The main structural protein in skin, tendons, ligaments, bone, and cartilage — what gives these tissues their tensile strength.The most abundant protein in the human body. Major collagen subtypes: Type I (skin, tendon, bone); Type II (cartilage); Type III (blood vessels, early wound healing). GHK-Cu, matrixyl (palmitoyl pentapeptide-4), and several injury recovery peptides have proposed mechanisms involving collagen synthesis stimulation or MMP regulation. The claim that a topical peptide "boosts collagen" requires specifying whether the evidence is from cell culture, animal models, or human skin biopsy — the difference matters enormously.