Addressing epidermal barrier function in canine atopic dermatitis
Classically, atopic dermatitis is thought to be caused by a genetic defect in the immune system, leading to a hypersensitivity to normal environmental allergens. Newer theories propose that genetic defects in skin lipids and proteins create a barrier defect that lets allergens into the body, stimulating an immune response.1 The study of skin barrier function in dogs is the subject of a lot of research, and measuring transepidermal water loss is commonly used to estimate barrier function. New topical treatments geared toward improving the skin barrier are emerging and may be useful additions to the classic atopic dermatitis treatment protocol.
TRANSEPIDERMAL WATER LOSSTo understand skin barrier defect research, a brief review of transepidermal water loss is necessary. A machine called an evaporimeter measures the amount of water evaporating from the skin. The evaporimeter can have an open or closed chamber, and a debate exists about which type is superior.2 Transepidermal water loss measurement is noninvasive and has been shown to be a good estimate of barrier function in dogs and people.
All animals lose a small amount of water through the skin, called perspiratio insensibilis.3 But atopic dogs lose higher amounts of water than normal dogs do.4,5 It is thought that the higher water loss dries out and irritates the skin, but more important, it signifies that the skin barrier is not working properly. If water is leaking out, allergens may be penetrating the barrier.
THE EPIDERMAL BARRIER
Most canine epithelial barrier research can be divided into two categories: research to determine the chemical content of skin and research with electron microscopy to determine the physical architecture of skin. Canine skin can be described as bricks and mortar, with epithelial cells making up the bricks and extracellular lipids and proteins making up the mortar.
The stratum granulosum of the epithelium produces lamellar bodies, which contain necessary lipids and enzymes needed for differentiation and desquamation of epithelial cells.1 The lamellar bodies are extruded into the extracellular space and form organized stacks called lamellae, which help prevent water loss and allergen penetration. Ceramides are a type of lipid that makes up a large portion of the lamellae. Dogs with atopic dermatitis have a skin deficiency of ceramides,5,6 and their lamellae are arranged in a disorderly manner.1