Bricks, Mortar, and More: A Formulator's Guide to How the Skin Barrier Works
“Rome wasn't built in a day”
And neither was our skin barrier.
Because good things take time to form, our skin needs at least 28 days to fully renew itself.
In these 28 days, the barrier gives you your best face forward.
So, slap on some barrier cream, grab your favorite drink and get ready for the lowdown on the skin barrier.
Table of Contents
Components of the Skin Barrier
How the Skin Barrier is Formed
The Skin Barrier Series
This topic is so hot, we’re making a series about it. Browse our collection of articles focused solely on learning about the skin barrier and what the latest research tells us.
What is the Skin Barrier
Your skin's barrier is generally thought of as the outermost layer of the skin, called the Stratum Corneum. It's basically the first line of defense against intruders like pollutants and toxins and works to keep our skin happy and healthy.
But it's worth taking a moment to appreciate just how much this thin layer is actually doing. The Stratum Corneum is only about 10-20 micrometers thick yet it's responsible for regulating transepidermal water loss (TEWL), resisting mechanical stress, maintaining a slightly acidic pH, and keeping a complex ecosystem of microorganisms in check. Not bad for a layer of dead cells.
What makes it so effective is not just what it's made of, but how it's organized. The classic "brick and mortar" model is a helpful starting point, corneocytes acting as the bricks and the surrounding lipid matrix acting as the mortar, but the reality is far more dynamic than that analogy suggests. The barrier is constantly being built, maintained, and shed in response to internal and external conditions, and even small disruptions to that process can have a measurable impact on how well it functions.
With more and more consumers understanding the importance of protecting this critical component of the skin, it is no wonder your brand wants to hop on the barrier train as well. In order to define a well-thought-out formula strategy around skin barrier health, we first need to understand the basics of how it works and how we can support it via topical products.
Components of the Skin Barrier
The skin barrier is composed of a variety of different complexes that all work together to ensure the barrier functions properly. In order to really understand this part of our skin we need to define exactly what these structures are.
Ready for the breakdown?
Corneocytes
Sometimes referred to as “dead skin cells,” Corneocytes were once live cells, called Keratinocytes, that got squished and died off. These flattened cells provide protection for the deeper layers of the skin.
Made mainly of a protein called Keratin and its intermediates, which organize into tough bundles that provide strength and rigidity to the skin. This mechanical strength is necessary to fight against the tough environment.
Corneocytes also offer a safe space for water to hang out, helping to keep the top layers of the skin hydrated. Without water, enzymes that are crucial to the proper functioning of the skin barrier can’t function (more on this later).
This tough layer of cells is sometimes referred to as the “bricks” of our skin barrier.
Lipid Matrix
The oily substance that fills the gaps between the corneocytes, helping to keep water in the skin. The predominant lipids are cholesterol, ceramides and free fatty acids, collectively referred to as the “mortar."
These lipids organize into bilayer structures that are linked to the outer structure of the corneocytes. The more "water loving" heads of the lipid molecules line up while the more "oil loving" tails align forming layers.
The composition of the lipids and the structural organization play a role in how well the barrier functions to keep water in and invaders out.
Cornified Envelope
The envelope is a highly insoluble complex that engulfs the corneocytes, providing a tough barrier surrounding the cell. This membrane replaces what was once the plasma membrane found in Keratinocytes.
It consists of linked proteins and bound lipids, notably ω-acylated-hydroxy-ceramides. This unique ceramide creates a starting point for the formation of the lipid matrix.
Corneodesmosomes
The main adhesive structures within the Stratum Corneum that hold the Corneocytes together. This adhesive prevents the Corneocytes from moving around, keeping the integrity of the barrier intact.
These protein structures are formed from a similar structure found in Keratinocytes called Desmosomes. These structures must be degraded for Corneocytes to shed off in the process called Desquamation (we will touch on this below).
