Cilium

Not to be confused with Psyllium.

SEM micrograph of the cilia projecting from respiratory epithelium in the lungs

A cilium (plural cilia) is an organelle found in eukaryotic cells. Cilia are tail-like projections extending approximately 5–10 micrometers outwards from the cell body.

There are two types of cilia: motile cilia, which constantly beat in a single direction, and non-motile cilia, which typically serve as sensory organelles. Along with flagella, they make up a group of organelles known as undulipodia.

Types and distribution

Cilia are rare in plants, occurring most notably in cycads. Protozoans (ciliates) possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface. Some ciliates bear groups of cilia that are fused together into large mobile projections called cirri (singular, cirrus).

Larger eukaryotes, such as mammals, have motile cilia as well. Motile cilia are rarely found alone, usually present on a cell's surface in large numbers and beating in coordinated waves. In humans, for example, motile cilia are found in the lining of the trachea (windpipe), where they sweep mucus and dirt out of the lungs. In female mammals, the beating of cilia in the Fallopian tubes moves the ovum from the ovary to the uterus.

In contrast to motile cilia, non-motile cilia usually occur one per cell. The outer segment of the rod photoreceptor cell in the human eye is connected to its cell body with a specialized non-motile cilium. The dendritic knob of the olfactory neuron, where the odorant receptors are located, is also carrying non-motile cilia (about 10 cilia / dendritic knobs). Aside from these specialized examples, almost all mammalian cells have a single non-motile primary cilium. Although the primary cilium has historically been one of the oldest cellular organelles to be studied (at least since 1898), only a small group of devotees have followed it until its importance began to become clear in the late 1990. Recent findings regarding its physiological roles in chemical sensation, signal transduction, and control of cell growth, have led scientists to acknowledge its importance in cell function and the problems caused where its development has been disrupted or inhibited (as in polycystic kidney disease).

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External links

• Brief summary of importance of cilia to many organs in human physiology
• The Ciliary Proteome Web Page at Johns Hopkins