The centriole of Tribolium spermatids has doublet microtubules. (a) High-pressure freezing (HPF), freeze substitution (FS) and serial section TEM analysis of Tribolium golfclub/S-shape spermatid showing the transition from a centriole lacking central microtubules to an axoneme with central microtubules. Abbreviations: Ax, axoneme; C, centriole; M, mitochondria; N, nucleus. (b) Magnification of axoneme and centriole cross-sections. Yellow arrowheads: the microtubules in the doublet microtubules; green line: accessory microtubule. (c) Model of the axoneme-attached centriole. The centriole is made of doublet microtubules labelled A and B in ninefold symmetry, which is different than Drosophila's sperm's axoneme-attached centriole's triplets, labelled A, B and C.