Cell center (centrosome)
The cell center is a crucial organelle present in animal cells. Notably, it is absent in higher plants, lower fungi, and some protozoa.
Structural features The organelle consists of a centrosome and two centrioles (mother and daughter). The centrioles are always located strictly perpendicular to each other. Structurally, each centriole is a hollow cylinder. Its wall is formed by 9 triplets (each triplet containing three fused microtubules). A characteristic feature of the cell center is its ability to self-replicate.
Topography in the cell The localization of the cell center directly depends on the phase of the cell cycle:
- In resting (non-dividing) cells, the centrosome is located near the nucleus or adjacent to the Golgi apparatus.
- In dividing cells, the centrioles diverge and are located at opposite poles of the cell.
Main functions of the centrosome:
- Organization and assembly of cytoskeletal microtubules.
- Formation of the mitotic spindle apparatus.
- Ensuring chromosome segregation to the poles in anaphase.
- Participation in the formation of basal bodies, from which flagella and cilia develop.
Microtubules: structural basis of the cytoskeleton
Microtubules are extremely fine cylindrical structures with a diameter of 25 nm. Their walls consist of globular subunits of the protein tubulin, which are arranged in a spiral.
The functional complex of microtubules includes the tubulin subunits themselves, associated proteins, and transported particles. A specific feature of microtubules is their sensitivity to colchicine, an alkaloid capable of causing their destruction.
Biological role of microtubules:
- Supportive: form a rigid framework (cytoskeleton) of the cell.
- Structural: part of centrioles, spindle fibers, as well as cilia and flagella.
- Transport: set strict directions for the movement of intracellular components.
- Motor: during nuclear division, they pull chromosomes to the poles of the cell.
Clinical significance and pathologies
Oncology and pharmacology Microtubules play a critical role in mitosis. In modern pharmacology, drugs that inhibit the formation of the mitotic spindle and block chromosome segregation are widely used. Under the action of such drugs, the cell (including tumor cells) exits the cell cycle and triggers apoptosis—programmed cell death.
Disorders of motor organelles Since microtubules form the basis of cilia and flagella, their damage leads to severe diseases:
- Male infertility: occurs due to immobility of spermatozoa (disrupted flagellar structure).
- Chronic bronchitis: develops due to defects in the ciliary apparatus of the respiratory epithelium, which impairs bronchial clearance.
Actin cytoskeleton
Actin filaments are another key component of the cytoskeleton. Their normal polymerization and depolymerization are critical for cell viability. If the actin apparatus is exposed to damaging agents that disrupt this dynamics for a long time, the cell inevitably dies.
Actin is of particular importance in oncogenesis. In malignant tumor cells, there is a decrease in the expression of proteins that normally regulate actin assembly. This leads to a pronounced change in tumor cell shape. Moreover, a direct correlation has been revealed: specific reorganization of the actin cytoskeleton in transformed cells is directly related to their metastatic potential.
Briefly: answers to topical questions
Which motor proteins are associated with microtubules for intracellular transport?
Translocator proteins (motors) dyneins and kinesins are associated with microtubules for intracellular transport. Microtubules serve as "tracks": transport is carried out not inside the microtubules, but along their outer surface (in the peritubular space). Dyneins and kinesins move organelles and vesicles, consuming ATP energy.
What types of intermediate filaments are part of the cytoskeleton of tissue cells?
Intermediate filaments consist of fibrous proteins; their protein composition varies depending on the tissue type and serves as an important diagnostic sign. In the epithelium—keratins: these structures are often called tonofilaments, they participate in keratinization. In connective tissue, endothelium, and vascular smooth myocytes—vimentin. In muscle tissue, except vascular—desmin. In nerve cells—neurofilaments formed by specific proteins. In the nucleus—lamins. Intermediate filaments are used for metastasis diagnosis.
It is easy to remember the structure of the centriole by the "9×3" formula: the cylinder is assembled from 9 bundles, each containing exactly 3 fused microtubules (a triplet).
Class and exam questions
In which cells is the cell center absent?
The cell center (centrosome) is absent in the cells of higher plants, lower fungi, and some protozoan organisms.
How are microtubules related to the development of chronic bronchitis?
Microtubules form the framework of respiratory tract epithelial cilia. When damaged, the cilia stop moving, the respiratory tract is not cleared of mucus, leading to inflammation.
How does the cytoskeleton affect the ability of tumors to metastasize?
In malignant cells, the regulation of actin cytoskeleton assembly is altered. This changes the cell shape and provides it with the ability to metastasize.
What else to study on this topic
- Mechanism of microtubule destruction under the influence of colchicine
- Microtubule assembly and tubulin dynamic instability
- Structure of basal bodies of flagella and cilia
- Molecular mechanism of apoptosis upon mitotic spindle damage
- Regulatory proteins of the actin cytoskeleton and their suppression in tumors
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