Final answer:
The interstitial fluid and extracellular matrix contain fibrous proteins such as collagen, reticular, and elastin fibers which provide strength, support, and elasticity to tissues, and are essential for the structure and function of connective tissues.
Step-by-step explanation:
The fibrous proteins found in the interstitial fluid and extracellular matrix include collagen fibers, reticular fibers, and elastin fibers. Collagen fibers are particularly abundant, making up about 25% of the total protein content in mammals and providing strength and flexibility to connective tissues. Reticular fibers form a supportive mesh that aids in the structure of organs, and elastin fibers contribute elasticity, allowing tissues to resume their shape after stretching or contracting. Additionally, these protein fibers are associated with other molecules like proteoglycans that contribute to the bulk of the extracellular matrix, and proteins such as fibronectin, which play a role in cell adhesion.