Numerous eukaryotic cells

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 carry whip-like appendages (cilia or eukaryotic flagella) whose inner core consists of a cytoskeletal

Cytoskeleton

The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...

 structure called the axoneme.
The axoneme serves as the "skeleton" of these organelles, both giving support to the structure and, in some cases, causing it to bend. Though distinctions of function and/or length may be made between cilia and flagella, the internal structure of the axoneme is common to both.

Structure

Inside cilia and flagella is a microtubule

Microtubule

Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

-based cytoskeleton

Cytoskeleton

The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...

 called the axoneme. The axoneme of primary cilia typically has a ring of nine outer microtubule doublets (called a 9+0 axoneme), and the axoneme of a motile cilium has two central microtubule doublets in addition to the nine outer doublets (called a 9+2 axoneme). The axonemal cytoskeleton acts as a scaffolding for various protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 complexes and provides binding sites for molecular motor

Molecular machine

A molecular machine, or nanomachine, is any discrete number of molecular components that produce quasi-mechanical movements in response to specific stimuli . The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level...

 proteins such as kinesin II

Kinesin

A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP . The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular...

, that help carry proteins up and down the microtubules.

Motile cilia

The building block of the axoneme is the microtubule

Microtubule

Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

; each axoneme is composed of several microtubules aligned in parallel

Parallel (geometry)

Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...

. More specifically, the microtubules are arranged in a characteristic pattern known as the “9x2 + 2," as shown in the image at right. Nine sets of "doublet" microtubules (a specialized structure consisting of two linked microtubules) form a ring around a "central pair" of single microtubules.

Besides the microtubules, the axoneme contains many protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s and protein complexes necessary for its function. The dynein

Dynein

Dynein is a motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell...

 arms, for example, are motor complexes which produce the force needed for bending. Each dynein arm is anchored to a doublet microtubule; by "walking" along an adjacent microtubule, the dynein motors can cause the microtubules to slide against each other. When this is carried out in a synchronized fashion, with the microtubules on one side of the axoneme being pulled 'down' and those on the other side pulled 'up,' the axoneme as a whole can bend back and forth. This process is responsible for ciliary/flagellar beating, as in the well-known example of the human sperm.

The radial spoke

Radial spoke

The radial spoke is a multi-unit protein structure found in the axonemes of eukaryotic cilia and flagella. Although experiments have determined the importance of the radial spoke in the proper function of these organelles, its structure and mode of action remain poorly understood.- Cellular...

 is another protein complex of the axoneme. Thought to be important in regulating the motion of the axoneme, this "T"-shaped complex projects from each set of outer doublets toward the central microtubules. The inter-doublet connections between adjacent microtubule pairs are termed nexin linkages

Nexin

Nexin is a proteinous inter-doublet linkage that prevents microtubules in the outer layer of axonemes from movement with respect to each other. Otherwise vesicular transport proteins such as dynein would dissolve the whole structure....

.

Non-motile/primary cilia

The axoneme structure in non-motile ("primary") cilia shows some variation from the canonical “9x2 + 2” anatomy. No dynein arms are found on the outer doublet microtubules, and there is no pair of central microtubule singlets. This organization of axoneme is referred as “9x2 + 0”. In addition, “9x2 + 1” axonemes, with only a single central microtubule, have been found to exist.
Primary cilia appear to serve sensory functions.

Clinical significance

Mutations or defects in primary cilia have been found to play a role in human diseases. These "ciliopathies" include Polycystic Kidney disease (PKD), retinitis pigmentosa

Retinitis pigmentosa

Retinitis pigmentosa is a group of genetic eye conditions that leads to incurable blindness. In the progression of symptoms for RP, night blindness generally precedes tunnel vision by years or even decades. Many people with RP do not become legally blind until their 40s or 50s and retain some...

, Bardet-Biedl syndrome and other developmental defects.