Cytoskeleton, cilia, flagella and centrioles

Like the iron rods laid inside a concrete roof, a cell has an inner framework of protein fibres, the cytoskeleton: microtubules, microfilaments and intermediate filaments. It gives mechanical support, helps the cell move, and keeps its shape.

Cilia and flagella are hair-like outgrowths of the cell membrane. Cilia are short and many; they beat like oars, moving the cell or the fluid around it. Flagella are longer and fewer, and move the cell. (Bacterial flagella are built quite differently.)

Inside each cilium or flagellum is a core, the axoneme, made of microtubules in a 9 + 2 pattern: nine pairs (doublets) around the edge and one central pair. The central pair sits in a central sheath, linked to the outer doublets by nine radial spokes; neighbouring doublets are joined by linkers. Each cilium or flagellum grows from a basal body at its base.

The centrosome contains two cylinder-shaped centrioles, lying at right angles to each other, in a cloud of pericentriolar material. Each centriole looks like a cartwheel: nine triplets of tubulin fibres around the edge, a central hub, and radial spokes linking them.

Centrioles do two jobs: they form the basal bodies of cilia and flagella, and in animal cells they form the spindle fibres that pull chromosomes apart during cell division.