Cilia (singular, Cilium)
Cilia are very small, hair-like structures that are found in or on many specialized cell types (for example, photoreceptors and ear, kidney, and lung cells). A mutation in a gene whose protein helps maintain the structure of these "hairs" can cause diseases with symptoms in one or more affected body systems, creating a syndrome. For example, some Usher syndromes involve vision and hearing problems, caused by altered cilia proteins found in both photoreceptors and ear cochlear cells.
There are two types of cilia: motile cilia, which constantly beat in a single direction, and non-motile cilia, which typically serve as sensors. Motile cilia are rarely found alone and are usually present on a cell's surface in large numbers where they beat 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 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 hair-like cells that detect sound in the ear's cochlea can be considered to be cilia-like (called stereocilia). In addition, a defect of the cilium of a renal (kidney) tube cell can lead to polycystic kidney disease (PKD). In a number of retinal degenerative diseases, mutations in structural proteins of the cilia can affect both eyes and other body organs. For instance, symptoms of Bardet-Biedl syndrome include problems with vision (photoreceptors) and kidney function, while Usher syndrome symptoms include vision problems (photoreceptors) and hearing loss (hair cells).