The piRNA protein Asz1 is essential for germ cell and gonad development in zebrafish and exhibits differential necessities in distinct types of germ granules.

Germ cells are essential for fertility, embryogenesis, and reproduction. Germline development requires distinct types of germ granules, which contains RNA-protein (RNP) complexes, including germ plasm in embryos, piRNA granules in gonadal germ cells, and the Balbiani body (Bb) in oocytes. However, the regulation of RNP assemblies in zebrafish germline development are still poorly understood.

The Balbiani body is formed by microtubule-controlled molecular condensation of Buc in early oogenesis.

Vertebrate oocyte polarity has been observed for two centuries and is essential for embryonic axis formation and germline specification, yet its underlying mechanisms remain unknown. In oocyte polarization, critical RNA-protein (RNP) granules delivered to the oocyte's vegetal pole are stored by the Balbiani body (Bb), a membraneless organelle conserved across species from insects to humans. However, the mechanisms of Bb formation are still unclear.

Germ cell development: Anchoring and pulling forces shape germline cyst construction.

A new study reports a 'tug-of-war' mechanism in mouse germline cyst formation, where cell motility and intercellular bridges balance fragmentation and stabilization of the cyst. These dynamic and opposing forces that anchor and pull cells apart shape cyst construction, advancing our understanding of mammalian oogenesis and reproduction.

Storage of Functional Sperm at Room Temperature in Zebrafish and Medaka.

The longevity of sperm in teleost such as zebrafish and medaka is short when isolated even in saline-balanced solution at a physiological temperature. In contrast, some internal fertilizers exhibit the long-term storage of sperm, >10 months, in the female reproductive tract. This evidence implies that sperm in teleost possesses the ability to survive for a long time under suitable conditions; however, these conditions are not well understood.

Tools to analyze the organization and formation of the germline cyst in zebrafish oogenesis.

Oocytes develop in the germline cyst, a cellular organization in which germ cells are tightly interconnected and surrounded by somatic cells. The cyst produces oocytes for follicle formation and is a hub for essential processes in meiosis and oocyte differentiation. However, the formation and organization of the cyst, and their contribution to oocyte production in vertebrates remain unclear.

The many faces of the bouquet centrosome MTOC in meiosis and germ cell development.

Meiotic chromosomal pairing is facilitated by a conserved cytoskeletal organization. Telomeres associate with perinuclear microtubules via Sun/KASH complexes on the nuclear envelope (NE) and dynein. Telomere sliding on perinuclear microtubules contributes to chromosome homology searches and is essential for meiosis.

Stage Specific Transcriptomic Analysis and Database for Zebrafish Oogenesis.

Oogenesis produces functional eggs and is essential for fertility, embryonic development, and reproduction. The zebrafish ovary is an excellent model to study oogenesis in vertebrates, and recent studies have identified multiple regulators in oocyte development through forward genetic screens, as well as reverse genetics by CRISPR mutagenesis. However, many developmental steps in oogenesis, in zebrafish and other species, remain poorly understood, and their underlying mechanisms are unknown.

Control of meiotic chromosomal bouquet and germ cell morphogenesis by the zygotene cilium.

A hallmark of meiosis is chromosomal pairing, which requires telomere tethering and rotation on the nuclear envelope through microtubules, driving chromosome homology searches. Telomere pulling toward the centrosome forms the "zygotene chromosomal bouquet." Here, we identified the "zygotene cilium" in oocytes.

A proteomics approach identifies novel resident zebrafish Balbiani body proteins Cirbpa and Cirbpb.

The Balbiani body (Bb) is the first marker of polarity in vertebrate oocytes. The Bb is a conserved structure found in diverse animals including insects, fish, amphibians, and mammals. During early zebrafish oogenesis, the Bb assembles as a transient aggregate of mRNA, proteins, and membrane-bound organelles at the presumptive vegetal side of the oocyte.

Live and Time-Lapse Imaging of Early Oogenesis and Meiotic Chromosomal Dynamics in Cultured Juvenile Zebrafish Ovaries.

Oocyte production is crucial for sexual reproduction. Recent findings in zebrafish and other established model organisms emphasize that the early steps of oogenesis involve the coordination of simultaneous and tightly sequential processes across cellular compartments and between sister cells. To fully understand the mechanistic framework of these coordinated processes, cellular and morphological analysis in high temporal resolution is required.

New roles for Wnt and BMP signaling in neural anteroposterior patterning.

During amphibian development, neural patterning occurs via a two-step process. Spemann's organizer secretes BMP antagonists that induce anterior neural tissue. A subsequent caudalizing step re-specifies anterior fated cells to posterior fates such as hindbrain and spinal cord.

Coordination of cellular differentiation, polarity, mitosis and meiosis - New findings from early 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.

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.

Methods for the analysis of early oogenesis in Zebrafish.

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.

Localization in Oogenesis of Maternal Regulators of Embryonic Development.

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.

Oocyte Polarization Is Coupled to the Chromosomal Bouquet, a Conserved Polarized Nuclear Configuration in Meiosis.

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.

A hindbrain-repressive Wnt3a/Meis3/Tsh1 circuit promotes neuronal differentiation and coordinates tissue maturation.

During development, early inducing programs must later be counterbalanced for coordinated tissue maturation. In Xenopus laevis embryos, activation of the Meis3 transcription factor by a mesodermal Wnt3a signal lies at the core of the hindbrain developmental program. We now identify a hindbrain restricting circuit, surprisingly comprising the hindbrain inducers Wnt3a and Meis3, and Tsh1 protein.

Focal adhesion kinase protein regulates Wnt3a gene expression to control cell fate specification in the developing neural plate.

Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase protein localized to regions called focal adhesions, which are contact points between cells and the extracellular matrix. FAK protein acts as a scaffold to transfer adhesion-dependent and growth factor signals into the cell. Increased FAK expression is linked to aggressive metastatic and invasive tumors.

Mesodermal Wnt signaling organizes the neural plate via Meis3.

In 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.

Xenopus Meis3 protein lies at a nexus downstream to Zic1 and Pax3 proteins, regulating multiple cell-fates during early nervous system development.

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.