Cilia. Computer graphic of a cross-section through bristle-like extensions called cilia which protrude from the surface of a cell. This type of cell surface is found, for example, in the human windpipe (trachea). Each cilium contains a bundle of parallel microtubules (tiny pink flame-like structures seen at the tip of each cilium) which originate from 'basal bodies' lying just inside the cell cytoplasm. Cilia perform a beating motion by sliding adjacent microtubules over each other. The function of cilia varies with cell type; some protozoa use cilia for locomotion; respiratory tract cilia help to keep the air passages clean. Magnification: x10,000 at 6x4.5cm size.