Vesicular fusion shapes yolk granule size and uniformity to support discoidal development

Organelle size is tightly regulated in cells, yet their functional importance for embryonic development remains largely unclear. Here we show that yolk granule size is essential for organizing early embryogenesis in zebrafish. Genetic disruption of vesicular fusion regulators leads to diverse alterations in yolk granule size and uniformity, resulting in defective blastodisc formation, compromised cleavage, and impaired maternal-zygotic transition.