The Lipid Bilayers of the Stratum Corneum: How the Brick-and-Mortar Model Actually Works

Bricks and mortar, literally

The outermost layer of the epidermis, the stratum corneum, is built from flattened dead cells called corneocytes embedded in a lipid matrix. The standard model describes corneocytes as bricks and the lipid matrix as mortar. The mortar is what seals water in and keeps irritants out.

Unlike most cell membranes, this lipid matrix is organized into stacked bilayers made of three main lipids in a tightly regulated ratio: ceramides, cholesterol, and free fatty acids. Research indicates the ratio matters as much as the amount.

The three lipids and filaggrin

• Ceramides, the largest fraction by mass, form the backbone of the bilayer structure.
• Cholesterol, provides fluidity and is often the first lipid depleted after an acute insult.
• Free fatty acids, contribute to the ordered packing and to the acidic surface pH the barrier depends on.
• Filaggrin, a protein that breaks down into natural moisturizing factors, the water-binding molecules that keep the corneocyte bricks hydrated.

What disrupts the bilayer

Studies associate several inputs with disordered bilayers and higher water loss: harsh surfactants that extract lipids, over-exfoliation, low humidity, and age-related decline in lipid production. Filaggrin gene variants also reduce the supply of natural moisturizing factors, which is why some people start with a more fragile barrier.

This is the mechanism behind Resilience. A barrier with intact bilayers in the right ratio tolerates far more daily load before it reacts. Research on barrier repair consistently supports replacing the three lipids together rather than any one alone, since the ratio is what restores the bilayer structure.