Your skin's outermost layer works like a brick wall: dead cells called corneocytes are the bricks, and a lipid matrix between them is the mortar. That mortar is built from three lipids—ceramides, cholesterol, and free fatty acids—in roughly a 1:1:1 molar arrangement, corresponding to approximately 40–50% ceramides, 25% cholesterol, and 10–15% fatty acids. When any of these is depleted, the wall leaks water, and dryness, irritation, and sensitivity follow.
The Skin Barrier: A 'Brick and Mortar' Foundation
Dermatologists routinely describe the stratum corneum—the skin's outermost layer—as a "bricks and mortar" structure. A clinical review of skin barrier physiology published in the PMC library explains that the corneocytes represent the bricks while the intercellular lamellar lipid membrane represents the mortar. Writing in Dermatology Times, dermatologists Zoe Diana Draelos and Cheri Frey describe the same structure as a "brick wall" whose lipid matrix prevents water loss and blocks irritants.
The lipid portion is not a minor ingredient. According to the clinical review, stratum corneum lipids represent about 20 percent of the total volume of the stratum corneum, and the intercellular membrane they form is vital to maintaining water flux and balance. In other words, the mortar does much of the protective work the "wall" is known for.
Ceramides: The Structural Backbone of the Mortar
Ceramides are the largest single component of the barrier's lipids. The clinical review states that the intercellular lipid membrane is "purposely designed to predominantly contain" ceramides at 40–50 percent, arranged in a lattice with the other key lipids. CeraVe's educational materials, drawing on dermatologist Jenny Liu, put the figure at about 50% of the skin's lipids, describing ceramides as helping "keep vital moisture in and harmful irritants out."
That structural role explains why ceramide depletion is so consequential. When ceramides become depleted, the same source notes, skin can become dull, dry, agitated, or cracked over time. The lattice loses its dominant scaffolding, and the membrane's organization suffers.
Cholesterol and Fatty Acids: Essential Partners in Barrier Function
Ceramides do not work alone. Cholesterol and free fatty acids are integral components of the same intercellular matrix, and the clinical review identifies all three as strategically arranged together to provide the barrier's physiological function. Cholesterol contributes to the structural integrity and organization of the lipid matrix, which acts as the permeability barrier and helps maintain proper water flux and balance.
Free fatty acids, likewise, are part of the lamellar bilayers surrounding corneocytes. The clinical review assigns them a 10–15 percent share of the membrane's lipids. Neither cholesterol nor fatty acids is a supporting extra; the membrane's design depends on all three classes being present together in their characteristic proportions.
