Skin Barrier: What is it and What does it do?

Last few days, I have been working on a blog post on “moisturizers”. But without talking about the skin barrier, it would have been incomplete.

Skin barrier in itself is a very complex topic and a summary would not have done justice to it. (I have included some graphics to make it easier to understand).

Let’s start with the basic skin anatomy:

Layers of the skin

Epidermis

It is the outermost layer of the skin and predominantly consists of epidermal cells arranged in several layers. From deep to superficial, these layers are:

1. Basal layer (stratum basale)
2. Prickle layer (stratum spinosum)
3. Granular layer (stratum granulosum)
4. Stratum corneum

Dermis

This layer supports the epidermis. It contains:

Fibroblasts produce collagen giving strength and endurance to the dermis.
Proteoglycans maintain the skin’s hydration.
Elastin fibers provide elasticity and flexibility to the skin.

Hypodermis (subcutaneous)

This layer sits under the dermis and consists of loose connective tissue, blood vessels, and fat cells.

What is a skin barrier?

The skin barrier is formed primarily by the stratum corneum, the so-called dead layer on the top of the skin. Our skin barrier does not perform just one function but different barrier functions.

Function

Our skin barrier performs different functions which include:

1. Permeability barrier: Maintenance of water content and balance

Most of the time when we use the term “Skin or epidermal barrier,” we are talking about the permeability barrier.

This is what regulates the transepidermal water loss (TEWL – the amount of water evaporating from the surface of the skin. In healthy skin, TEWL is directly proportional to skin hydration. However, with damage to this barrier, TEWL increases).

This comprises several components:

• Formation of stratum corneum lipids in specific ratio from precursor lipids
• Formation of natural moisturizing factors (NMFs)
• Formation of cornified and lipid envelope around the skin cells – corneocyte
• Maintenance of acid mantle (acidic pH)
• Response of cytokines to impairment of permeability barrier [1]

2. Antimicrobial barrier: Prevention and responses to invasion by microbial organisms and antigens

3. Immune response barrier: Immune cells protect from the external aggressors

4. Photo-protection barrier: Reduction of the effects of ultraviolet (UV) light exposure

5. Antioxidant barrier: Protects from oxidative stresses

So let’s focus on the permeability barrier, the skin barrier function which we often damage using harsh products.

Permeability barrier

This skin barrier function maintains the integrity of the skin by maintaining proper water balance and protecting it from exogenous stressors like the environment. It can be better explained by the “Brick and mortar model” as below.  [1] 

Brick and Mortar model

The stratum corneum consists of 15–20 layers of flattened dead cells (corneocytes) embedded in the lipid matrix. This structure is called a “brick and mortar” model and constitutes a major barrier function.

“The Bricks”

In this barrier layer, the ‘‘bricks’’ are terminally differentiated skin cells – keratinocytes (corneocytes). [1] [5] The corneocytes are held together in the lipid matrix by corneodesmosomes.

Inside these cells are humectants called natural moisturizing factors, that are essential for stratum corneum hydration, barrier function, and desquamation. These NMFs are made up of free amino acids and various derivatives of these amino acids such as inorganic salts, sugars, lactic acid, and urea. [1] [5]

“The Mortar”

The ‘‘mortar’’ includes intercellular lipids arranged into lamellar layers. [5]

It consists of three major lipid components present in a relative ratio of approximately 3:1:1 based on stratum corneum lipid content (% by weight):

• Ceramides (40–50%)
• Cholesterol (25%)
• Fatty acids (10–15%)

These major stratum corneum lipids are produced enzymatically within the stratum corneum from specific precursor lipids which are derived from lamellar bodies (LBs) within the granular layer. [1] [6]

How does it repair itself?

