An unbranched tubule identified by electron microscopy in a wide variety of cells. Microtubule numbers vary considerably in different kinds of cell and through the life of a cell. They may either be arranged in an orderly manner or randomly distributed through the cytoplasm. Their outer diameter is about 25 nm and they have an apparently hollow core, 4 nm in diameter. The walls are composed of subunits of a protein, tubulin, which are arranged linearly to form protofilaments. Thirteen protofilaments arranged in a cylinder form the wall of each microtubule. Microtubules form the axoneme fibres of *flagella
and cilia and the *spindle
fibres in dividing cells. They are also thought to organize the microfibrils of cell walls and may be involved in the transport of materials within and between cells, e.g. through sieve tubes.