Structure of the Skin in Cats
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The skin is the largest organ of your cat’s body. It provides a protective barrier against the environment, regulates temperature, and gives your cat its sense of touch. Depending on the species and age, the skin may be 12 to 24% of a cat’s body weight. The skin has 3 main layers: the epidermis or outer layer, the dermis or middle layer, and the subcutis or innermost layer. Other important parts of the skin include skin appendages (such as hair and claws) and subcutaneous muscles and fat.
The epidermis is the top skin layer. It provides protection from foreign substances. The epidermis is constantly regenerating. New skin cells are created near the base of the epidermis and migrate upwards in a process called keratinization. This produces a compact layer of dead cells on the skin surface. This layer keeps in fluids, salts, nutrients, and water, while keeping out infectious or noxious agents. The top layer of dead skin cells are continuously shed and replaced by lower cells. The rate of cell replacement is affected by nutrition, hormones, tissue factors, immune cells in the skin, and genetics. Disease and inflammation also alter normal cell growth and keratinization.
Cells in the epidermis can be damaged by excessive ultraviolet light exposure. Healthy skin cells contain a skin and hair pigment called melanin. The presence of melanin helps protect the cells from the damaging rays of the sun.
The dermis supports and nourishes the epidermis and skin appendages. The network of blood vessels that supply the epidermis with nutrients is in the dermis. Blood vessels are also responsible for regulating 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 produces collagen and elastin proteins that give support and elasticity to the skin. There are also immune cells in the dermis that defend against infectious agents that pass through the epidermis.
Hair follicles, oil and sweat glands, and claws are skin appendages that grow out of the epidermis and dermis. The hair follicles of cats are compound. The follicles have a central hair surrounded by 3 to 15 smaller hairs all exiting from a common pore. Cats are born with simple hair follicles that develop into compound hair follicles.
The growth of hair is affected by nutrition, hormones, and change of season. Cats normally shed hair in the early spring and early fall. They may also shed in response to changes in temperature or the amount of sunlight. 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 it also helps regulate body temperature. Trapping dead air space 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 paws, back of the neck, rump, chin, and tail area. They are part of the cat’s scent-marking system. Cats mark territories by rubbing their face on objects and depositing a layer of sebum laced with feline facial pheromones. Sebum is a mixture of fatty acids. It is important for keeping the skin soft and pliable and for maintaining proper hydration. Sebum gives the hair coat sheen and has antimicrobial properties.
Cats have sweat glands on the feet that may have a minor role in cooling the body. Cats also will sweat through their paws when excited; this is most commonly seen as wet paw prints on surfaces, such as shiny countertops or floors.
The subcutis is the innermost layer of the skin. It contains the subcutaneous fat and muscles. (The word subcutaneous means “beneath the skin.”) The twitch muscle is the major muscle immediately beneath the skin. The subcutaneous fat provides insulation; a reservoir for fluids, electrolytes, and energy; and a shock absorber.