Cytoplasmic components regulating cortical granule biology. (A) Schematic representation of a mouse oocyte. In mouse oocytes, Rab27a-associated CGs are synthesized in the central cytoplasm and translocated to the cortex through an actin-mediated mechanism driven by myosin Va. Additionally, Rab11a vesicles facilitate the rapid translocation of CGs to the cortex via a similar actin-mediated mechanism, powered by myosin Vb. Once at the cortex, CGs are anchored by the subcortical maternal complex (SCMC). Upon egg activation, cortical actin depolymerizes, enabling the release of CG contents into the perivitelline space. This process modifies the zona pellucida to establish a block against polyspermy. (B) Schematic representation of a zebrafish oocyte. CGs are synthesized in the central cytoplasm of the oocyte and translocated to the actin-rich cortex as the oocyte matures. This translocation is driven by a cytoplasmic flow (blue arrows), generated by the fusion of yolk globules. In the cortical region, microtubule (MT) asters, which form in a wave from the animal pole to the vegetal pole during oogenesis, work alongside Rab11 vesicles to ensure the proper exocytosis of CGs. This process is critical for chorion elevation following egg activation. For CGs to release their contents into the perivitelline space, cortical actin must first undergo depolymerization.
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