Microfilaments: Structure and Mechanisms of Motility
Microfilaments are formed by globular molecules of the protein actin, which assemble into a double helix 5–7 nm thick. In the cytoplasm, they form a dense network, and beneath the plasmalemma, they constitute a prominent cortical network. Actin filaments attach to the plasmalemma via accessory proteins such as α-actinin, and cross-link within the network nodes using filamin.
Cell movement and changes in cell shape occur through two main mechanisms:
- Changes in microfilament length — driven by actin polymerization or depolymerization (e.g., during pseudopodia formation).
- Interaction with myosin — sliding of actin filaments relative to myosin, similar to muscle contraction.
These mechanisms drive cell migration, phagocytosis and pinocytosis, axon outgrowth, and cytokinesis. A specialized organization of microfilaments is found in the microvilli of the intestine and renal tubules, where a core of about forty microfilaments operates via ATP-dependent sliding of filaments.
Intermediate Filaments and Their Tissue Specificity
Intermediate filaments have a diameter of 10 nm, occupying an intermediate position between microfilaments and microtubules. Their primary role is structural support. A unique feature of intermediate filaments is the strict tissue specificity of their protein composition, making them valuable diagnostic markers:
- Epithelium: protein keratin (forming structures known as tonofilaments).
- Connective tissue, endothelium, and smooth muscle cells: protein vimentin.
- Muscle tissue: protein desmin.
- Nervous tissue: specific proteins forming neurofilaments.
Inside the cell, cytoplasmic intermediate filaments run parallel to the nucleus and integrate into cell junctions. In the nucleus, a specialized class of intermediate filaments forms the nuclear lamina on the inner surface of the nuclear envelope, serving as a site for chromosome attachment.
Microtubules: Organization, Transport, and Division
Microtubules are hollow cylinders with an outer diameter of 24 nm, whose walls consist of 13 tubulin protofilaments. They radiate from the centrosome to the periphery and are stabilized by MAPs (microtubule-associated proteins). Microtubules exhibit pronounced polarity: the minus-end is typically anchored at satellites in the centrosome (microtubule-organizing center), while the plus-end is the growing zone driven by tubulin polymerization.
Key functions of microtubules:
- Maintaining the rigid framework of the cell.
- Intracellular transport: Microtubules act as tracks along which motor proteins dyneins and kinesins move, transporting vesicles and organelles utilizing ATP energy.
- Cell division: Formation of the mitotic spindle and segregation of chromosomes.
Pharmacological agents such as the alkaloid colchicine induce microtubule depolymerization, causing the cell to lose its shape and completely blocking mitosis.