In a centriole, each "arm" (a triplet microtubule) is attached from its A-tubule to the C-tubule of the adjacent arm. What is the type of fiber that connects each arm ("connecting fibrils" in the image below)?
Cross-sectional images of centrioles and basal bodies show a linker between the A- and C-tubules in many different organisms... Surprisingly, candidate molecules serving as constituents of these linkers have not yet been identified.
That's as of 2014. I don't have the time right now, but I'll try at some point to find if there are more recent developments.