Our skin is made up of three main layers: the epidermis, the dermis and the hypodermis. The epidermis consists of 5 smaller layers: the stratum basale, the stratum spinosum, the stratum granulosum, the stratum lucidum and the uppermost layer, known as the stratum corneum, which is the skin barrier.
In order 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 called corneocytes (anucleated keratinocytes) and the intercellular lipid lamellae which surround them are the mortar. This lipid lamellae is composed of phospholipids, cholesterol and glucosylceramides. In healthy skin, this brick wall is solid without any cracks or crumbling mortar, as shown in the healthy skin side of Fig 1.
A number of factors contribute to a healthy skin barrier. Firstly, the corneocytes contain a substance called NMF (Natural Moisturising Factor), a collection of low molecular weight compounds that include amino acids, urea and inorganic salts, which acts as a humectant and attracts water into the corneocytes. Secondly, there is the lipid lamellae (composed of phospholipids, cholesterol and glucosylceramides) which surround the corneocytes helping them to retain water inside them. The overall effect is that the corneocytes are swollen and so sit tightly against each other with the lipid lamellae filling any gaps so producing a smooth and strong semi-permeable barrier that allows some water loss (Transepidermal Water Loss or TWL) whilst keeping irritants, allergens and pathogens out.
In skin that is dry or suffering from eczema, this skin barrier is no longer effective as a result of the corneocytes containing less NMF and the breakdown of the lipid lamellae. The reduction in NMF in the corneocytes means that they cannot attract as much water and so start to shrink which results in gaps forming between them. Furthermore, due to the breakdown of the lipid lamellae the corneocytes are no longer completely surrounded (the mortar is crumbling) and cracks start to appear in the skin. We now have a barrier which has cracks and gaps that irritants, allergens and pathogens can penetrate and which causes more water loss than the normal Transepidermal Water Loss (see the Eczema side of Fig 1). This results in itching and inflammation leading to scratching and exacerbation of the itch-scratch cycle (shown in Fig 2) and worsening of the condition. Scratching can also break the skin leaving it open to infections, which can further complicate the eczema or other dry skin condition.