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Structure and Function of the Skin and Hair Coat in Dogs

By: Virginia Wells

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What Are the Skin and Hair Coat?

The skin and hair coat comprise the outer covering of the body and the skin is the largest organ of the body. Together with the claws, pads and skin glands, they form the integumentary system.

Where Are the Skin and Hair Coat Located?

The skin is located on the outer part of the body and covers the muscles, skeleton and internal organs. The hair coat is located on the external part of the skin.

What Is the General Structure of the Skin and Hair Coat?

The skin consists of an outer cellular, avascular layer called the epidermis, and an inner fibrous corium or dermis that rests on a supporting layer of fat and very thin muscle.

The epidermis is the body's environmental shield. It is made up of tough keratinized cells that are continuously formed and shed from the surface. In some animals and in some areas of the body, epidermis contains dark brown pigment that screens the body from harmful rays of the sun and gives the dog his distinctive color. The dermis is composed of a network of connective tissue that also contains nerves, blood vessels, hair follicles, and sweat and oil glands.

Dogs have several different types of hair. On the back are both guard hairs and under-hairs. The guard hairs are longer, thicker and stiffer. The smaller underhairs provide most of the insulation and softness. The basic unit of hair production is the hair follicle (folliculus pili). Each follicle has one guard hair and up to 15 secondary hairs emerging from the same follicle. These underhairs grow as a tiny tuft of hair and can be seen to sprout from the pore of the follicle.

The type and length of hair coat vary widely among dog breeds. Smooth-haired terriers and toy poodles have the highest density of follicles, giving them lots of guard hairs and the fewest number of underhairs coming from each follicle. German shepherd dogs, Airedales and rottweilers have only half as many follicles but have twice as many under-hairs coming from each follicle.

What Are the Functions of the Skin and Hair Coat ?

The skin and hair coat combined are the dog's largest sensory organ, monitoring the environment and influencing body temperature.

The skin has many important functions. It prevents desiccation and dehydration, acts as a sensory organ, and is the receptor for the perception of touch, pressure, vibration, heat, cold and pain. It prevents trauma, protects against invasion of microorganisms and noxious chemicals, and regulates temperature changes within the body. It also acts as the site of vitamin D synthesis. The subcutaneous tissues serve as a reservoir for fat, electrolytes, water, carbohydrates and proteins. Secretions from skin glands waterproof and lubricate the skin and function as pheromones (substance secreted by one individual that allows a second individual to recognize it). Skin may reflect the state of health of the animal, as well as indicate the presence of internal diseases.

The hair coat serves as an insulating layer between the dog's skin and the external environment. It protects him from the cold in winter, and the heat and sun in the summer. It also serves as aggressive display, as when his "hackles rise" on the back when the dog is threatened or frightened.

What Are the Common Diseases of the Skin and Hair Coat?

Many skin diseases are lifelong problems requiring continual or frequent treatments by owners and veterinarians. Common diseases that affect the skin are:

  • Dermatitis or inflammation of the skin is caused by numerous agents like irritants, allergens, and bacterial, viral, parasitic and fungal infections. These diseases include contact dermatitis, flea allergy dermatitis, acute moist dermatitis, atopy, and pyoderma.

  • Parasitic diseases are also common. They include red mange or demodicosis, sarcoptic mange, the migration of parasite larvae, and flea, tick, and lice infestation.

  • Numerous degenerative diseases of the skin and hair coat may occur. These include various forms of alopecia, such as pattern alopecia, flank alopecia, color mutant alopecia, and excessive shedding.

  • Some skin diseases arise from abnormal immune reactions, and form areas of persistent, sterile inflammation. Examples include sterile pyogranulomas, sebaceous adenitis, puppy strangles, eosinophilic dermatopathies, and injection reactions.

  • Immune-mediated skin diseases may also develop, such as the pemphigus complex, systemic lupus erythematosus, discoid lupus, atopy (inhalant allergy), and vasculitis.

  • Skin and hair coat changes are a common manifestation of hormonal (endocrine diseases), such as hypothyroidism, hyperadrenocorticism ( Cushing's disease), and sex hormone abnormalities.

  • Nutritional deficiencies, such as protein deficiency, fatty acid deficiency, vitamin A deficiency, and zinc deficiency, often cause skin and hair coat changes.

  • Numerous forms of skin cancer also occur in the dog. While many skin tumors are generally benign, a variety of malignant tumors may also occur.

    What Types of Diagnostic Tests Are Used to Evaluate the Skin and Hair Coat?

  • Complete blood count, biochemistry profile, urinalysis, and fecal tests to evaluate general health
  • Hormonal assays
  • Serum immune tests
  • Microscopic examination of skin scrapings
  • Microscopic examination of hair shafts
  • Fungal and bacterial cultures
  • Skin biopsies
  • Intradermal and serum allergy testing

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