Structure and Function of the Stratum Corneum Layer

The thickness of the stratum corneum layer is highest on the palms and soles (around 0.059 inches), while it is lowest on the eyelids (0.0020 inches).

The skin is the outer covering of the body, and it consists of different layers. Considered as one of the most complex organs of the body, the skin is an intricate system of dense layers. It primarily consists of 3 layers; epidermis, dermis, and the hypodermis.

We all know that the skin acts like a waterproof barrier and helps in maintaining stable body temperature by insulating our body from the constantly changing external environment.

However, do you know what part of the skin plays an important role in serving as a protective barrier of the skin? Well, it is the stratum corneum, the layer located in the epidermis. It is discussed in detail below.

The epidermis, the outermost layer of the skin, is divided into 5 sub-layers. Of these sub-layers, the stratum corneum is the topmost layer of the skin. The skin that we scratch or apply a skin care product on is the stratum corneum layer.

It acts as an effective barrier against harmful microbes including viruses, bacteria, and fungi, and maintains adequate moisture levels in the skin.

It protects the inner layers of the skin from mechanical damage. However, any damage to this outer layer is bound to cause infection, the most common being boils, a bacterial infection.

In general, the protective function of the skin as well as toughness imparted to the skin primarily comes from the stratum corneum layer.

• The stratum corneum layer consists of tightly packed sheets of corneocytes, dead cells that assist in skin barrier function. Corneocytes are flat, anucleur, plate-like structures that have a thickness of around 0.5 -1 um and are approximately 10-30 um across. These cells contain keratin filaments.

• The layer is composed of 12-16 sheets of corneocytes. These cells, apart from containing keratin filaments, store the natural moisturizing factor (NMF), which serves in retaining moisture. NMF is primarily composed of amino acids, organic acids, peptides, calcium, lactate, and glucosamine.

• The corneocytes are encased within a protein envelope (cell envelop). These cell envelopes are cross-linked by protein structures referred to as corneodesmosomes. They essentially promote cellular adhesion and ensure that the corneocytes are tightly connected. The lipids constitute the extracellular space of stratum corneum layer.

• Free fatty acids (FFAs) and ceramides are the two types of lipids found in this layer. They combine to form the lipid bilayer in the extracellular environment that serves as a barrier.
• The corneocytes are sloughed off continuously, which are eventually replaced by new corneocytes in 4 to 6 weeks. This is how the renewal of the epidermis occurs.

• The palms and soles of the feet are subjected to a lot of mechanical stress. Hence, the stratum corneum layer at these areas is thickest, which provides greater protection against mechanical trauma.

• The brick and mortar model is often used to understand the structure of the stratum corneum layer. When drawing this comparison, the corneocytes are viewed as bricks, while the mortar is represented by the extracellular space of lipid layers.