The structure of mammalian sperm axoneme provides insights into motility and disease mechanisms.

Mammalian sperm are highly specialized cells consisting of a head containing paternal genome and a flagellar tail responsible for propulsion. At the core of the flagellum lies the axoneme-an evolutionarily conserved microtubule-based organelle essential for generating motility. Since the first visualization of the canonical "9 + 2" axonemal architecture by transmission electron microscopy (TEM) in the 1950s, advancements in cryo-electron microscopy (cryo-EM) have revealed unprecedented details of its structural organization and molecular composition. These breakthroughs have illuminated how axonemal elements-including doublet microtubules (DMTs), dynein arms, radial spokes (RSs), the central pair complex (CPC), and the nexin-dynein regulatory complex (N-DRC)-coordinate to generate bending motions that drive sperm motility. Moreover, structural defects in the axoneme are increasingly recognized as major contributors to male infertility. This review traces the historical development of axonemal-structural studies, highlights the latest high-resolution architectures of mammalian sperm axonemes, and examines how these structural insights have advanced our understanding of motility regulation and disease mechanisms.