Lysosome-Related Organelles Orchestrate Biogenic Crystal Formation in Pigment Cells

Here, we show that iridosomes - optically active, guanine crystal-forming organelles in zebrafish iridophores - constitute a previously unrecognized class of LROs. Using transcriptomic profiling of purified iridophores, CRISPR-Cas9 gene perturbation, and cryogenic transmission electron microscopy, we identify a conserved molecular program governing iridosome biogenesis.

Key LRO regulators, including rab32a, ap3M2, and hps5, are required for crystal formation, with gene disruptions leading to reduced organelle number and distorted crystal morphology. We also observe hallmark LRO features such as intraluminal vesicles and dynamic pH regulation during maturation. Cross-species transcriptomic analysis reveals that the LRO identity of iridosomes is conserved in vertebrates including fish and reptiles, suggesting ancient evolutionary roots.