Final answer:
The cross-bridges that hold intermediate filaments together are predominantly composed of keratin, providing structural support to cells.
Step-by-step explanation:
The cross-bridges that hold intermediate filaments together in cells are composed of keratin. Intermediate filaments, which are thicker than actin filaments but thinner than microtubules, play a crucial role in maintaining cell integrity and structure. They consist of long fibrous subunits of keratin that intertwine and provide resistance to tension, thus reinforcing cell shape, anchoring organelles, and forming cell-to-cell junctions such as desmosomes. Amongst the options provided, keratin is the correct answer, as actin and myosin are involved in muscle contractions and are components of microfilaments, while collagen is a structural protein found in connective tissues but not in intermediate filaments.