Implications for cumulative and prolonged clinical improvement induced by cross‐linked hyaluronic acid: An in vivo biochemical/microscopic study in humans

Abstract

AbstractIn photoaged human skin, type I collagen fragmentation impairs dermal extracellular matrix (ECM) integrity, resulting in collapsed/contracted fibroblasts with reduced type I procollagen synthesis. Injections of cross‐linked hyaluronic acid (CL‐HA) reverse these deleterious changes. To investigate the time course and effects of biochemical changes induced by injected CL‐HA, particularly whether fibroblast activation leads to accumulation/deposition of dermal collagen, we injected CL‐HA into photoaged skin of human participants over 60 years‐old and performed biochemical/microscopic analyses of skin samples. Beginning 1 week post‐injection and lasting 6–9 months, fibroblasts exhibited activation, including increased immunostaining and gene expression of markers of type I collagen synthesis, such as heat shock protein 47 and components of the transforming growth factor‐β pathway. At 1 week post‐injection, multiphoton microscopy revealed elongation/stretching of fibroblasts, indicating enhanced dermal mechanical support. At 4 weeks, second‐harmonic generation microscopy revealed thick collagen bundles densely packed around pools of injected CL‐HA. At 12 months, accumulation of thick collagen bundles was observed and injected CL‐HA remained present in substantial amounts. Thus, by occupying space in the dermal ECM, injected CL‐HA rapidly and durably enhances mechanical support, stimulating fibroblast elongation and activation, which results in thick, densely packed type I collagen bundles accumulating as early as 4 weeks post‐injection and continuing for at least a year. These observations indicate that early and prolonged clinical improvement following CL‐HA injection results from space‐filling and collagen deposition. As type I collagen has an estimated half‐life of 15 years, our data provide the foundations for optimizing the timing/frequency of repeat CL‐HA injections.