The stratum corneum is the outermost epidermal layer of the skin, which consists of dead cells called corneocytes. It is the major barrier of the skin when it comes to chemical transfers. The main purpose of the stratum corneum is to protect the underlying tissues by forming a barrier. The underlying tissues are protected from infection, chemicals, mechanical stress, and dehydration.
The cells are also shed from the surface of stratum corneum through the process called desquamation, which balances the keratinocytes that proliferate and form in the basal layer of the epidermis. These cells migrate toward the surface through the epidermis. This journey will take around 14 days.
Function of the Stratum Corneum
During the process of cornification, nonliving corneocytes are formed from living keratinocytes. In this process, a layer of ceramides replaces the cell membrane. This layer is linked to the cornified envelope or an envelope of structural protein. The cells in the stratum corneum are surrounded by this complex structure, which helps in the barrier function of the skin.
The adhesive structures that hold the corneocytes together called corneodesmosomes link adjacent cells within the stratum corneum. Proteases eventually degrade these complexes, so that cells can be shed at the surface. For the maintenance of skin homeostasis, both desquamation and the formation of the cornified envelope are required. Skin disorders may develop when these processes are not correctly regulated.
Stratum corneum cells are composed of a protein called keratin, which prevents water evaporation, and thus, keeps the skin hydrated. Water can also be absorbed by the skin, which will further help in hydration. The skin also becomes stretchy due to this layer. The skin is pulled back to its natural shape by a weak glutenous protein.
Throughout the body, the thickness of stratum corneum varies. To grasp objects and avoid injury, the palms of the hands and the soles of the feet require additional protection, so these layers are typically more cohesive and thicker. Generally, the stratum corneum is made up of 15-20 dead cell layers with a thickness of 10-40 micrometer.
The stratum corneum is permanent in reptiles and is only replaced during the phase of rapid growth called molting or ecdysis. Beta-keratin is also present and provides rigidity to the skin layer.
The outermost layer of the skin is composed of five layers, of which stratum corneum is the outermost layer that acts as a barrier. It was earlier believed that the stratum corneum protects the lower layer of the skin and is like a thin plastic sheet.
Structure of the Stratum Corneum
After a few decades, the stratum corneum's chemical and biological activities were found to be actually quite complex and intricate. The following components make the stratum corneum:
- Corneocyte - The structure of stratum corneum is like that of a brick and mortar. The stratum corneum is made of protein complexes, which act as a brick, thus giving the stratum corneum a brick and mortar type of structure. These protein complexes are also known as corneocytes. Each corneocyte measuring about a mean thickness of 1 micrometer is made of thin fiber-like keratin, which is placed on an organized matrix. Between the fibers or threadlike structures, large amounts of water are held by the keratin. About 12-16 layers of corneocytes are present in the stratum corneum. The thickness of the corneocyte depends on the anatomical location, age, and exposure to UV radiation.
- Lamellar Bodies - These are formed in the keratinocytes of the stratum granulosum and stratum spinosum. When they reach the stratum corneum, they are in the mature form, and thus, the outer envelope of these lamellar bodies is degraded by the enzymes. Free fatty acids and ceramides, which are types of lipids, are released through the degradation process.
- Intercellular Lipids - A continuous layer of lipids is formed when the free fatty acids and ceramides fuse in the stratum corneum. This layer is referred as the lamellar lipid bilayer, which plays a significant role when it comes to the maintenance and support of the barrier properties of the skin.
- Cornified Envelope - A protein shell that surrounds each corneocyte. This protein shell is called as the cell envelope, which is composed of two proteins: involucrin and loricrin. Between the two proteins, there are extensive links, so the cell envelope becomes an insoluble structure. The cell envelope has two subtypes. One is rigid and the other one is fragile, and depends on how the cell envelope interacts with the lamellar lipid bilayer.
- Cornified Envelope Lipids - A layer of ceramide lipids are attached to the cell envelope and repel water. Water molecules are held between the lipid bilayer and the cell envelope lipids, thus maintaining the balance of water in the stratum corneum instead of being absorbed into the lower epidermal layers.
- Corneodesmosomes - These are specialized protein structures that hold the corneocytes together. They are a part of the mortar when it comes to the brick and mortar analogy. They are also the structures that should be degraded for the skin to undergo desquamation.
- Natural Moisturizing Factors - These are water-soluble compounds, which can only be found in the stratum corneum. These compounds nearly comprise 20-30 percent of the dry weight of the corneocyte. They also keep the skin's outer layer well-hydrated and protected. If the skin repeatedly comes into contact with water, it tends to become dry since these components easily leach from the skin. To prevent the loss of these compounds, the corneocytes are sealed by the lipid layer that surrounds them.
- Desquamation Process - This process is also known as the exfoliation process. It is a very complex process, which is not completely understood as a whole. It is known that corneodesmosomes are degraded by several enzymes and that they undergo degradation in a specific pattern. However, it is still unknown how the enzymes get activated to start the process or what the nature of these enzymes is. When it comes to the activity of these enzymes, pH and water play significant roles.
The body is protected by the skin, which keeps away unwanted influences from the environment. The skin even protects the body from excessive loss of water. The barrier function of the skin is in the stratum corneum.