As the skin is exposed to multiple exogenous factors such as “improper skin care, exposure to skin irritants, occupational exposures, application of certain topical agents, and low ambient humidity”, the skin barrier gets impaired. [7]

However, the stratum corneum has a variety of self-repair mechanisms. Once TEWL (trans-epidermal water loss) is increased beyond the normal level, multiple things happen:

1. Immediate release of precursor lipids into the stratum corneum (from lamellar bodies within the granular layer) produces a partial reversal of water loss within minutes, providing recovery of approximately 20 percent of the overall permeability barrier function.
2. Increase in the synthesis of physiologic lipid precursors and conversion to stratum corneum lipids.
3. Increased formation of NMFs. These NMFs, primarily amino acids, maintain hydration within the stratum corneum, which decreases TEWL by inhibiting loss into the atmosphere.
4. Increased TEWL initiates an inflammatory pathway with the release of some cytokines, which increases the epidermal thickness to decrease TEWL.
5. When microbial organisms attack the stratum corneum, specific antimicrobial peptides may be activated, thus providing protection from the development of infection. [1] [7]

On a daily basis, this process can repair and maintain balance within the skin.

Some individuals with underlying skin conditions, such as atopic dermatitis, rosacea (like me), psoriasis, diabetes, and increased age, have underlying stratum corneum barrier impairment, which makes them more susceptible to increased TEWL. When adversely affected by the additional exogenous factors, the barrier damage is further increased. [1] [7]

Damaged skin barrier symptoms

Most of the time, our skin can repair itself. However, sometimes the damage becomes more than our skin can repair.

At this stage, certain changes occur in the skin which can be seen clinically. It includes:

Scaly and flaky skin: The function of stratum corneum is dependent on proper hydration, since many enzymes involved in its function are hydrolytic, meaning they don’t work well in the absence of water.

The proteases are the enzymes that degrade the corneodesmosomes (proteins keeping the cells together on the skin surface) and facilitate proper shedding or exfoliation of surface dead cells.

Since these don’t work efficiently without water, the dead cells or corneocytes will clump before shedding. This results in roughness with visible scaling and flaking.

Cracking or fissuring of skin due to loss of skin elasticity and increased rigidity.

Thickened skin due to epidermal proliferation. [7]

Read more: Skin barrier damage and how to repair it

Summary

The skin barrier is not just one single entity, but a collection of different barrier functions. Most of the time when we talk about the skin barrier, we are talking about the “permeability skin barrier” which maintains a water balance.

Many things in our environment are attacking this barrier without us even knowing. To maintain this barrier, we need gentle skin care including a pH-balanced cleanser and a well-formulated moisturizer.

If you are using any harsh skin treatments like exfoliants, retinoids, or benzoyl peroxide; the importance of protecting this barrier becomes even more important.

If you are seeing any signs of barrier damage: that scaly, dry, or flaky skin with cracks and thickening, it’s time to take a look at your routine. If you think you are doing everything right and it’s still not working, it’s time to make an appointment with your dermatologist.

References

• Del Rosso JQ, Levin J. The clinical relevance of maintaining the functional integrity of the stratum corneum in both healthy and disease-affected skin. J Clin Aesthet Dermatol. 2011;4(9):22-42.
• Braff M, DiNardo A, Gallo R. Keratinocytes store the antimicrobial peptide cathelicidin in lamellar bodies. J Invest Dermatol. 2005;124:94–400.
• Elias P. Stratum corneum defensive functions: an integrated view. J Invest Dermatol. 2005;5:183–200.
• Schroder JM, Harder J. Antimicrobial skin peptides and proteins. Cell Mol Life Sci. 2006;63:469–486.
• Kahraman E, Kaykın M, Şahin Bektay H, Güngör S. Recent Advances on Topical Application of Ceramides to Restore Barrier Function of Skin. Cosmetics. 2019; 6(3):52.
• Harding CR. The stratum corneum: structure and function in health and disease. Dermatol Ther. 2004;17:6–15.
• Rosso JD, Zeichner J, Alexis A, Cohen D, Berson D. Understanding the Epidermal Barrier in Healthy and Compromised Skin: Clinically Relevant Information for the Dermatology Practitioner: Proceedings of an Expert Panel Roundtable Meeting. J Clin Aesthet Dermatol. 2016;9(4 Suppl 1):S2-S8.

Disclaimer: This blog post is for information purposes only and does not replace medical advice.

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