cells

It turns out that not all the hairlike cilia projecting from the surfaces of many cells in the human body are equal--there are the myriad ones for sweeping, swimming and other functions, and then there is the until recently mysterious primary cilium.

Nearly all human cells contain these numerous microscopic projections. The more abundant variety of cilia are motile; they act like oars, paddling in coordinated waves to help propel cells through fluid, or to sweep material across cellular surfaces (as in the respiratory system, where millions of cilia lining the airways help to expel mucus, dead cells and other bodily debris). By contrast, cells also contain a single, nonmotile cilium known as the primary cilium. Its presence on cells has been known for more than a century, but many believed it was a functionless evolutionary remnant.