Stratum corneum

The three main layers of the skin are the epidermis, the dermis, and the hypodermis (see "The skin : Layers and their functions"). The epidermis, the outermost layer, is itself divided into several sub-layers with very different properties and functions. 

THE STRATUM CORNEUM

Starting from the surface and working towards the deepest layer, we can observe :

• the stratum corneum
• the stratum granulosum
• the stratum spinosum
• the stratum basale

The stratum corneum is the main protective element of the skin. Because of its structure and chemical composition, it forms a virtually impermeable barrier, a storage place for lipidic compounds and plays a major role in skin hydration.

The stratum corneum owes its strength to keratin, an extremely resistant fibrous protein. The stratum corneum is made up of dead, flattened cells without nucleus, called corneocytes, which are constituted almost exclusively of keratin. These cells overlay each other like tiles on a roof, giving the stratum corneum its high mechanical strength. Between the corneocytes is an extracellular lipid matrix, also called lipid cement, whose composition in ceramides, cholesterol and fatty acids is responsible for the skin's hydric barrier function.

The stratum corneum contributes significantly to skin hydration by maintaining a hydrolipidic film on its surface, but also NMFs (natural moisturizing factors). These NMFs are contained within the corneocytes and are composed of a mixture of free amino acids, amino acid derivatives (pyrrolidone carboxylic acid (PCA), lactic acid), urea, inorganic salts and sugars. This mixture allows NMFs to retain the water that crosses the cell layers during diffusion, but also to attract and bind water from the atmosphere, drawing it into the corneocyte.

Thus, the maintenance of a strong and well-hydrated skin requires the concomitant care of the stratum corneum, the lipidic matrix and the NMF.

References :

Martini, Marie-Claude. 2003. Introduction à la dermopharmacie et à la cosmétologie (3e ed.). Lavoisier.

Fowler, Joseph. 2012. « Understanding the Role of Natural Moisturizing Factor in Skin Hydration ». Practical  Dermatology.

Clar, E. J., et A. Fourtanier. 1981. « Pyrrolidone Carboxylic Acid and the Skin ». International Journal of Cosmetic Science 3 (3): 101‑13.

Fondation Dermatite Atopique. 2013. « Peau et galénique ». https://www.fondationeczema.org/sites/default/files/2020-06/leaflet_peau_et_galenique_eczema_atopique_0.pdf.

Par Valentine Du bois

Master's degree in medical biology, University of Lausanne, Switzerland

Valentine holds a Master's degree in medical biology from the University of Lausanne, following a Bachelor's degree from the University of Geneva. During her Masters, Valentine specialised in the fields of pharmacology and toxicology, focusing on applied laboratory research for the development of new treatments.