It is easy to forget that the skin is an organ in its own right, equating to a huge 10 to 15 percent of dogs’ and cats’ body weight. It is therefore not surprising that skin health can have a huge impact on our patients’ general health and well-being. The skin and its accessory structures (including hair, nails, hooves, feathers, sweat glands, sebaceous glands and scales) make up the integumentary system, which is the main boundary between the body and the outside world. The skin has several varied functions ranging from protective and immunoregulatory roles to involvement in sensation and homeostasis.
The skin has several varied functions ranging from protective and immunoregulatory roles to involvement in sensation and homeostasis.
Structure of the skin
The skin structure has three main layers: the epidermis, dermis and hypodermis (Figure 1).
Starting with the deepest layer, the hypodermis consists mainly of fat tissue which can act as a store of energy or an insulating layer, or can provide physical protection to underlying tissues.
The next layer is called the dermis, which gives the skin its strength, elasticity, turgor and resilience. It is also the layer that contains blood vessels, nerves, sweat glands and hair follicles, hence it is important for thermoregulation and sensation. The dermis also contains sebaceous glands which produce sebum, an oil that lubricates the skin and coat and regulates the skin surface environment.
The outermost and final layer of the skin is called the epidermis. The epidermis consists of four main layers made up of specialist cells called keratinocytes. Keratinocytes originate from the stratum basale, the deepest part of the epidermis; as they migrate outwards through the stratum spinosum and stratum granulosum they become tougher and stronger.
The outermost layer of the epidermis is called the stratum corneum; by the time they reach this layer, the keratinocytes have lost their nuclei and become very hard and flattened. They are then known as corneocytes. This process is called cornification, which in Latin means “turning to horn”.
The keratinocytes also produce the extracellular lipid matrix, which consists primarily of ceramides along with cholesterol and fatty acids. This fills the space between the corneocytes, forming a waterproof barrier. We often liken the epidermis to a brick wall, where the corneocytes are equivalent to the bricks, and the waterproof lipid matrix to the cement. Together they create a strong, waterproof barrier acting as the first line of defence against environmental pathogens, while also preventing excessive moisture loss from the body.
Functions of the skin
The skin has several important functions that are vital not only for survival, but also in regulation of bodily processes via feedback to the brain.
Immunity
The skin, similar to the gut, has a diverse ecosystem of microorganisms living on its surface known as the microbiome. This consists of a fine balance of commensals that act to prevent the multiplication of pathogenic bacteria, yeast and fungi. The skin microbiome can easily be disrupted if the underlying skin is diseased or damaged, allowing harmful bacteria to proliferate and infections to occur. Furthermore, the epidermis and dermis contain Langerhans cells and dendritic cells respectively, which are specific white blood cells that help to determine the appropriate immune response (inflammation or tolerance) following exposure to exogenous substances.
Protection
The fur provides the first level of protection against trauma and UV light, with the skin beneath acting as the second barrier. The skin provides further protection of the underlying organs and structures from external threats, while also sensing stimuli that could be potentially damaging.
Hydration
One of the most important functions of the skin is to keep our patients hydrated. The corneocytes and extracellular lipid matrix of the epidermis create a “brick wall”-like waterproof barrier which acts to prevent transepidermal water loss (TEWL). As the name suggests, TEWL describes the evaporation of water from the skin to the environment. Water is held within the dermis bound to hyaluronic acid, maintaining skin hydration and giving the skin its turgor. Without this combination of hyaluronic acid in the dermis and a healthy waterproof epidermis, our patients would dehydrate very quickly, with their skin becoming dry, flaky and itchy.
Without this combination of hyaluronic acid in the dermis and a healthy waterproof epidermis, our patients would dehydrate very quickly, with their skin becoming dry, flaky and itchy
Sensory
The basal layer of the epidermis contains Merkel cells of neuroectodermal origin, which enable the sensation of light touch. All other nerve endings are located in the dermis, allowing sensations of heat, cold, pressure and pain. Sensory information is fed back to the brain and results in physiological and/or behavioural reactions, such as panting or moving to a cooler spot in the house if they sense they are too warm.
Temperature regulation
The skin plays a huge role in thermoregulation. In cold temperatures, piloerection enables a layer of insulating air to be trapped against the animal’s skin, thereby increasing its body temperature. Depending on the body’s requirements, dermal capillaries are able to vasoconstrict or vasodilate in order to conserve or lose heat respectively, as needed. It is worth noting that dogs and cats are unable to sweat (except through their paws), so rely on panting as their main method of cooling. This is one reason why dogs are so prone to heatstroke in hot weather.
What happens in disease?
With all these functions in mind, it is clear why supporting our patients’ skin is important, not only for the sake of the skin itself but also for their general health. When the skin’s protective barrier is damaged, these normal functions are disrupted and disease may result, which can present in several ways:
- Dry, flaky skin: Loss of structural integrity of the epidermis results in a dysfunctional waterproof barrier and subsequent transepidermal water loss leaving the skin dry, flaky and itchy. Skin structure can be disrupted by disease, such as physically by burrowing mites or in atopic dermatitis, where skin ceramide levels have been shown to be lower, or by the act of scratching and itching itself
- Pruritus (itching): when exogenous substances cross the damaged skin barrier they initiate inflammatory pathways in the skin, which can lead to itching. Unfortunately, scratching results in further damage and irritation to the skin leading to more itching and exacerbation of the itch–scratch cycle
- Alopecia (hair loss): one of the first dermatological signs that owners may notice in their pets is hair loss; this can be secondary to scratching or due to hormonal/systemic conditions. Close physical examination of the animal can provide important information to direct further work-up, such as signs of erythema, excoriations and broken hair shafts often seen with itching, or other clinical signs noticed with systemic disease
- Erythema (redness) and irritation: Erythema may occur as a direct result of scratching, or a rash may develop following an allergic reaction or infection, among several other causes. A detailed history and physical examination may help to ascertain the cause
- Exudate/abnormal smell: a clear exudate may be seen following skin trauma due to scratching; however, a coloured or smelly exudate is likely to be due to a bacterial or yeast infection. Skin infections can occur following breakdown of the skin barrier, as certain commensal microorganisms are then able to invade and colonise the deeper tissue.
Summary
The skin is a very important organ with many vital functions. Disruption to the barrier function of the skin can be a cause or result of skin disease. Supporting the barrier function of the skin is an important consideration in these cases.
