microtubules

Microtubules are cylindrical tubes 20-25 nm in diameter, which are composed of linear polymers (protofilaments) of the globular protein tubulin. The tubulin molecules form heterodimers of alpha and beta tubulin and linear rows of tubulin dimers form the protofilaments. Microtubules act as scaffolding to maintain cell shape, and extend throughout the cytoplasm of eukaryotic cells. image_filamentous actin microtubules nuclei : image_filamentous actin & microtubules : image_microtubules nuclei endothelial tc : : image_filamentous actin microtubules nuclei fibroblast mouse : image_tubulin microtubules

In addition to this cytoskeletal role, microtubules act as cellular conveyer belts, employing special attachment proteins to move chromosomes, granules, endosomal vesicles, and organelles such as mitochondria through the cytoplasm.

Microtubules may work alone, or be joined with other proteins to form more complex structures such as cilia and flagella and centrioles. Within cilia and flagella, microtubules are assembled in a 9 + 2 arrangement. animation - inside flagellum. The tubulin is coupled to dynein arms to enable locomotion (spermocytes, protozoa) or movement of liquid over the cell (important in embryologic differentiation). image_spermatozoa (mouse) : image_sperm (Dv) : diagram - mechanism of ciliary motility : animation - cilia & flagella

Arranged in orthogonal paired tubes of 9 fibers, microtubules form centrioles during cell division, or basal bodies at the root of cilia and flagella. animation - spinning centriole pair : tour centriole : zoom in on centriole. During mitosis microtubules form spindle fibres along which chromosomes assemble then separate. animation - mitosis : animation ~ mitosis : link to animation - mitosis : kyrk animation _ mitosis : image_mitosis microtubules kinetochores DNA : image_mitosis : image_aberrant division mammalian cell : image_anaphase : image_golgi apparatus DNA microtubules dividing cells : image_mitotic spindle : image - spindle : animation - mitosis animation - meiosis : kyrk animation _ meisosis : Google cytoskeleton : Google microtubule

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