Abstract

Embryogenesis demands an increasing nutrient supply, yet how maternal reserves are temporally delivered remains unclear. In egg-laying vertebrates, reserves are pre-deposited in the yolk, providing a closed system to dissect this problem. Using zebrafish, we quantified mobilization of the two major yolk stores—protein and lipid—across embryogenesis. Protein catabolism is continuous from cleavage to hatching, whereas bulk lipid release is increased during embryogenesis. This increase is coupled to zygotic activation of h nf4a/b , which promotes a yolk-syncytial lipoprotein assembly/transport. The acidic lipases Pla2g15 and Lipf contribute to yolk-granule lipid hydrolysis, while Mttp-dependent lipoprotein export mediates delivery of yolk-derived lipids to the embryo through the circulation. Inhibiting this lipid-delivery pathway has minimal early effect but impairs post-segmentation growth. Comparative transcriptomic analyses further reveal a similar Hnf4-lipoprotein expression program in the mammalian yolk sac. Together, these findings define a stage-tuned, transcriptionally gated strategy for maternal nutrient utilization during early development.