Saturday, January 5

Stratum corneum layer in skin - Function of stratum corneum cells

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Stratum corneum, the outermost layer of the epidermis, has the skin barrier function and is formed by the death of keratinocyte cells. The stratum corneum (SC) is multi layered and is composed of dead, anucleated, flattened corneocytes.
The stratum corneum has a thickness between 10 and 40 μm and may contain 15-20 layers. These corneocytes are embedded within the multiple lamellar sheets of lipid matrix formed by specialized lipids.

Formation of stratum corneum and desquamation

Epidermis is the outermost layer of skin and it consists of five sublayers. Its main function is to protect underlying tissues and organs from pathogens, toxic agents, oxidants, dehydration, chemicals, environment, ultraviolet radiation and physical stress. Epidermis functions very dynamically and renews continuously.

A continuous process of cell proliferation takes place in the lowermost layer of epidermis known as basale layer. These cells moving outwards undergo maturation and after a number of changes become keratinocyte cells . Eventually these cells die and form the stratum corneum to function as a barrier and get replaced and shed by the desquamation process.

The process of living keratinocytes undergoing transformation into non-living corneocytes is called cornification. As the living keratinocyte cells die, they lose the nuclei and organelles. Their cell membranes undergo changes and ceramides replace them and a cornefied envelope of structural proteins is linked to them.

Desmosomes (corneodesmosomes) which are molecular complexes of cell adhesion proteins and cell linking proteins, bind these corneocytes in the outer st. corneum. As these protein complexes move towards outer region of st. corneum, protein degrading proteases, break them up which leads to desquamation.

Function of stratum corneum

This layer functions as a barrier protecting the inner layer of epidermis from dehydration and also from invasion of pathogens. Stratum corneum contain a dense network of a protein known as keratin. Keratin keeps the epidermis hydrated by absorbing moisture as well as by preventing water evaporation.

The st. corneum by the nature of its structure, makes the epidermis elastic and a glutenous protein bond pulls the epidermis back to its original natural shape.

Another important action of S. corneum is in providing protection from mechanical injuries. To carry out this action, this layer is typically thicker in palms and soles. To some extent it helps in the absorption of moisturizers, holding them back from immediate loss.

Effects of defects in stratum corneum

The formation of corneocytes and their desquamation in a regulated manner is very essential for the maintenance of healthy skin. Any failure or set back in these regulated processes can give rise to skin disorders and ailments. A collection of low-molecular-weight water-soluble components, known as the natural moisturizing factor (NMF) present in st. corneum act as a humectant absorbing moisture by direct contact and also from atmosphere.

These barrier and moisturizing properties can be deranged by intrinsic factors like skin diseases and excessive sebum production or xerosis. Extrinsic factors like weather, temperature, low humidity, surfactants and chemicals can also impair the moisturizing barrier. Mental and physical stress can also have effects on the function of stratum corneum cell layers by the increase in the circulating stress hormones in the skin.
Reference:
Harding, C. R. (2004), The stratum corneum: structure and function in health and disease. Dermatologic Therapy, 17: 6–15. doi: 10.1111/j.1396-0296.2004.04S1001.x
Image source:
http://en.wikipedia.org/wiki/File:Epidermal_layers.png
Author: Mikael Häggström, based on work by Wbensmith
License: CC BY-SA 3.0

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