There are three layers of the skin – the epidermis, the dermis and the hypodermis (subcutaneous tissue). The epidermis is the outermost skin compartment, which has a thin hydro-lipid layer on top of it. This film consists of secretions of the sebaceous and sweat glands, and it helps maintain the skin’s firmness. It serves also as an additional barrier to prevent the excessive loss of transcutaneous water and the entry of harmful substances.
The epidermis itself is composed of 5 different layers. The top layer of the epidermis is the stratum corneum, which is made up of dead, keratinized cells that shed about every two weeks in a process known as desquamation.
Skin hydration is important for normal functioning of the skin and is primarily linked to the ability of the stratum corneum to retain water. While the amount of water in the inner layers of the skin is relatively constant, the water content of the corneum depends on different factors:
– the rate at which the water in the dermis reaches the stratum corneum
– the rate at which the water is eliminated by evaporation (transepidermal water loss – TEWL)
– the ability of the stratum corneum to retain water.
The last factor is linked to the presence of natural hygroscopic agents (collectively known as a natural moisturizing factor (NMF)) and the organization of the intercellular lipids, which form a barrier to water loss.
Claims Related to Hydration