1. When the water content in the skin is too low, cracks and fissures will appear. This is because water is the main plasticizer in the skin. If the skin barrier function is disrupted, resulting in increased water loss through the epidermis, the skin becomes dry. In general, stain removers, acetone, hot water, frequent air travel, friction from clothing, pollution, and air conditioning can disrupt the skin’s natural barrier, leading to increased loss of trans-epidermal water and dry skin. When the skin is severely dehydrated, the stratum corneum hardens to form cracks, which in turn form fissures, and the skin becomes irritable, inflamed and itchy. 2. The barrier of the epidermis is like a mortar and brick wall structure – the cells of the stratum corneum are like a brick wall, while the lipids outside the cells constitute the mortar. If the barrier function is damaged, the skin’s ability to resist external pathogens, irritants and allergens is greatly reduced, and infectious and allergic diseases are easily formed. In areas where sebaceous glands are relatively sparsely distributed, such as the extremities and trunk, the skin is prone to dryness and dehydration. 3.If the water in the stratum corneum is insufficient, then the activity of the enzyme that breaks down the bridge particles is inhibited, and the normal shedding of the stratum corneum is hindered, forming large scales visible to the naked eye, and the skin becomes rough and dry. 4, the water content of the stratum corneum is mainly regulated by the natural moisturizing factor. The natural moisturizing factor is a natural hydrophilic moisture-absorbing substance in the stratum corneum, which is the metabolic product of filaggrin, a water-soluble low-molecular substance that plays an important role in skin moisturizing. The main components of natural moisturizing factor are amino acid, pyrrolidone carboxylic acid and lactate. 5.In addition to the simple diffusion of water into and out of the skin, water is also transported through Aquaporins (AQPs) on the cell membrane. Thirteen water channel proteins have been identified in mammals. Animal studies have confirmed that if rats are defective in AQP3, then there is a decrease in epidermal water and glycerol, dehydration of the stratum corneum, and a decrease in skin elasticity and barrier function. There are many ways to increase the water content of the skin, mainly by topical application of various moisturizers or encapsulants such as ceramides and glycerol.