2.1.2.1　Pathogenesis

1.Ultraviolet can induce degration of skin collagen

（1）Reactive oxygen species（ROS）：Ultraviolet can cause an increase of ROS.Excessive ROS can cause oxidative damage to cells，which resultantly contributes to collagen breakdown.

（2）Mitogen-activated protein kinase（MAPK）：MAPK holds a key role in the regulation of cell growth and procollagen Ⅰ formation.Ultraviolet radiation upregulates the MAPK signal transduction pathway，which causes downstream signal transduction and the formation of activator protein 1（AP-1）.AP-1 can inhibit the synthesis of collagen and promotes its breakdown.

（3）Matrix metalloproteinases（MMPs）：The expression of metalloproteinases is increased，instigating the degradation of the dermal extracellular matrix，especially procollagens Ⅰ andⅢ.Collagen synthesis is also reduced.

2.Ultraviolet can lead to genetic and molecular changes of the cells in the skin(https://www.daowen.com)

Ultraviolet can damage deoxyribonucleic acid（DNA）and mitochondrial DNA（mtDNA）in direct and indirect ways.This causes telomere shortening in DNA and ATP generation reduction in mtDNA，which induce acceleration of skin aging.

3.Ultraviolet has paradoxical effects on blood vessels

Ultraviolet can induce the skin neovascularization.However，these new blood vessels are hyperpermeable，producing inflammatory factors that leak into the intercellular stroma and trigger cutaneous inflammation.This leads to extracellular matrix degradation and reduces dermal vasculature，accelerating photoaging.