26. Cytoskeleton. Actin and intermediate filaments.
Covers actin microfilaments first, with their diameter, polarity and treadmilling, their dynamic assembly and disassembly, and their roles in motility, cytokinesis, junctions and microvilli. It then describes intermediate filaments: their size, structure, high stability and locations, followed by a list of protein types such as keratins, vimentin, desmin, GFAP, neurofilaments and lamins, with their tissue distributions.
Topic 26 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 3 sections, about 375 words (3 min of reading), with 13 flashcards.
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Cytoskeleton. Actin and intermediary filaments.
Actin Filaments (Microfilaments)
Intermediate Filaments
Actin Filaments (Microfilaments)
Structure: These are the thinnest filaments in the cytoskeleton, with a diameter of 5-7 nm. They are helical, two-stranded polymers assembled from globular actin (G-actin) subunits, which in the presence of K+ and Mg2+ form filamentous F-actin.
Polarity: They are polarized, with G-actin subunits added rapidly at the (+) end and removed at the (–) end, a process called treadmilling.
Dynamics: They are highly dynamic structures, meaning they undergo continuous assembly and disassembly.
Location: Often concentrated near the cell membrane (cell cortex) and in cellular extensions like microvilli. In skeletal muscle, they are arranged with thick myosin filaments.
Functions:
Responsible for cellular motility and most contractile activity in cells.
Allow cells to change shape.
Involved in endocytosis, exocytosis, and cell migration.
Form contractile rings with myosin II that constrict to produce two cells at the end of mitosis (cytokinesis).
Help form cell junctions.
Support microvilli, increasing surface area for absorption.
Interact with myosin motors to transport cargo along the filaments.
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