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Microtubule Definition

plural: microtubules
mi·cro·tu·bule, mī’krō-tū’byūl

A cytoplasmic tubule made up of polymers of alpha- and beta-tubulin dimers, and serves as the structural component of cytoskeleton, cilia, and eukaryotic flagella

Etymology: Greek mīkrós (“small”). Synonym: actin filament.


Cytoskeleton is a cytoplasmic structure composed of protein filaments and microtubules in the cytoplasm, and has a role in controlling cell shape, maintaining intracellular organization, and in cell movement. In eukaryotes, there are three major types of cytoskeleton, namely (1) microfilaments, (2) microtubules, and (3) intermediate filaments. For an overview of the differences between them, see table below.

type of cytoskeleton Features Functions
Microfilaments helical polymer of actin sub-units
(e.g. actin)
Cell shape
Cell locomotion (via filopodia, pseudopodia, or lamellipodia)
Intracellular movement or transport
Cytokinesis (by aiding centrosomes at opposite poles)
Muscle contraction (with myosin filaments)
Cytoplasmic streaming
Microtubules tubular structure with a diameter of 25nm and length ranging from 200nm to 25μm; exhibits polarity; in cilia and flagella, 9+2 microtubular arrangement
(e.g. alpha-tubulin and beta-tubulin)
Intracellular shape
Cell locomotion (as axoneme of cilia and flagella)
Intracellular transport of organelles (e.g. mitochondria) via dyneins and kinesins
Spindle fiber formation
Intermediate filaments two anti-parallel helices or dimers of varying protein sub-units with diameters ranging from 8 to 12 nm
(e.g. vimentin (mesenchyme), glial fibrillary acidic protein (glial cells), neurofilament proteins (neuronal processes), keratins (epithelial cells), and nuclear lamins)
Cell shape (by bearing tension)
“Scaffolding” for cell and nucleus
Nuclear lamina formation
Anchor organelles
Cell-cell connections (when with proteins and desmosomes)


The microtubules are cytoplasmic tubules that serve as structural components of cytoskeleton, cilia, and eukaryotic flagella. In eukaryotes, a microtubule is made up of polymers of alpha– and beta-tubulin dimers. It is a tubular structure (i.e. a hollow tube) with diameter of 25 nm, length ranging from 200 nm to 25 μm, and wall thickness of 5 nm. The hollow space inside the microtubule is referred to as the lumen. Since a microtubule is made up of α-/β-tubulin dimers, it exhibits polarity as one end could be α-tubulin (- end) and the other end, β-tubulin (+ end). Microtubule is organized by a microtubule organizing centers (e.g. centrioles and basal bodies). Similar to other major cytoskeletons, the microtubules have proteins bound to them. Examples of proteins that bind to microtubules are motor proteins (e.g. kinesin and dynein), severing proteins (e.g. katanin), and other regulatory proteins.


There are three main subgroups of microtubules: the polar microtubules (those extending across the cell, as in from centrosome to centrosome), the astral microtubules (those that anchor the spindle poles to the cell membrane), and the kinetochore microtubules (those that extend from the centrosome to the kinetochore protein in the centromere of the chromosome).

Common biological reactions

The microtubule forms from the polymerization of two types of globular proteins (particularly α- and β-tubulin) forming linear filaments called protofilaments. Typically, a single microtubule would be a hollow structure comprised of about 13 protofilaments that associate laterally. The length of the microtubule varies. It could extend by the addition of α-/β-tubulin dimers at the end of the polymer. In prokaryotes though the microtubule is often comprised of five instead of 13.
In cilia and flagella, microtubules assemble to form a 9+2 arrangement.

Biological functions

Microtubules are cytoplasmic tubules that serves as the structural component of cytoskeleton, cilia, and eukaryotic flagella. Their functions are associated with providing intracellular shape, locomotion, and transport. Particularly in cellular division, the microtubules are a source of spindle fibers. They give rise to the spindle apparatus that plays an important role in moving and separating chromosomes during cell division. Microtubules are also involved in intracellular movement of organelles, secretory vesicles, and other cytoplasmic structures.

Related terms

See also

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