Generic structure of cilia and flagella

Many protozoans, but also sperm cells, are propelled by cilia and flagella. Additionally, cilia on the epithelial cells of our air ways and in the ventricles of the brain remove contaminants. Hair cells in the inner ear carry one kinetocilium in addition to the sensory stereocilia (which have a different structure).

Cilia and flagella have the same basic structure, but cilia are shorter and tend to occur in greater number on a cell than flagella.

The travelling speeds of protozoans can be up to 1 mm/s, about 10 - 100 body lengths. Flagella from some insect sperm cells can reach a length of 2 mm.

The movement of cilia and flagella is complicated and can be studied with a high speed video camera mounted onto a microscope. These patterns ensure maximum resistance against the medium during the forward and minimum resistance during the backward stroke.

The basic structure of most flagella and cilia is a "9+2" arrangement of microtubules. 2 central tubulin singlets are surrounded by 9 peripheral tubulin doublets (see fig. 12.6).

Flagellum (cross section)

Flagellum (cross section)

Figure 12.6. Structure of flagella and cilia. 2 singlet and 9 doublet microtubules (purple) form the core of the structure (hence "9+2"-pattern). Each doublet is composed of an A-tubule with 13 and a B-tubule with 10 protofilaments. They are linked by nexin-bridges (green) and carry a radial spoke (cyan) and dynein arms (yellow). The two singlet-tubules in the centre are connected by a bridge (blue). The entire flagellum is about 250 nm in diameter in all species, but the length can vary between 1 pm in ciliates to 2 mm in insect sperm. The plus-end of the microtubules points to the distal end of the flagellum.

Figure 12.6. Structure of flagella and cilia. 2 singlet and 9 doublet microtubules (purple) form the core of the structure (hence "9+2"-pattern). Each doublet is composed of an A-tubule with 13 and a B-tubule with 10 protofilaments. They are linked by nexin-bridges (green) and carry a radial spoke (cyan) and dynein arms (yellow). The two singlet-tubules in the centre are connected by a bridge (blue). The entire flagellum is about 250 nm in diameter in all species, but the length can vary between 1 pm in ciliates to 2 mm in insect sperm. The plus-end of the microtubules points to the distal end of the flagellum.

Inside the cell each flagellum connects to a basal body, a structure resembling a centriole. They contain 9 triplet microtubules. In a triplet microtubule we have A-, B- and C-tubules (see fig. 11.4). A- and B-tubules continue into the flagellum, but the C-tubule terminates in a transition zone between basal body and flagellum.

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