Widely in animals and plants, gamete polarization lays the foundations for early embryonic development. However, while vertebrate oocyte polarity has been observed for over two centuries, how it is generated is unknown. In most vertebrates, including zebrafish, oocytes are polarized along an animal-vegetal axis which is established during oogenesis and is essential for embryonic development. RNA-protein (RNP) granules of embryonic patterning factors, including dorsal- and germline fate determinants localize to the oocyte and embryo vegetal pole, from which they later establish the global embryonic body axes and the germline lineage.
Those factors localize to the vegetal pole during oogenesis via a membraneless organelle, called the Balbiani body (Bb), which forms close to the oocyte nucleus and then translocates to the future vegetal pole. The single known essential protein for Bb formation in any species, is the zebrafish Bucky ball (Buc) protein. Loss of the Bb in buc mutants, results in radially symmetrical eggs and early embryonic lethality.
The Bb is conserved in oocytes from insects to humans, and forms in equivalent stages of oogenesis, exhibiting similar dynamics. The human Bb is similar to the zebrafish Bb in morphology, content of organelles, and cellular and developmental dynamics, but its functions in oogenesis are unclear. In mice, the Bb is associated with proper formation of the primordial follicle, suggesting a critical role, but molecular and genetic understanding of the mammalian Bb is lacking. Despite the Bb: 1) critical importance for oogenesis, embryogenesis, and reproduction, 2) wide conservation, and 3) discovery already in 1845, mechanisms of Bb formation are still unknown.
We answered this two centuries-old question by deciphering the formation of the Bb, which provides insight into the origins of embryonic polarity in the early oocyte. We demonstrated that the Bb forms by molecular condensation, and resolved a multi-step regulation by microtubules over the complete condensation process, from initial seeding to ripening of the mature compartment (Kar, Deis, et al., 2024, biorxiv): first by dynein mediated trafficking of early Bb condensing granules, then by scaffolding condensed granules likely by serving as molecular crowding agents, and finally by caging the mature condensate to prevent its over-growth and shape distortion.
Our work highlights a paradigm for cellular control over phase-separation self-assembly, and proposes a new framework for physiological and pathological condensation. By studying our repository of identified novel Buc and Bb regulators, each offering critical insight into various facets of condensate biology, we are working to construct mechanisms of molecular condensation and oocyte polarization in-vivo.
