Centrin-POC5 inner scaffold provides distal centriole integrity for sperm flagellar assembly

Centrioles undergo marked transformations during spermatogenesis that are essential for sperm motility and male fertility. Despite their importance, the molecular mechanisms and ultrastructural dynamics underlying these transformations remain largely unknown. Here, we apply ultrastructure expansion microscopy and reveal previously unrecognized centriolar architectural changes in mouse male germ cells, including geometry switching between the two centrioles and stage-specific removal of distal tip proteins such as centrin and SFI1. We further identify the centrin-POC5 inner scaffold as a key structure selectively augmented at the distal centriole, which directly forms and anchors the flagellum. Functional analyses of <i>Poc5</i> knockout mice demonstrate that this inner scaffold is essential for distal centriole integrity and flagellar assembly in spermatids but dispensable in somatic cells and spermatocytes. Our findings provide a spatiotemporal molecular atlas of centriole remodeling during spermatogenesis and uncover the critical physiological role of the centriolar centrin-POC5 inner scaffold in mammalian reproduction.