Spatial gradients of lipase activity govern lipid droplet remodeling in the zebrafish embryo

Early zebrafish embryos rely on maternally supplied yolk lipids to fuel growth before the onset of feeding; yet, how these lipids are mobilized and redistributed remains poorly understood. Here, we combine live imaging with the solvatochromic dye Nile Red to map lipid composition in space and time through changes in the emission peak, a readout of local polarity. We show that lipid droplets (LDs) in the blastodisc are highly heterogeneous in size and polarity at the "one-cell stage" but progressively homogenize as development proceeds. LDs originate at the yolk-blastodisc interface, where localized lipase activity drives their biogenesis and initial composition. As LDs migrate toward the animal pole, their polarity increases, reflecting continuous lipolysis within a spatially confined metabolic zone. Smaller LDs display greater lipolytic efficiency than larger ones, linking droplet geometry to metabolic turnover. Inhibition of lipase activity disrupts both LD formation and lipid cycling, demonstrating that shared enzymatic machinery underlies droplet synthesis and degradation. Together, our findings reveal a spatially organized and developmentally regulated lipid metabolism in the zebrafish blastodisc, where local enzyme activity, droplet mechanics, and lipid composition are dynamically coupled. This live-imaging approach establishes a framework for studying lipid regulation in vertebrate development and disease. DIGEST SUMMARY: By visualizing lipids in living zebrafish embryos, this study reveals that lipid droplets are not passive fat stores but dynamic organelles whose size, composition, and behavior are shaped by local enzyme activity at the yolk-blastodisc interface, offering new insight into how developing cells control their energy reserves.