The metabolism of the epidermis is that the basal keratinocytes gradually migrate upward as the cells differentiate, eventually dying to form a stratum corneum with no cell nucleus, which then falls off. It is generally believed that as the age increases, the basal layer and the spinous layer are structurally disordered. The epidermal dermal junction is flattened and the thickness of the epidermis is reduced. As the outermost barrier of the human body, the epidermis directly contacts the external environment and is most susceptible to various external factors. Epidermal aging is the easiest to visually reflect the effects of age and external factors on human aging.
Epidermal structure and biochemical changes in skin aging
In the aging skin epidermis, the variability of basal cell size, morphology and staining properties increases. In ten years, the junction is gradually flattened, the skin nails are reduced, and the thickness of the skin is reduced. Skin thickness changes were about 6.4% exposed and decreased or even faster in women. The thickness of the epidermis decreases with age. This change is most pronounced in the exposed area, including the extensor surface of the face, neck, hands, and forearm. Keratinocytes change shape as the skin ages, becoming shorter and fatter. The keratinocytes become larger due to the short periplasmic transition, and the renewal time of the aged epidermis increases. The proliferative activity of epidermal cells is degraded and the epidermis is thinned. Thereby the skin loses its elasticity and wrinkles.
Due to these morphological changes, the epidermal dermis is not tightly connected and is susceptible to external damage. The number of melanocytes decreased gradually after the age of 30, the proliferative power decreased, and the enzymatic activity of melanocytes decreased at a rate of 8% to 20% per decade. Although the skin is not easy to tan, the melanocytes tend to proliferate locally to form a color shift, especially in the exposed parts of the sun. Langerhans cells are also reduced, which reduces the immune function of the skin and is prone to infectious diseases.
Effect of skin aging on epidermal physiological function
- Epidermal cell turnover rate
In vitro cell culture demonstrated that aged keratinocytes have a low response to growth factors and limited proliferative capacity. During the 30-70 age period, the epidermal cell turnover rate was reduced by about 50%, and the stratum corneum barrier function was weakened.
- Barrier function
The transdermal absorption of certain substances varies with age. Although the stratum corneum of aging skin is intact, its barrier function has been impaired.
- Immune function
Cell-mediated immunity decline with aging has been recognized. The Langerhans cells that make up 3% to 4% of epidermal cells are reduced by 20% to 50% in the non-exposure area of the elderly, and are reduced more in the exposed area. The effect of aging on the immune function of the body leads to an increase in the susceptibility of the elderly to infection, and the increase in the incidence of thinking tumors.
Anti-aging cosmetics and epidermal aging
Skin physiology aging is characterized by thinning of the epidermis, dryness, relaxation, lack of elasticity, and involvement in fine lines. At the same time, under the influence of external factors, the above process will be accelerated. Based on the relationship between aging and epidermis, it is concluded that the normal metabolism of the epidermis is impaired, lipids are reduced, proteins and metabolic enzymes are disordered, inflammation is produced, and barrier damage occurs. Therefore, in the development of anti-aging related cosmetics, it may be considered to add raw materials related to skin barrier damage to better delay skin aging.
- Classical “rejuvenating agents” such as vitamin A and lactic acid are often used to solve the problem of slowing down the metabolic rate of epidermal cells, and the effect is affirmed by consumers.
- The maintenance of skin barrier is the first consideration for anti-aging cosmetics. How to achieve water and oil balance is the key to moisturizing.
There are several types of humectants:
- Emollients, lanolin, mineral oil, petroleum to increase corneal cell cohesion;
- Blocking agent, paraffin, soybean oil, propylene glycol, squalene, lanolin to reduce transdermal moisture loss (TEWL);
- Moisturizing substances, glycerin, urea, hyaluronic acid increase the horny layer hydration. The oxidation of the epidermis and the breaking of the antioxidant system seriously affect the skin aging process, and it is necessary to use anti-oxidant components for anti-aging cosmetics. Commonly used antioxidants are vitamin C, vitamin E, nicotinamide, a-lipoic acid, coenzyme Q10, green tea polyphenols, and the like. In recent years, the research on the mechanism of skin aging caused by epidermal immune dysfunction has been progressing rapidly. The anti-inflammatory and immune function of many plant extracts or Chinese herbal compound extracts has been verified and achieved good results in application.