MICROTUBULES: STRUCTURE, FUNCTIONS AND CLINICAL IMPORTANCE - BIOLOGY - 2025

The microtubules are shaped cellular structures that play key cylinder support - related functions, cell motility and cell division, among others. These filaments are present inside eukaryotic cells.

They are hollow and their internal diameter is in the order of 25 nm, while the external one measures 25 nm. The length varies between 200 nm and 25 µm. They are quite dynamic structures, with a defined polarity, capable of growing and shortening.

Structure and composition

Microtubules are made up of protein molecules. They are made from a protein called tubulin.

Tubulin is a dimer, its two components are α-tubulin and β-tubulin. The hollow cylinder is made up of thirteen chains of this dimer.

The ends of a microtubule are not the same. That is, there is a polarity of the filaments. One extreme is known as plus (+) and the other as minus (-).

The microtubule is not a static structure, the filaments can change size quickly. This growing or shortening process takes place mainly at the extreme; This process is called self-assembly. The dynamism of microtubules allows animal cells to change their shape.

There are exceptions. This polarity is indistinct in the microtubules inside the dendrites, in the neurons.

Microtubules are not homogeneously distributed in all cell forms. Its location depends mainly on the cell type and the state of it. For example, in some protozoan parasites, microtubules form an armor.

Likewise, when the cell is in interface, these filaments are dispersed in the cytoplasm. When the cell begins to divide, the microtubules begin to organize on the mitotic spindle.

Features

Cytoskeleton

The cytoskeleton is made up of a series of filaments, including microtubules, intermediate filaments, and microfilaments. As its name indicates, the cytoskeleton is in charge of supporting the cell, motility and regulation.

Microtubules associate with specialized proteins (MAPs) to fulfill their functions.

The cytoskeleton is particularly important in animal cells, since they lack a cell wall.

Mobility

Microtubules play a fundamental role in motor functions. They serve as a kind of clue for movement-related proteins to move. Similarly, microtubules are roadways and proteins are cars.

Specifically, kinesins and dynein are proteins found in the cytoplasm. These proteins bind to microtubules to carry out movements and allow the mobilization of materials throughout the cell space.

They carry vesicles and travel long distances through microtubules. They can also transport merchandise that is not in the vesicles.

Motor proteins have a kind of arms, and through changes in the shape of these molecules, movement can be carried out. This process is dependent on ATP.

Cellular division

Regarding cell division, they are essential for the proper and equitable distribution of chromosomes. The microtubules assemble and form the mitotic spindle.

When the nucleus divides, the microtubules carry and separate the chromosomes to the new nuclei.

Cilia and flagella

Microtubules are related to cellular structures that allow movement: cilia and flagella.

These appendages are shaped like thin whips and allow the cell to move in their environment. Microtubules promote the assembly of these cell extensions.

Cilia and flagella have an identical structure; however, the cilia are shorter (10 to 25 microns) and tend to work together. For movement, the applied force is parallel to the membrane. The cilia act like "paddles" that push the cell.

In contrast, the flagella are longer (50 to 70 microns) and the cell generally has one or two. The applied force is perpendicular to the membrane.

The cross-sectional view of these appendages presents a 9 + 2 arrangement. This nomenclature refers to the presence of 9 pairs of fused microtubules that surround a central, unfused pair.

Motor function is the product of the action of specialized proteins; dynein is one of these. Thanks to ATP, protein can change its shape and allow movement.

Hundreds of organisms use these structures to get around. Cilia and flagella are present in unicellular organisms, in spermatozoa and in small multicellular animals, among others. The basal body is the cellular organelle from which the cilia and flagella originate.

Centrioles

The centrioles are extremely similar to the basal bodies. These organelles are characteristic of eukaryotic cells, except for plant cells and certain protists.

These structures are barrel-shaped. Its diameter is 150 nm and its length is 300-500 nm. The microtubules in the centrioles are organized into three fused filaments.

The centrioles are located in a structure called the centrosome. Each centrosome is made up of two centrioles and a protein-rich matrix called the pericentriolar matrix. In this arrangement, the centrioles organize the microtubules.

The exact function of the centrioles and cell division is not yet known in detail. In certain experiments, the centrioles have been removed and said cell is capable of dividing without major inconvenience. The centrioles are responsible for forming the mitotic spindle: here the chromosomes are joined.

Plants

In plants, microtubules play an additional role in cell wall arrangement, helping to organize cellulose fibers. Likewise, they help cell division and expansion in plants.

Clinical significance and drugs

Cancer cells are characterized by high mitotic activity; thus, finding drugs that target microtubule assembly would help stop such growth.

There are a number of drugs responsible for destabilizing microtubules. Colcemide, colchicine, vincristine, and vinblastine prevent microtubule polymerization.

For example, colchicine is used to treat gout. The others are used in the treatment of malignant tumors.

References

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