The source of symmetry breaking in vertebrate oocytes is unknown. Animal—vegetal oocyte polarity is established by the Balbiani body (Bb), a conserved structure found in all animals examined that contains an aggregate of specific mRNAs, proteins, and organelles. The Bb specifies the oocyte vegetal pole, which is key to forming the embryonic body axes as well as the germline in most vertebrates. How Bb formation is regulated and how its asymmetric position is established are unknown. Using quantitative image analysis, we trace oocyte symmetry breaking in zebrafish to a nuclear asymmetry at the onset of meiosis called the chromosomal bouquet. The bouquet is a universal feature of meiosis where all telomeres cluster to one pole on the nuclear envelope, facilitating chromosomal pairing and meiotic recombination. We show that Bb precursor components first localize with the centrosome to the cytoplasm adjacent to the telomere cluster of the bouquet. They then aggregate around the centrosome in a specialized nuclear cleft that we identified, assembling the early Bb. We show that the bouquet nuclear events and the cytoplasmic Bb precursor localization are mechanistically coordinated by microtubules. Thus the animal—vegetal axis of the oocyte is aligned to the nuclear axis of the bouquet. We further show that the symmetry breaking events lay upstream to the only known regulator of Bb formation, the Bucky ball protein. Our findings link two universal features of oogenesis, the Bb and the chromosomal bouquet, to oocyte polarization. We propose that a meiotic—vegetal center couples meiosis and oocyte patterning. Our findings reveal a novel mode of cellular polarization in meiotic cells whereby cellular and nuclear polarity are aligned. We further reveal that in zygotene nests, intercellular cytoplasmic bridges remain between oocytes and that the position of the cytoplasmic bridge coincides with the location of the centrosome meiotic—vegetal organizing center. These results suggest that centrosome positioning is set by the last mitotic oogonial division plane. Thus, oocytes are polarized in two steps: first, mitotic divisions preset the centrosome with no obvious polarization yet, then the meiotic—vegetal center forms at zygotene bouquet stages, when symmetry is, in effect, broken.

SUMMARYIn vertebrates, canonical Wnt signaling controls posterior neural cell lineage specification. Although Wnt signaling to the neural plate is sufficient for posterior identity, the source and timing of this activity remain uncertain. Furthermore, crucial molecular targets of this activity have not been defined. Here, we identify the endogenous Wnt activity and its role in controlling an essential downstream transcription factor, Meis3. Wnt3a is expressed in a specialized mesodermal domain, the paraxial dorsolateral mesoderm, which signals to overlying neuroectoderm. Loss of zygotic Wnt3a in this region does not alter mesoderm cell fates, but blocks Meis3 expression in the neuroectoderm, triggering the loss of posterior neural fates. Ectopic Meis3 protein expression is sufficient to rescue this phenotype. Moreover, Wnt3a induction of the posterior nervous system requires functional Meis3 in the neural plate. Using ChIP and promoter analysis, we show that Meis3 is a direct target of Wnt/-catenin signaling. This suggests a new model for neural anteroposterior patterning, in which Wnt3a from the paraxial mesoderm induces posterior cell fates via direct activation of a crucial transcription factor in the overlying neural plate.

Oocyte differentiation is a highly dynamic and intricate developmental process whose mechanistic understanding advances female reproduction, fertility, and ovarian cancer biology. Despite the many attributes of the zebrafish model, it has yet to be fully exploited for the investigation of early oocyte differentiation and ovarian development. This is partly because the properties of the adult zebrafish ovary make it technically challenging to access early stage oocytes. As a result, characterization of these stages has been lacking and tools for their analysis have been insufficient. To overcome these technical hurdles, we took advantage of the juvenile zebrafish ovary, where early stage oocytes can readily be found in high numbers and progress in a predictable manner. We characterized the earliest stages of oocyte differentiation and ovarian development and defined accurate staging criteria. We further developed protocols for quantitative microscopy, live time-lapse imaging, ovarian culture, and isolation of stage-specific oocytes for biochemical analysis. These methods have recently provided us with an unprecedented view of early oogenesis, allowing us to study formation of the Balbiani body, a universal oocyte granule that is associated with oocyte survival in mice and required for oocyte and egg polarity in fish and frogs. Despite its tremendous developmental significance, the Bb has been little investigated and how it forms was unknown in any species for over two centuries. We were able to trace Balbiani body formation and oocyte symmetry breaking to the onset of meiosis. Through this investigation we revealed novel cytoskeletal structures in oocytes and the contribution of specialized cellular organization to differentiation. Overall, the juvenile zebrafish ovary arises as an exciting model for studies of cell and developmental biology. We review these and other recent advances in vertebrate oogenesis in an accompanying manuscript in this issue of Developmental Biology. Here, we describe the protocols for ovarian investigation that we developed in the zebrafish, including all experimental steps that will easily allow others to reproduce such analysis. This juvenile ovary toolbox also contributes to establishing the zebrafish as a model for post-larval developmental stages.

