intermediate filaments

Meaning: Intermediate filaments are cytoskeletal fibers between actin microfilaments (thinnest) and microtubules (thickest) in diameter. They are built from rope-like fibrous proteins that vary by cell type: keratin in epithelial cells, vimentin in connective tissue cells, neurofilaments in neurons, and lamins in the nuclear lamina. Their role is mechanical: resisting tension, holding organelles such as the nucleus in place, and anchoring cells at desmosomes and hemidesmosomes.

On the MCAT: Expect questions that match a cytoskeletal element to its protein, function, or cell type, or that compare all three. Microtubules (tubulin; kinesin and dynein; spindle, cilia, flagella) and microfilaments (actin; myosin; cytokinesis, muscle contraction, microvilli) handle transport and movement, while intermediate filaments provide structural support. In a drug or mutation experiment, disrupting them mainly weakens mechanical integrity, for example in skin cells under stretch, rather than blocking division or vesicle transport.

Common confusion: Students treat intermediate filaments as dynamic and motor-driven like the other two elements. Microtubules and actin are comparatively dynamic and serve as tracks for motor proteins; intermediate filaments are comparatively stable and are not generally used as motor tracks. Lamins are a notable exception to the stability rule: they disassemble during mitosis when the nuclear envelope breaks down.