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.
The healthy skin barrier is due to a number of factors. Firstly, the skin cells contain a substance called NMF (Natural Moisturising Factor) which attracts water into them and secondly, they are surrounded by extracellular lipids which help to keep the water inside them. The overall effect is that the cells are swollen and so sit tightly against each other producing a smooth and strong barrier. It is important to remember that even in healthy skin this barrier is not absolute and that there is some water loss (this is known as Transepidermal Water Loss or TWL) through it.
In skin that is dry or suffering from eczema, this skin barrier is no longer effective due to there being less NMF in the skin cells and the breakdown of the extracellular lipid layers. As there is less NMF in the skin cells they cannot attract as much water and so start to shrink which results in gaps beginning to form between them. The breakdown of the extracellular lipid layers has a number of effects, firstly cracks start to appear and secondly the skin cells are no longer completely surrounded (the mortar is crumbling). We now have a barrier that has cracks and gaps which allows irritants, allergens and pathogens in and causes more water loss compared to the normal Transepidermal water loss (see the Eczema side of Fig 1). This causes inflammation and itching that results in scratching, which in turn damages the skin resulting in the release of chemicals that causes further inflammation and itching, leading to more scratching so you have yourself an itch-scratch cycle as shown in Fig 2.