Undulipodium

Diagram of a cross-section of the axoneme microtubule array present in all undulipodia. 1-A. and 1-B. Tubulin dimer units. 2. Central pair inside the central sheath. 3. Inner and outer arm of dynein. 4. Radial spoke. 5. Nexin. 6. Plasma membrane.

Cross section of an axoneme

An undulipodium or 9+2 organelle is an intracellular projection of a eukaryotic cell containing a microtubule array. Both eukaryotic flagella and eukaryotic motile cilia are considered undulipodia.

Eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length.

Flagella use a whip-like action to create movement of the whole cell, such as the movement of sperm in the reproductive tract, and also create water movement as in the choanocytes of sponges.^{[citation needed]}

Motile (or secondary) cilia are more numerous, with multiple cilia per cell, move in a wave-like action, and are responsible for movement in organisms such as ciliates and Platyhelminthes, but also move extracellular substances in animals, such as the ciliary escalator found in the respiratory tract of mammals and the corona of rotifers.

Primary cilia function as sensory antennae, but are not undulipodia as primary cilia do not have the rotary movement mechanism found in motile cilia.

Undulipodia are an extension of the cell membrane containing both cytoplasm and a regular arrangement of microtubules known as an axoneme. At the base of the extension lies a structure called the kinetosome or basal body which is attached via motor proteins to the microtubules. The kinetosome mediates movement through a chemical reaction, causing the microtubules to slide against one another and the whole structure to bend.^{[citation needed]}

See also

• flagella
• cilia