Dermal Collagen
Dermal collagen is the fibrillar protein network that makes up most of the dry weight of the dermis and provides its tensile strength, dominated by type I with a minority of type III.
Collagen is roughly three quarters of the dry weight of the dermis. Type I is the overwhelming majority, with type III a much smaller share that runs proportionally higher in fetal and early granulation tissue. Each molecule is a triple helix built on a repeating glycine-X-Y motif, with proline and hydroxyproline common at X and Y; hydroxylation requires vitamin C, which is why ascorbate deficiency destabilises the helix. Fibroblasts secrete procollagen, propeptides are cleaved at both ends, and copper-dependent lysyl oxidase forms the crosslinks that give mature fibrils their strength.
Ultraviolet exposure activates AP-1 signalling and induces matrix metalloproteinase-1, the interstitial collagenase that cleaves type I collagen; the fragmented network then gives fibroblasts less mechanical tension to attach to, further reducing their synthetic output, a self-reinforcing loop. Topical tretinoin was shown to restore collagen formation in photodamaged human skin in a controlled study reported in the New England Journal of Medicine in 1993, one of the few interventions with biopsy-level evidence behind it.
That biopsy point is the decision. Procollagen propeptide or hydroxyproline in a skin biopsy measures collagen directly; wrinkle scores, elasticity ratios, and ultrasound density are downstream and confounded by hydration. Almost no cosmetic study takes biopsies, so almost no cosmetic product has direct evidence of altering dermal collagen.
The claims to distrust are topical and oral collagen as replacements. Intact collagen is around three hundred kilodaltons and cannot cross the stratum corneum, so topically it acts as a film-former. Ingested collagen is digested to amino acids and small peptides such as prolyl-hydroxyproline; whatever the merits of that, it is not delivery of collagen to the dermis.