More complex microtubule structures

Singlet microtubule, built from 13 protofilaments. In some cases singlet MTs can contain 11 or 15 protofilaments. Doublet or triplet microtubules are found in specialised structures (centrioles), the latter of which form the base unit of cilia and flagella. These contain an A tubule, which has 13 protofilaments and a B tubule (in doublets) or a B and C tubule (in triplets) which have either 10 or 9 protofilaments, respectively. The additional protofilaments come from the A or B tubule. Note that the lumen of the B and C tubule is distinct. Doublets are flexible but less so than singlets and triplets are completely inflexible - particularly because they are short.