Natural Moisturizing Factors (NMF)
Water-attracting molecules that are formed from the breakdown of a protein called Filaggrin. These molecules are our skin’s natural humectants and are located within the Corneocytes. They help to keep water within the cell allowing for important processes to occur, preventing the cells from drying out, and helping to maintain the skin’s pH. These molecules include things like PCA, urea, lactic acid and free amino acids.
How the Skin Barrier is Formed
Now that we have a better understanding of the key structures and components of the skin barrier, we can dive into the basics of how the barrier is formed.
Cornification
It all starts deeper in our Epidermis with the Keratinocytes. These cells gradually make a complex journey upwards through the Epidermis.
Over their travels they start slowly changing in structure. Production of certain lipids and proteins is ramped into high gear to create the materials needed for forming some of the components we defined above.
When the Keratinocytes have traveled as far as they can (they have reached the Stratum Granulosum), a few important things occur in order to be welcomed into the Stratum Corneum.
High levels of calcium present in the Stratum Granulosum cause the release of profilaggrin, which is further processed into Filaggrin. The Filaggrin then facilitates the aggregation of the keratin fibers causing the collapse and flattening of the cell.
The calcium also causes the rupture of a sack of lipids, coined lamellar bodies, that is produced when the Keratinocytes are still alive. This rupture releases key lipids and enzymes that are integrated into the Cornified Envelope and the lipid matrix. The enzymes break down the released lipids and transform them into the cholesterol, free fatty acids and ceramides.
During this process the plasma membrane is replaced by the cornified envelope and the maturation to the final Corneodesmosome structure also occurs.
And just like that, the Keratinocytes have found their new home within the skin.
Desquamation (AKA skin shedding)
Once the Keratinocytes have officially become Corneocytes the work isn’t over yet.
The Corneocytes continue to migrate to the surface of the skin and eventually flake off through a process called desquamation (AKA skin cell shedding). This is the final destination.
(Okay but side note- does that scene with the log coming through the window in Final Destination live rent free anyone else's head?)
The process of skin cell shedding is thought to be a part of our skin’s physical innate immune system. Since cells are always being removed, any harmful microorganisms can go with it.
During this final migration more important stuffhappens within the skin barrier.
When the water activity is just right within the cells, our bestie filaggrin is broken down into our NMF. There are a few important molecules that form that I want you to know: trans-urocanic acid (UCA) and pyrrolidone-5-carboxylic acid (PCA). These two acids will become critical for maintaining the pH within the Stratum Corneum. Another cool feature about UCA is that it also has photoprotective effects against UV rays (the more you know guys!!).
Maintaining proper pH is critical for the processing and organization of the extracellular lipid matrix and for regulating enzymatic activity during this whole shedding event.
Alteration of the lipid matrix structure starts to reduce the cohesion between the cells. Then enzymes come in and start to eat away at the Corneodesmosomes. With the main adhesives weakened the cells reach their final destination (aka your clothes, the pillowcase, the floor... ok sorry if I grossed you out).
This marks the end of the journey for the cells that form our skin barrier.
The skin barrier has become one of the most talked about topics in skincare but talking about it and truly formulating for it are two very different things. Now that you have a clearer picture of what the barrier actually is and how it forms, the challenge is letting that knowledge shape your decisions at the bench.
References
[1] Dissecting the formation, structure and barrier function of the stratum corneum | International Immunology | Oxford Academic (oup.com)
[4] The skin barrier in healthy and diseased state - ScienceDirect
[5] The Discovery and Function of Filaggrin - PMC (nih.gov)
[6] Structure and Function of the Stratum Corneum Extracellular Matrix - PMC (nih.gov)
[7] The mechanisms by which lipids coordinately regulate the formation of the protein and lipid domains of the stratum corneum - PMC (nih.gov)
[8] Understanding the physical properties that affect the lipid organization in the skin barrier - Leiden University (universiteitleiden.nl)
