A collagen orientation switch reshapes fin architecture

Fibrillar-collagen architecture is a fundamental determinant of vertebrate tissue morphology; yet, its organizational principles remain poorly understood because collagen structures are challenging to visualize in vivo. Actinotrichia, the fibrillar-collagen structures in fish fins, provide an ideal system: they are readily visualized and exhibit a simple, tractable spatial pattern optimal for studying collagen organization. Using high-sensitivity 3D imaging, we revealed the essential role of the fish-specific actinodin genes in establishing this pattern. Our findings demonstrate that actinodin1 and actinodin2 are indispensable for forming the fin's collagen architecture and for generating its thin, elongated morphology. Loss of actinodin function reorients collagen fibers and transforms the fin bud into a limb-bud-like configuration. A similar collagen organization is observed in developing amphibian limbs, which lack actinodin genes. These findings suggest that alterations in fibrillar-collagen patterning have contributed to morphological divergence and position actinodin mutants as promising models for investigating collagen matrix formation.