Microtubules

Microtubules are hollow tubes approximately 25 nanometers wide, composed of 13 protofilaments arranged side by side around a hollow core. Each protofilament consists of repeating alpha- and beta-tubulin heterodimers—globular protein subunits linked together in linear rows. This arrangement gives microtubules their characteristic cylindrical structure and polarity.

Microtubules are polar structures with distinct plus and minus ends. The plus end exposes beta-tubulin and is the fast-growing end where new dimers are typically added. The minus end exposes alpha-tubulin, is less dynamic, and is often anchored to a microtubule-organizing center. This polarity supports directional transport and organization within the cell.

Dynamic instability is the rapid growth and shrinkage of microtubules through polymerization and disassembly of tubulin molecules. This behavior is critical for remodeling the cytoskeleton during cell division and for axon extension in growing neurons. The ability to quickly assemble and disassemble allows cells to rapidly reorganize their internal structures in response to changing needs.

Motor proteins such as kinesin and dynein interact with microtubules to transport organelles and vesicles directionally through the cytoplasm. These proteins move along the microtubule track, carrying cargo like neurotransmitter-filled vesicles from the cell body down the axon to their destination. The polarity of microtubules ensures cargo moves in the correct direction.

Microtubule-associated proteins bind along the sides of microtubules to help control their stability and organization. These proteins enable microtubules to remain stable when needed, allowing the cell to maintain its shape and structural integrity. They also regulate the dynamic properties of microtubules to support various cellular functions.

In eukaryotic cells, microtubules form the core of cilia and flagella with a characteristic 9+2 arrangement—nine outer doublet microtubules surrounding two central microtubules. These structures extend outside the cell and move fluidly to propel cells through their environment, as in sperm, or to move fluid across cell surfaces, such as in lung epithelial cells.

Microtubules carry out multiple critical functions including providing cell structure and support, transporting organelles throughout the cytoplasm, enabling cell motility through cilia and flagella, and separating chromosomes during cell division. As the thickest cytoskeletal elements, microtubules work with microtubule-associated proteins to maintain eukaryotic cell organization and enable complex cellular processes.