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Our skin is made up of three main layers: the epidermis, the dermis and the hypodermis. The epidermis is actually made up of 5 smaller layers and the uppermost or surface layer, known as the stratum corneum or horny layer, is the skin barrier.
To help us understand what happens in eczema and other dry skin conditions we first need to know what a healthy skin barrier looks like and what it does. The best way to think of this barrier is as a brick wall whose function is to keep out irritants, allergens and pathogens whilst preventing the loss of water and other substances from the skin. The individual bricks within this wall are the skin cells (called corneocytes) and the extracellular lipids that surround these cells are the mortar. In healthy skin, this brick wall is solid without any cracks or crumbling mortar, as shown in the healthy skin side of Fig 1.