It has been recognized for approximately 7 decades that the stratum corneum (SC) exhibits biological properties that contribute directly to maintaining and sustaining healthy skin.
The stratified structure is maintained by cell division at the innermost part of the epidermis, overproduction of specialized lipids that are transported to the extracellular space, overproduction of keratin proteins, followed by the cell death and eventual desquamation at the outermost part. It takes 14 days for a daughter cell from the stratum basale to differentiate into a SC cell, these cells retained further 14 days prior to shedding.
Typically, the SC comprises only 10 to 15 cell layers and is around 10 micrometers thick when dry, although it may swell to several times this thickness when wet. SC thickest on the palms and soles and is thinnest on the lips.
The SC is a composite material made up of proteins and lipids structurally organized “bricks and motor”. It is the upper most layer of epidermis.
Bricks= Corneocyte (this cell is without intracellular organelles)
Motor= Hydrophobic lipids in extracellular space
The keratinocytes are polygonal, elongated and relatively flat-approximately 0.2 to 1.5 µm thick with diameter of 34-36 µm.
Barrier Functions of SC
The barrier nature of SC depends on its constituents; 75-80% is protein, 5-15% is lipid with 5-10% unidentified on a dry weight basis. This protein contains around 70% alpha keratin with some beta-keratin (around 10%) and a proteinaceous cell envelope (around 5%). Enzymes and other proteins account for approximately 15% of the protein component.
Not only the amount of lipids or the correct equimolar ratios of the major lipid species important, but the precise ceramide chemicals composition and physical organization of the total lipid matrix is vital for healthy barrier. SC creates a dense membrane network by the covalent ester bonds between the ceramides of intercellular lipids matrix and the structural proteins of corneocyte, which decides which molecule should entre and which one is not.
The cell envelop is highly insoluble and is very resistant to chemical attack.
Following are some main barrier functions of SC:
It prevents the excess loss of water from skin which is measure by trans epidermal water loss (TEWL).
SC keeps the skin smooth and flexible by binding the water molecule even in an extreme dry environment.
SC completes many vital barrier tasks, including but not limited to regulating epidermal water content and magnitude of water loss.
Reduces exogenous oxidants that can damage components of skin via our natural anti-oxidant system.
Preventing cutaneous infection via multiple anti-microbial peptides.
Protecting its neighboring cutaneous cells and structures that lie beneath from damaging effects of UV radiation.
Stratum Corneum and Cosmetics Products
Cosmetics products such as cleanser, perfumes, changing the appearance, body odor corrector, protectors and skin’s good condition keeper all are directly worked on or through the SC.
We well known about the solubility of SC, because of its highly insolubility and semi permeable nature the cosmetician smartly uses raw materials in cosmetic product which can perform outside the SC, such as large aggregates, polymers and proteins. For example, coloring pigments, high molecular weight UV filters.
The gaps between intercellular matrixes are how most hydrophilic and lipophilic molecules penetrate the SC.
Cosmetics products which contain the scrub particles that mechanically remove the outer ‘dirty’ desquamating SC layer and parallel the creams make a smoother skin surface. Skin care cleansing products are also directly work on the SC either by removing excessive oil, dirt, dust etc., or by providing a moisturizing layer on SC.
Sunscreens work by different mechanisms. Physical sunscreen works by forming layer on the SC and reflects the UV rays. Chemical sunscreen works by absorbing the UV light and converting it into heat and releases into the body.
Anti-aging, anti-pigmentation products contain active ingredients that penetrate into skin through the SC and fight against free radicals, repairs the cells and also protects from UV damage.
Moisturizing products improves the hydration of SC and increases the water content in it. It forms occlusive layer on the SC and prevents TEWL, covers skin fissures, soothes the skin and protect against friction.
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