Abstract
Eukaryotic flagella are complex cellular extensions involved in many human diseases gathered under the term ciliopathies. Currently, detailed insights on flagellar structure come from studies on protozoa. Here, cryo-electron tomography (cryo-ET) of intact human spermatozoon tails showed a variable number of microtubules in the singlet region. Inside their lumen, a novel left-handed interrupted helix which extends several micrometers at their plus ends was discovered. This structure was named Tail Axoneme Intra-Lumenal Spiral (TAILS) and binds directly to 11 protofilaments on the internal microtubule wall, coaxial with the surrounding microtubule lattice. It leaves a gap over the microtubule seam, which was directly visualized in both singlet and doublet microtubules. We suggest that TAILS may stabilize microtubules, enable rapid swimming or play a role in controlling the swimming direction of spermatozoa.