Stratum corneum barrier function -- the brick and mortar model

The stratum corneum is only about 0.02mm thick -- less than half the thickness of plastic wrap. Yet this ultra-thin layer functions as the critical barrier between the outside world and the body's interior. The key to understanding its structure is the "brick and mortar model."

Bricks = Corneocytes

Corneocytes are "dead cells" that have lost their nuclei and flattened at the final stage of turnover. These stack in multiple layers like bricks. Inside corneocytes, NMF (Natural Moisturizing Factor) holds water. Key NMF components include PCA (pyrrolidone carboxylic acid), UCA (urocanic acid), lactic acid, and amino acids.

Mortar = Intercellular lipids

The "mortar" filling gaps between bricks is intercellular lipids. Their composition is ceramides:cholesterol:fatty acids = 50:25:25. Ceramides are the most critical -- when deficient, barrier function drops dramatically.

Lamellar structure -- precise layers of water and oil

Intercellular lipids aren't just a "blob of fat" -- they form a lamellar structure with alternating water and lipid layers. The LPP (Long Periodicity Phase) with its 13nm periodicity plays a decisive role in barrier function.

Measuring barrier function with TEWL

TEWL (Transepidermal Water Loss) measures how much moisture evaporates from the skin. High TEWL = compromised barrier; low TEWL = healthy barrier. It's a crucial metric for scientifically evaluating skincare product effectiveness.