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An undulipodium or undulopodium (Greek: "swinging foot"; plural undulipodia), or a 9+2 organelle is a motile filamentous extracellular projection of eukaryotic cells.It is basically synonymous to flagella and cilia which are differing terms for similar molecular structures used on different types of cells, and usually correspond to different waveforms.
The latter formation is commonly referred to as a "9+2" arrangement, wherein each doublet is connected to another by the protein dynein. As both flagella and cilia are structural components of the cell, and are maintained by microtubules, they can be considered part of the cytoskeleton. There are two types of cilia: motile and non-motile cilia.
The building-block of the axoneme is the microtubule; each axoneme is composed of several microtubules aligned in a characteristic pattern known as the 9+2 axoneme 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 ...
Cilia also can change structure when introduced to hot temperatures and become sharp. They are present in large numbers on each cell and move relatively slowly, making them intermediate between motile and primary cilia. In addition to 9+0 cilia that are mobile, there are also solitary 9+2 cilia that stay immobile found in hair cells. [39]
Cilia Structure. Primary cilia are found to be formed when a cell exits the cell cycle. [2] Cilia consist of four main compartments: the basal body at the base, the transition zone, the axenome which is an arrangement of nine doublet microtubules and considered to be the core of the cilium, and the ciliary membrane. [2]
Microtubule arrangement in a 9+2 axoneme of bronchiolar cilia. Microtubule-organizing centers function as the site where microtubule formation begins, as well as a location where free-ends of microtubules attract to. [2] Within the cells, microtubule-organizing centers can take on many different forms.
The flagellar axoneme itself is a bundle of nine pairs of microtubule doublets surrounding two central microtubules, termed the 9+2 axoneme, [10] and cross-linking dynein motors, powered by ATP hydrolysis, perform mechanical work by promoting the relative sliding of filaments, resulting in bending deformations.
Eukaryotic flagella and motile cilia are identical in structure, but have different lengths, waveforms, and functions. Primary cilia are immotile, and have a structurally different 9+0 axoneme rather than the 9+2 axoneme found in both flagella and motile cilia.