What is the Stratum Corneum? "The brick wall"
The stratum corneum is the most outer layer of the skin.
The stratum corneum can be thought of as a brick wall that protects you.
The "bricks" are the skin cells, called corneocytes, and the "mortar" that holds the bricks together is the lipids or fats, that together create the outer barrier.
The stratum corneum is the most outer layer of the epidermis and is the layer directly exposed to the external environment.
There are approximately 15 to 30 layers of corneocytes in the stratum corneum.
What is the Stratum Corneum made of?
The stratum corneum is composed of skin cells, water, and lipids.
On a weight basis, the stratum corneum contains approximately 70% protein, 15-25% water and 15% lipids (Ananthapadmanabhan et al, 2013).
What are Corneocytes? "The bricks of the wall"
"The bricks of the wall" are corneocytes, skin cells that compose most if not all of the stratum corneum.
Corneocytes are terminally differentiated from skin cells called keratinocytes, which populate the lower levels of the skin barrier.
Corneocytes are regularly replaced through desquamation and renewal from lower epidermal layers, making them an essential part of the skin barrier property.
Cell Turnover and Loss
Desquamation is the natural process of shedding skin cells.
New skin cells are formed at the base layer of the skin, and they differentiate and migrate towards the skin surface, in a process that takes approximately 30 days. Nearly a billion cells are lost each day from the surface of adult skin (Milstone et al, 2004).
What are Keratinocytes?
Keratinocytes are the major cell type in the epidermis, and make up 95% of the cell population. Keratinocytes are formed at the basal layer just above the dermis. They are metabolically active cells with normal constituents such as a nucleus and cytoplasm.
As keratinocytes mature, they differentiate into corneocytes, which serve as a physical barrier, and protect your body from the external environment.
What are Corneocytes made of? Proteins, AAs and NMFs
Corneocytes are mostly made up of the protein keratin, which is also found in hair and nails.
Corneocytes in the stratum corneum are filled with keratin filaments as well as amino acids (AAs) and other small molecules, collectively referred to as natural moisturizing factors (NMFs), derived from the breakdown of filaggrin, a protein that surrounds the keratin filaments (Verdier-Sévrain et al, 2007).
What are Natural Moisturizing Factors?
Natural moisturizing factors are a mix of humectants or hydroscopic molecules that help attract and hold onto water and maintain hydration in the skin.
Natural moisturizing factors include amino acids and their derivatives (pyrrolidone carboxylic acid and urocanic acid) made from the breakdown of epidermal filaggrin. Other components found within but also external to the corneocytes include lactates, urea, and electrolytes (Table 1).
Natural moisturizing factors are present in high concentrations within corneocytes and represent up to 20% to 30% of the dry weight of the outer skin layer (Verdier-Sévrain et al, 2007).
Proper hydration of the skin is required for maintaining the aesthetic properties of skin, such as moisturization, softness, smoothness, lack of flaking, etc.
Furthermore, proper hydration of the stratum corneum is also necessary for the critical processes of lipid biosynthesis, desquamation and natural moisturizing factor (NMF) production, that take place within these non-living, yet biochemically active, layers of the skin (Loden et al, 2001; Harding et al, 2004; Feingold et al, 2007; Bowser et al, 1986).
Lipids "The mortar that holds the bricks together"
Epidermal lipids are are a mixture of ceramides, free fatty acids and cholesterol. These lipids are released from skin cells and fill the spaces between the cells, like mortar or cement (Pappas, 2009).
The stratum corneum lipids consist of an equimolar mixture of ceramides (45–50% by weight), cholesterol (20–25%), and free fatty acids (10-15%), with lower quantities of cholesterol sulfate and nonpolar lipids (Verdier-Sévrain et al, 2007).
Free Fatty Acids in the Stratum Corneum
The main free fatty acids in the lamellar membranes are palmitic acid (C16:0) by 10% (mass/mass), stearic acid (C18:0) by 10% (mass/mass), behenic acid (C22:0) by 15% (mass/mass), lignoceric acid (C24:0) by 25% (mass/mass), and hexacosanoic acid (C26:0) by 10% (mass/mass) (Kang et al, 2006; Lin et al, 2017).
Other free fatty acids that are also found naturally in the stratum corneum include oleic acid (C18:1, n-9), eicosapentaenoic acid (C20:5, n-3), arachidonic acid (C20:4, n-6), docosahexaenoic acid (C22:6, n-3), linoleic acid (C18:2, n-6) as well as its derivatives of linolenic acids [α-linolenic acid (C18:3, n-3), γ-linolenic acid (C18:3, n-6) and dihomo-γ-linolenic acid (C20:3, n-6)] (Lin et al, 2017).
Linoleic acid is the most abundant polyunsaturated fatty acid in the skin barrier (Ansari et al, 1970).
Most of the free fatty acids can by synthesized by the keratinocytes and are released into the stratum corneum.
However, linoleic acid and arachidonic acid are essential fatty acids that must be provided externally through diet, supplements or topically through skincare products (Lin at al, 2017).
The skin barrier includes the outermost layers of skin, which is called the stratum corneum.
The stratum corneum can be thought of as a brick wall that protects you.
The "bricks" are the skin cells, called corneocytes.
Corneocytes in the stratum corneum are filled with proteins, amino acids and natural moisturizing factors (NMFs), that help hold moisture in your skin.
The "mortar" that holds the bricks together is the lipids (fats), including ceramides, cholesterol and fatty acids, which coat and lubricate your skin cells.
Together, the corneocytes and lipids, or "bricks" and "mortar" create the outer skin barrier that protect you from the external environment.
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