Understanding the role of water in the skin.

Skin is an important organ that retains a large proportion of the body’s water. However, water distribution between skin layers differs and reflects distinct biological functions. The dermis, the skin’s middle layer, acts as a true water reservoir. It contains about 80% of the water in the skin. This high concentration results from the abundance of hydrophilic macromolecules in the extracellular matrix, such as collagen, proteoglycans and especially glycosaminoglycans (GAGs). These, such as hyaluronic acid, have strong hygroscopic properties. Due to its numerous negative charges, this molecule binds sodium ions and attracts dozens of water molecules around it, forming a semi-fluid gel.

The dermis is a vascularised layer, which favours water supply from the circulatory system. In contrast, the epidermis is avascular: it depends on passive diffusion of nutrients and water from the dermis, which limits its storage capacity. Dermal water is non-mobilisable, as it is bound to matrix macromolecules, including hyaluronic acid. Although it cannot circulate in the body or evaporate, it supports tissue pliability and density.

The stratum corneum, the outermost layer of the epidermis, contains only 10 to 20 per cent water. Yet superficial water content is a key factor in theskin hydration state perceived visually and felt. When the skin is dehydrated for example, it is due to a disruption of the stratum corneum’s water balance. Unlike water in the dermis, that in the epidermis is mobile. It moves in response to osmotic or transepidermal gradients. Some of this water reaches the surface, where it evaporates at a slow rate through a process known as 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 capacity to store water but also on the epidermis’s capacity to distribute and retain it.

To counter continuous water losses, notably via insensible water loss, the skin relies on several regulatory mechanisms. The first line of defence against skin dehydration is based on the natural moisturising factors (FNH), called natural moisturizing factors in English (NMF). Present in corneocytes, the cells of the stratum corneum, these hygroscopic compounds derive from filaggrin degradation, a key protein in the keratinisation process. They include free amino acids, urea, lactates and pyrrolidone carboxylic acid, capable of retaining water in the stratum corneum, preserving skin suppleness.

Water moves through the various skin layers via membrane transport proteins, the aquaporins, the most abundant of which in the skin is aquaporin-3. Localised in epidermal keratinocytes, this protein facilitates the diffusion of water and glycerol between the deep and superficial skin layers. By ensuring a uniform distribution of moisture across the epidermis, aquaporins support skin water homeostasis and maintain its barrier function.

The skin provides a physical barrier to water evaporation thanks to the presence of intercellular lipids in the epidermis, especially within the stratum corneum. These lipids, arranged in lamellar sheets between corneocytes, act as an impermeable cement. They consist mainly of ceramides, free fatty acids, and cholesterol. They are synthesised and then extruded by mature keratinocytes. Their three-dimensional organisation limits water loss to the exterior, provided lipid synthesis and organisation remain intact. Added to this is the hydrolipid film, a natural emulsion on the skin surface composed of sweat and sebum. It forms a lightweight occlusive layer that reduces surface water evaporation. The hydrolipid film of dry skin, which has reduced sebum production, is often fragile.

The skin retains water by combining chemical mechanisms (natural moisturising factors), cellular mechanisms (aquaporins), and structural mechanisms (intercellular lipids and the hydrolipidic film).