elegans and Drosophila. The zebrafish juvenile ovary is an attractive model where these individual processes can be investigated, but also revealing how they are inter-coordinated in oocyte differentiation. A conserved cellular organizer was discovered in the zebrafish oocyte that seems to function at a nexus of oocyte differentiation. This organizer, termed the Meiotic Vegetal Center (MVC), is composed of the oocyte centrosome, and couples meiotic chromosomal pairing with oocyte polarization and Balbiani body formation. The MVC breaks the oocyte symmetry, is regulated by upstream mitotic division in the cyst and nucleates Balbiani body mRNPs prion-like aggregation downstream. These processes can shed new light on broad questions in biology, such as how mitosis contributes to cell polarity, and how prion aggregation which lead to neurodegenerative disease when awry, is regulated in a physiological context. Furthermore, novel cytoskeletal structures can unravel cytoplasmic mechanical functions in chromosomal pairing. Finally, together with recently developed tools, genome editing technology now enables a robust genetic analysis of these fundamental processes in the zebrafish, paving the way for a comprehensive cell and developmental view of vertebrate oogenesis.