FAK is linked to aggressive tumors, but its normal function is not clear. FAK knockdown early in Xenopus development anteriorizes the embryo via a loss of Wnt signaling. Wnt3a expression is FAK dependent in both embryos and human breast cancer cells, suggesting that a FAK–Wnt linkage is highly conserved.

In Xenopus embryos, XMeis3 protein activity is required for normal hindbrain formation. Our results show that XMeis3 protein knock down also causes a loss of primary neuron and neural crest cell lineages, without altering expression of Zic, Sox or Pax3 genes. Knock down or inhibition of the Pax3, Zic1 or Zic5 protein activities extinguishes embryonic expression of the XMeis3 gene, as well as triggering the loss of hindbrain, neural crest and primary neuron cell fates. Ectopic XMeis3 expression can rescue the Zic knock down phenotype. HoxD1 is an XMeis3 direct-target gene, and ectopic HoxD1 expression rescues cell fate losses in either XMeis3 or Zic protein knock down embryos. FGF3 and FGF8 are direct target genes of XMeis3 protein and their expression is lost in XMeis3 morphant embryos. In the genetic cascade controlling embryonic neural cell specification, XMeis3 lies below general-neuralizing, but upstream of FGF and regional-specific genes. Thus, XMeis3 protein is positioned at a key regulatory point, simultaneously regulating multiple neural cell fates during early vertebrate nervous system development.

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.

A mechanistic dissection of early oocyte differentiation in vertebrates is key to advancing our knowledge of germline development, reproductive biology, the regulation of meiosis, and all of their associated disorders. Recent advances in the field include breakthroughs in the identification of germline stem cells in Medaka, in the cellular architecture of the germline cyst in mice, in a mechanistic dissection of chromosomal pairing and bouquet formation in meiosis in mice, in tracing oocyte symmetry breaking to the chromosomal bouquet of meiosis in zebrafish, and in the biology of the Balbiani body, a universal oocyte granule. Many of the major events in early oogenesis are universally conserved, and some are co-opted for species-specific needs. The chromosomal events of meiosis are of tremendous consequence to gamete formation and have been extensively studied. New light is now being shed on other aspects of early oocyte differentiation, which were traditionally considered outside the scope of meiosis, and their coordination with meiotic events. The emerging theme is of meiosis as a common groundwork for coordinating multifaceted processes of oocyte differentiation. In an accompanying manuscript we describe methods that allowed for investigations in the zebrafish ovary to contribute to these breakthroughs. Here, we review these advances mostly from the zebrafish and mouse. We discuss oogenesis concepts across established model organisms, and construct an inclusive paradigm for early oocyte differentiation in vertebrates.

Cell polarity generates intracellular asymmetries and functional regionalization in tissues and morphogenetic processes. Cell polarity in development often relies on mechanisms of RNA localization to specific subcellular domains to define the identity of future developing tissues. The totipotent egg of most animals illustrates in a grand way the importance of cell polarity and RNA localization in regulating multiple crucial developmental events. The polarization of the egg arises during its development in oogenesis. RNAs localize asymmetrically in the early oocyte defining its animal-vegetal (AV) axis, which upon further elaboration in mid- and late-oogenesis stages produces a mature egg with specific localized factors along its AV axis. These localized factors will define the future anterior-posterior (AP) and dorsal-ventral (DV) axes of the embryo. Furthermore, AV polarity confines germ cell determinants to the vegetal pole, from where they redistribute to the cleavage furrows of the 2- and 4-cell stage embryo, ultimately specifying the primordial germ cells (PGCs). The sperm entry region during fertilization is also defined by the AV axis. In frogs and fish, sperm enters through the animal pole, similar to the mouse where it enters predominantly in the animal half. Thus, AV polarity establishment and RNA localization are involved in all the major events of early embryonic development. In this chapter, we will review the RNA localization mechanisms in vertebrate oocytes that are key to embryonic patterning, referring to some of the groundbreaking studies in frog oocytes and incorporating the current genetic evidence from the zebrafish.