Cilia are long, tubular organelles found on the surface of many eukaryotic cells. They have a complex structure and a mechanism allowing them to wave in a circular pattern or snap in a whiplike fashion.

Cilial action is used by single-celled organisms for locomotion and generally for moving fluids, while cilia that don't move are used for sensory input.

Cilia have many similarities to flagella in that they are hairlike extensions from a cell, protruding through the cell plasma membrane.

Differences of cilia vs. flagella include location, movement and length. A large number of cilia tend to be located over a wide area of the cell surface while flagella are either solitary or few in number.

Cilia move together, in a coordinated way, while flagella move independently. Cilia tend to be shorter than flagella.

Flagella are usually found at one end of the cell, and while they may be sensitive to temperature or certain substances, they are mainly used for cell movement. Cilia have several possible sensory functions, especially when part of nerve cells, and they may not move at all.

Cilia are found only in eukaryotes while flagella are found in both eukaryotic and prokaryotic cells.

Cilia in eukaryotic cells have a complicated tubular structure enclosed in a plasma membrane. The tubules are composed of of linear polymer proteins making up nine outer microtubule doublets placed symmetrically around a central pair of inner tubules.

The inner pair are two separate tubules while the outer nine doublets each share a common tubule wall.

The sets of 9 + 2 microtubules are arranged in a cylindrical structure called an axoneme and are attached to the cell at a part of the cilium called the basal body or kinetosome. The basal body is in turn anchored to the cytoplasmic side of the cell membrane. The microtubules are held in place by protein arms, spokes and links inside the cilia.

These protein structures give the cilia their stiffness and are an important part of their mobility system.

The motor protein dynein is found in the arms and spokes linking the microtubules, and it drives the movement of the cilia. The dynein molecules are attached to one of the microtubules through the arms and links.

They use energy from adenosine triphosphate (ATP) to move one of the other microtubules up and down. The variable sliding motion of the microtubules produces a bending motion.

Cilia come in two basic types, but each type can fulfill several cilial functions. Depending on their function, they have different characteristics and capabilities.

All cilia are either motile or non-motile, meaning they can move or not. Non-motile cilia are also referred to as primary cilia, and almost every eukaryotic cell has at least one. Motile cilia move, but their functions are varied, and only one type is locomotive in that its motion moves the associated cell.

The different types and functions are as follows:

• Primary cilia, chemical sensors: The cilia are stationary, but they sense the presence of substances such as proteins and send corresponding signals to cells such as kidney cells.
• Primary cilia, physical sensors: The cilia of these cells are sensitive to touch and movement. Such cilia are responsible for detecting sound in the inner ear.
• Primary cilia, signaling: The cilia detect cell signaling such as Hedgehog (Hh) signaling, a key factor in the development of mammalian cells and tissue.
• Motile cilia, locomotion: The cilia allow cells to move about in search of food and to avoid danger, especially in single-cell organisms such as the paramecium.
• Motile cilia, transportation: Cilia use their movement to promote the transport of fluid through a tube or channel as in the oviduct.
• Motile cilia, contaminant removal: Cilia use their motion to hand off contaminating particles and move them to the outside, such as in the respiratory system.

The cilia found on most cells are used as a way to interact with the surroundings and with other cells, whether through motion or sensory means. The different types of cilia help cells fulfill functions they would otherwise have trouble carrying out.

Since primary cilia don't have to move, their structure is simpler than that of other cilia. Instead of the 9 + 2 structure of motile cilia, they lack the two central pairs of microtubules and have a 9 + 0 structure. They don't need the dynein motor protein and they lack many of the arms, spokes and links associated with cilial movement.

Instead, their sensory capabilities often come from being nerve cell cilia and using nerve signaling functions to carry out their sensory tasks. Most eukaryotic cells have at least one of these primary or non-motile cilia.

If cilia or the cells associated with them are defective or absent, the lack of their specialized functions can result in serious diseases.

For example, cilia on kidney cells help kidney function, and problems with these cells cause polycystic kidney disease. Primary cilia in the eyes help cells detect light, and defects can cause blindness from a disease called retinitis pigmentosa. Other cilia on olfactory neurons are responsible for the sense of smell.

Specialized functions such as these are carried out by primary cilia throughout the body.

Cells with motile cilia can use the movement capabilities of their cilia in several ways. Their original purpose was to help single-cell organisms move, and they still play this role in primitive life forms such as ciliates.

When multicellular organisms evolved, cells with cilia were no longer needed for organism locomotion and took on other tasks.

Cilial motion has several characteristics that help make their movement useful. They typically beat in a coordinated back-and-forth fashion across several rows of cilia, making up an efficient transport mechanism.

Most cells involved in transport have a large number of cilia on one of their surfaces, making quick transport of significant volumes possible. While not moving the cells directly, they can help with the motion of other substances.

Typical examples are:

• Respiratory system: Cells with up to 200 cilia line parts of the respiratory system such as the trachea. Their coordinated wave motion transports mucus out of the respiratory tract, bringing any particles or dirt with it.
• Fallopian tubes: The beating of cilia in the walls of the fallopian tubes propels the ovum down the tube into the uterus where it becomes attached and grows. If the cilia are defective, the ovum doesn't enter the uterus and an ectopic pregnancy can result.
• Middle ear: Ciliated cells on the epithelium of the middle ear help with hearing development. Defects in these motile cilia can result in a disease called otitis media and can lead to hearing loss.

Motile cilia are found on the epithelium of many parts of the body, and although their function is sometimes not well understood, they assume critical roles in organism development and cell processes.

Their complex structure, the complicated internal sliding mechanism and their coordinated movement demonstrates that motion is a difficult biological function to realize, and a breakdown in their operation often results in disease for the organism.

Related cell biology content:

• Cell cycle
• Signal transduction
• Cell division
• Epithelial cells