“Bricks and mortar” is a model dermatologists use to describe how your skin’s outermost layer, the stratum corneum, is built. The “bricks” are flat, dead skin cells called corneocytes, and the “mortar” is the layer of fats (lipids) packed between them. Together, they form a waterproof, protective wall that keeps moisture in and irritants out. While the phrase is commonly used in business to describe physical retail stores, in biology and skincare it refers to one of the most important structural concepts in skin health.
How the Bricks and Mortar Structure Works
Your skin constantly produces new cells in its deeper layers. These cells, called keratinocytes, undergo a process of terminal differentiation as they migrate upward. By the time they reach the surface, they’ve flattened out, lost their internal machinery, and become corneocytes: tough, interlocking plates filled with a protein called keratin. They stack in overlapping layers, much like bricks in a wall.
The mortar between these bricks is a carefully organized mix of lipids, primarily ceramides, cholesterol, and free fatty acids. These fats aren’t just passively filling space. Each corneocyte has a lipid envelope chemically bonded to its outer surface, creating what researchers describe as a “Teflon-like coating.” These envelopes interlock with the surrounding lipid layers in a Velcro-like fashion, anchoring the bricks firmly into their mortar. The result is a barrier that is both flexible and remarkably waterproof.
Holding the bricks to each other are tiny protein rivets called corneodesmosomes. Think of them as rebar connecting one brick to the next. When your skin is healthy, these connections are strong in the lower layers of the stratum corneum and gradually break down only at the very surface, allowing dead cells to shed naturally in a controlled process called desquamation.
What the Bricks Actually Do
Corneocytes are far more than passive shields. They provide mechanical reinforcement against physical damage, protect the living cells beneath them from ultraviolet radiation, help regulate inflammation, and maintain hydration by holding onto water within their keratin structure. Each corneocyte retains keratin filaments embedded in a protein matrix, giving it rigidity without brittleness.
The layered stacking of these cells is what gives your skin its toughness. A single corneocyte is thin and fragile, but dozens of interlocking layers create a composite material that resists tearing, chemical penetration, and water loss simultaneously.
What the Mortar Is Made Of
The lipid mortar has a specific composition that matters for barrier function. Ceramides make up the largest share, roughly 50% or more of the total lipid weight in the stratum corneum. The remaining mortar consists of cholesterol and free fatty acids in smaller proportions, along with some omega-hydroxy acids. This ratio isn’t random. The three main lipids organize into flat, repeating sheets called lamellae, and their precise proportions determine how tightly packed and impermeable those sheets are.
In healthy skin, lipids account for about 10% of the stratum corneum’s weight. That number may sound small, but it’s the difference between a functional barrier and one that leaks. In people with atopic dermatitis (eczema), that lipid content can drop to roughly 5%, and the ceramide fraction drops disproportionately. This is why eczema-prone skin loses water faster and lets allergens in more easily.
How a Damaged Barrier Shows Up
One of the clearest signs of a compromised bricks-and-mortar structure is increased transepidermal water loss, or TEWL. This is exactly what it sounds like: water escaping through the skin into the air. Healthy forearm skin might lose only a few grams of water per square meter per hour, while damaged or naturally thinner skin (like the armpits, where TEWL can reach 44 grams per square meter per hour) loses far more. Higher TEWL generally signals a weaker barrier.
When the mortar is depleted, skin feels dry, tight, and irritated. When the rebar (corneodesmosomes) fails, the consequences can be more dramatic. Genetic mutations that prevent the body from making key corneodesmosome proteins cause conditions like peeling skin disease, where entire sheets of stratum corneum separate from the layers below, leaving raw, unprotected areas. Other mutations in these structural proteins lead to severe dermatitis, multiple allergies, and metabolic problems because the barrier can no longer prevent immune-triggering substances from entering the body.
A condition called Netherton syndrome illustrates another way the system breaks down. Normally, enzymes that break down corneodesmosomes are kept in check by an inhibitor protein. When that inhibitor is missing, the enzymes chew through the rebar too quickly, causing premature shedding, chronic inflammation, and severe barrier impairment.
Repairing the Mortar Layer
Because the mortar is made of specific lipids in specific proportions, effective barrier repair means supplying those same lipids back to the skin. Research on atopic dermatitis suggests that topical products should contain ceramides, cholesterol, and free fatty acids at a final concentration of at least 5% to meaningfully replenish what’s missing. The ideal ratio is ceramide-dominant: about 3 parts ceramide to 1 part free fatty acid to 1 part cholesterol. This mirrors the natural composition and allows the applied lipids to integrate into the existing lamellae rather than just sitting on the surface.
Some formulations also include ceramide precursors or ingredients that stimulate the skin’s own ceramide production, such as compounds derived from eucalyptus oil. The goal is not just to coat the skin temporarily but to restore the mortar’s architecture so the barrier can function on its own between applications.
Simple petroleum-based moisturizers work differently. They create an occlusive film on top of the skin that physically blocks water loss, but they don’t rebuild the lipid structure between corneocytes. Both approaches reduce TEWL, but lipid-based repair creams address the underlying problem rather than masking it.
Why This Model Matters for Skincare
Understanding the bricks-and-mortar model changes how you think about skin damage and repair. Harsh cleansers, over-exfoliation, and prolonged water exposure strip the mortar. Cold, dry air pulls moisture through a thinned lipid layer. Aging reduces both the speed of corneocyte replacement and the volume of lipids produced, gradually weakening the wall from both sides.
When you see a skincare product marketed for “barrier repair,” it’s targeting the mortar. When a dermatologist diagnoses a “barrier defect,” they’re saying the bricks, the mortar, or the connections between them aren’t holding up. The bricks-and-mortar framework gives both patients and clinicians a shared language for understanding what’s gone wrong and what needs to be restored.