Axoneme

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Axoneme

Numerous eukaryotic cells

Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
carry whip-like appendages (cilia or eukaryotic flagella) whose inner core consists of a cytoskeletal

Cytoskeleton

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Numerous eukaryotic cells

Cell (biology)

Cytoskeleton

The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within the cytoplasm. The cytoskeleton is present in all cells; it was once thought this structure was 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.

The building block of the axonmene is the microtubule

Microtubule

Microtubules are one of the components of the cytoskeleton. They have a diameter of 25 Nanometre and length varying from 200 nanometers to 25 micrometers....
; each axoneme is composed of several microtubules aligned in parallel

Parallel

From Greek language: pa???????? Parallel may refer to:...
. 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 organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s and protein complexes necessary for its function. The dynein

Dynein

Dynein is a motor protein in biological cells which converts the chemical energy contained in Adenosine triphosphate into the mechanical energy of movement....
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 axonmene 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....
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 axoneme structure in non-motile primary cilium 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.

Axoneme

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