Depressed scars, are scars caused by defects in the dermis and subcutaneous tissue of the skin. Acne, surgery, trauma, infection, chicken pox and smallpox are common causes of depressed scars. These causes lead to skin dermis and subcutaneous tissue defects, and the subsequent loss of collagen and elastin during the healing process will leave permanent depressed scars. The normal structure of human skin: depressed scars are composed of epidermis, dermis and subcutaneous tissue. The epidermis has no nerves or blood vessels. Dermis is located under the epidermis dermis contains nerves and blood vessels. The formation of scars, the elasticity and toughness of the skin depends entirely on the dermis, in which collagen, elastin, reticular fibers, hyaluronic acid, etc. are the main components that constitute the connective tissue of the skin and maintain the elasticity and toughness of the skin. 2. Mechanism of depressed scar formation: Acne, surgery, trauma, infection, chicken pox and smallpox are common causes of depressed scars. These causes lead to skin dermal layer and subcutaneous tissue defects, and the loss of collagen and elastin in the subsequent healing process will leave permanent depressed scars.