Structure of the Skin in Horses
The skin is the largest organ of your horse’s body. It provides a protective barrier against the environment, regulates temperature, and gives your horse its sense of touch. Depending on the species and age, the skin may be 12 to 24% of an animal’s body weight. The skin has 3 major layers: the epidermis or outermost layer, the dermis or middle layer, and the subcutis or innermost layer. Other important components include skin appendages (such as hair and hooves), and subcutaneous muscles and fat.
The epidermis is the outer layer of skin, which is composed of several layers of cells. It provides a barrier of protection from foreign substances. The epidermis is thickest in large animals like horses. It includes multiple types of cells, including keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Each of these cells has special functions.
Keratinocytes provide a protective layer that is constantly being renewed in a process called keratinization. In this process, new skin cells are created near the base of the epidermis and migrate upwards. This produces a compact layer of dead cells on the skin surface. This layer keeps in fluids, electrolytes, and nutrients, while keeping out infectious or noxious agents. The top layer of dead skin cells are continuously shed and replaced by cells from lower layers. The rate of cell replacement is affected by nutrition, hormones, tissue factors, immune cells in the skin, and genetics. Disease and inflammation can also change normal cell growth and keratinization.
Melanocytes are located at the base of the epidermis, the outer root sheath of hairs, and the ducts of the sebaceous and sweat glands. The melanocytes produce the skin and hair coloring (pigment) called melanin. Production of melanin is controlled by both hormones and the genes received from parents. Melanin helps protect the cells from the damaging rays of the sun.
Langerhans cells are part of the immune system. These cells are damaged when exposed to excessive ultraviolet light and glucocorticoids (anti-inflammatory drugs). Langerhans cells play an important role in the skin’s response to foreign substances and contribute to such things as the development of rashes when an animal is exposed to irritating materials.
Merkel cells are specialized cells associated with the sensory organs in the skin. In particular, Merkel cells help provide animals with sensory information from whiskers and the deep skin areas called tylotrich pads.
The dermis supports and nourishes the epidermis and skin appendages. The blood vessels that supply the epidermis with nutrients are located in the dermis. Blood vessels also regulate skin and body temperature. Sensory nerves are located in the dermis and hair follicles. The skin responds to the sensations of touch, pain, itch, heat, and cold. The dermis secretes the proteins collagen and elastin that give support and elasticity to the skin. Also present are immune cells that defend against infectious agents that pass through the epidermis.
Hair follicles, oil and sweat glands, and hooves are all skin appendages that grow out of the epidermis and dermis. The hair follicles of horses are simple—the follicles have 1 hair emerging from each pore.
The growth of hair is affected by nutrition, hormones, and change of season. The size, shape, and length of hair are controlled by genetics and hormones. Disease, drugs, nutrition, and environment also affect the health of hair.
The hair coat protects the skin from physical and ultraviolet light damage, and helps regulate body temperature. Trapping air between secondary hairs conserves heat. This requires that the hairs be dry and waterproof. The cold-weather coat of many animals is often longer and finer to facilitate heat conservation. The hair coat can also help cool the skin. The warm-weather coat has shorter thicker hairs and fewer secondary hairs. This anatomic change allows air to move easily through the coat, which facilitates cooling.
Oil glands (also called sebaceous glands) secrete an oily substance called sebum into the hair follicles and onto the skin. They are present in large numbers near the hooves, back of the neck, rump, mouth, and tail area. Sebum is important for keeping the skin soft, moist, and pliable. It gives the hair coat sheen and has antimicrobial properties.
Sweat glands are part of the horse’s system to regulate body temperature. They are found over most of the body except the legs. The evaporation of sweat from the skin is the primary cooling mechanism of the body for horses.
The subcutis is the innermost major layer of skin. It contains the subcutaneous fat and muscles. (The word subcutaneous means “beneath the skin.”) The twitch muscle is the major subcutaneous muscle. The subcutaneous fat provides insulation; a reservoir for fluids, electrolytes, and energy; and a shock absorber. Nerves and blood vessels that supply the skin are also found in the subcutis.