STRATUM CORNEUM: GENERAL CHARACTERISTICS, HISTOLOGY, FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2025

The stratum corneum, or squamous layer, is the outermost layer of the epidermis of terrestrial vertebrates, in which cells called corneocytes are filled with keratin. This layer is an evolutionary innovation of tetrapods that helps them survive in dry and abrasive terrestrial environments.

The epidermis, on the surface, and the dermis, below it, form the skin or integument, which is one of the longest organs in the body. The epidermis can be differentiated into hairs, feathers, horny scales, horns, claws and nails, beaks, and in the filter system of the whale's mouth.

Source: Rjelves

General characteristics

The corneocytes of the stratum corneum are dead cells, that is, they lack a nucleus and cellular organelles. These epidermal cells are formed by mitosis in the deep basal layer. They push pre-existing cells to the surface, where they die in an orderly fashion. They are exfoliated and continually replaced by cells from underlying layers.

During cell death, the protein keratin accumulates inside the cell. This process is called keratinization or cornification, and the cells that produce keratin are called keratocytes. Keratin gradually replaces the metabolically active cytoplasm, and the cells transform into cornified cells, called corneocytes.

Corneocytes have an insoluble envelope that replaces the plasma membrane. This envelope is composed of fatty acids, sterols and ceramides. These lipids are produced by lamellar bodies, organelles present in keratocytes that have not begun to cornify.

The lipid envelope constitutes the scaffold for the molecular organization of extracellular lipids that form bilayer sheets in the spaces between corneocytes. These layers of lipids offer resistance to the absorption of chemicals and other water soluble substances. They avoid the loss of water by evaporation.

Histology

The skins of reptiles, birds, and mammals are composed of stratified squamous epithelium. The epidermis of these vertebrates differs in the number of layers or regions that make it up.

In reptiles, the epidermis has three regions: stratum basalis, stratum granulosa, and stratum corneum. Crocodiles and turtles shed very little skin, while snakes experience the removal of large regions of the epidermis surface.

In birds, the epidermis has two regions: stratum basalis and stratum corneum. Between the two layers is a transient layer of cells that undergo keratinization.

In mammals, the epidermis has four regions: stratum spinosum, stratum granulosa, stratum lucidum, and stratum corneum. Keratinization is greatest in regions where there is more friction, such as the palms of the hands and the soles of the feet.

In vertebrates, the horny layer consists of 20–30 rows of flattened corneocytes (30–40 µm). Using a microscope, it is observed as a layer of fibers that looks like a brick wall, 0.75 to 1.5 mm thick. Corneocytes are "ghosts" of cells with bundles of keratin inside.

General functions

The stratum corneum is organized into two morphologically and functionally different systems of compartments: corneocytes and the extracellular matrix (made up of neutral lipids).

Corneocytes provide mechanical resistance to cutting or impact, they are a barrier against ultraviolet light, being the site where inflammation begins (activation of cytokines) and photoimmunosuppression.

The extracellular matrix is ​​responsible for the integrity of the stratum corneum, cohesion and desquamation. It works as an antimicrobial barrier (innate immunity) and provides selective absorption. The corneocytes and the lipid matrix act as barriers that impede permeability and hydration.

The function of the stratum corneum depends on its biochemical composition and the structure of the tissue. Before dying, the keratocytes of the stratum granulosa are responsible for producing the substances that are responsible for the functions performed by the stratum corneum.

Keratocytes, in addition to producing lipids, generate: enzymes that process these lipids, proteolytic enzymes, glycoproteins, enzyme inhibitors and antimicrobial peptides.

Water conservation and protection against the entry of pathogens

The ability of the skin to prevent the loss of water and the entry of pathogens depends on the four characteristics of the extracellular matrix of the stratum corneum: 1) absolute amount of lipids; 2) lipid distribution; 3) hydrophobic properties; and 4) supramolecular organization of lipids. It is estimated that in humans this barrier prevents the loss of 300–500 ml / day.

The amounts of lipids in the stratum corneum are: ceramides, 50%; fatty acids, 25% (they can be essential and non-essential; they contribute to acidify the layer); cholesterol, 25%. These lipids form a lamellar structure that closes the intercellular spaces within the stratum, forming an impermeable barrier.

In the extracellular matrix there are other components, in addition to the lamellar structure, that contribute to form this barrier: the corneocyte envelope; ω-hydroxyceramide monolayers surrounding corneocytes; enzymes; antimicrobial peptides; and structural proteins secreted by the lamellar bodies of keratocytes.

Antimicrobial peptides include beta-defensin, which has potent antimicrobial activity against gram-positive bacteria, yeast, and viruses, and cathelicidin, which has activity against a wide variety of bacteria (including Staphyloccous aureus) and viruses.

Hydration, UV filtering, and immunosuppression

Within the corneocytes there are many hygroscopic substances, which together with simple sugars and electrolytes, are called natural wetting factors (NHF). They play an important role in maintaining the hydration of the stratum corneum.

The degradation of filaggrin produces NHF, made up of: 1) free amino acids such as histidine, glutamine and arginine (product of proteolysis); and 2) carboxylic acid of pyrrolidine, urocanic acid, citrulline, ornithine and aspartic acid (product of the action of enzymes on free amino acids).

Through the enzyme histidine ammonolyase, histidine produces trans-urocanic acid (tUCA), which is photoisomerized by UV-A to cis-urucanic acid (cUCA). This last molecule acts as a sunscreen and is also a powerful immunosuppressant that participates in the pathogenesis of skin cancer caused by ultraviolet (UV) light.

Peeling

One of the characteristics of the stratum corneum is desquamation, which consists of the proteolytic degradation of corneodesmosomes, whose nature is protein and therefore are responsible for keeping the corneocytes together.

This can be evidenced morphologically by the loss of corneodesmosomes and the disappearance of other proteins, such as desmocholine 1.

There are at least ten types of serine proteases that are found in the stratum corneum and are involved in desquamation. For example, chymotrypsin and the stratum corneum tryptic enzyme. The activation of these enzymes depends on the presence of endogenous inhibitors and the physiological state of the stratum corneum (low pH; Ca ⁺² poorly hydrated).

References

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