Understanding the role of water in the skin.

The skin is an important organ that holds much of the body's water. However, water distribution in the skin varies across layers and reflects distinct biological functions. The dermis, the skin's middle layer, acts as a true water reservoir. It contains 80% of the water present in the skin. This concentration stems from abundant hydrophilic macromolecules in the extracellular matrix, such as collagen, proteoglycans, and especially glycosaminoglycans (GAGs). These, such as the hyaluronic acid, have high hygroscopic capacity. Due to many negative charges, this molecule binds sodium ions and attracts dozens of water molecules, forming a semi-fluid gel.

The dermis has high vascular density, which promotes water delivery from the circulatory system. In contrast, the epidermis is avascular. It relies on passive diffusion of nutrients and water from the dermis, which limits its storage capacity. Dermal water is non-mobilizable because it is bound to matrix macromolecules, especially hyaluronic acid. Although it cannot circulate freely or evaporate, it supports tissue flexibility and density.

However, the stratum corneum, the outermost epidermal layer, contains only 10 to 20% water. Yet it is this superficial water content that determines the skin hydration level perceived visually and by touch. When skin is dehydrated, it results from a disruption of the stratum corneum’s water balance. Unlike water in the dermis, epidermal water is mobile. It moves along osmotic or transepidermal gradients. Some of this water reaches the surface, where it evaporates in a process called insensible water loss. This process is constant, even without visible sweat, and causes skin dehydration when the epidermal barrier is compromised.

Skin hydration depends not only on the dermis’s ability to store water but also on the epidermis’s ability to distribute and retain it.

To counter permanent water loss, especially via insensible water loss, the skin relies on multiple regulatory mechanisms. The first line of defense against skin dehydration depends on natural moisturizing factors (NMF). Present in corneocytes, the cells of the stratum corneum, these hygroscopic compounds stem from filaggrin degradation, a key protein in the keratinization process. They include free amino acids, urea, lactates, and pyrrolidone carboxylic acid, capable of retaining water in the stratum corneum, ensuring skin suppleness.

Water also circulates through the skin’s layers via membrane transport proteins, the aquaporins, the most abundant of which in the skin is aquaporin-3. Located in epidermal keratinocytes, this protein facilitates diffusion of water and glycerol between deep and superficial skin layers. By ensuring an even distribution of moisture within the epidermis, aquaporins support water homeostasis and maintain the skin’s barrier function.

Skin resists water evaporation through the presence of intercellular lipids in the epidermis within the stratum corneum. These lipids, organized into lamellar sheets between corneocytes, act as a waterproof cement. They consist of ceramides, free fatty acids, and cholesterol. Mature keratinocytes synthesize and extrude them. Their three-dimensional organization limits water loss to the outside if lipid synthesis and organization remain intact. This is complemented by the hydrolipidic film, a natural emulsion on the skin surface composed of sweat and sebum. It forms a light occlusive layer that limits water evaporation. The hydrolipidic film of dry skin, with lower sebum production, can be fragile.

The skin combines chemical mechanisms, including natural moisturizing factors; cellular mechanisms, including aquaporins; and structural mechanisms, including intercellular lipids and the hydrolipidic film, to retain water.