The skin is the largest organ of the body and, depending on the species and age of the animal, can represent 12–24% of the animal’s body weight. The skin has many functions, including provision of an enclosing barrier and environmental protection, regulation of temperature, production of pigment and vitamin D, and sensory perception.
Anatomically, the skin consists of the following structures: epidermis, basement membrane zone, dermis, appendageal system, and subcutaneous muscles and fat (see normal skin and skin with hair images).
Courtesy of Dr. Karen Moriello.
Courtesy of Dr. Gheorghe Constantinescu.
Epidermis in Animals
The epidermis is composed of multiple layers of cells consisting of keratinocytes, melanocytes, Langerhans cells, and Merkel cells.
The vitamin D precursor 7-dehydrocholesterol forms in the epidermis. The epidermis is thickest in large animals.
Keratinocytes function to produce a protective barrier. They are produced from columnar basal cells attached to a basement membrane. The rate of cell mitosis and subsequent keratinization are controlled by a variety of factors, including nutrition, hormones, tissue factors, immune cells in the skin, and genetics.
Keratinocytes play a key role in the skin's immune system and in regulating cell growth and renewal. The dermis can also exert appreciable control over epidermal growth. Photoperiod and reproduction cycles have been hypothesized to affect the epidermis in animals. Glucocorticoids decrease mitotic activity; disease and inflammation also alter normal epidermal growth and keratinization.
As keratinocytes migrate upward, they undergo a complex process of programmed cell death called keratinization. The goal of this process is to produce a compact layer of dead cells called the stratum corneum, which functions as an impermeable barrier to the loss of fluids, electrolytes, minerals, nutrients, and water, while preventing infectious or noxious agents from penetrating the skin. The structural arrangement of keratin and the lipid content of the skin are critical to this function. The stratum corneum is continuously shed or desquamated.
Melanocytes are located in the basal cell layer, outer root sheath (outermost epithelial layer of a hair follicle), and ducts of sebaceous and sweat glands. They produce pigment (melanin) in the skin and hair. Pigment production is under hormonal and genetic control. Melanocytes provide constitutive pigmentation (genetically programmed pigment) and facultative pigmentation, which develops as a result of stimulation from hormones, UV light, inflammation, etc.
Langerhans cells are mononuclear dendritic cells that are intimately involved in regulating the immune system of the skin. They are damaged by excessive UV light exposure and glucocorticoids. Antigenic and allergenic material is processed by these cells and transported to local and nodal T cells to induce hypersensitivity reactions. Epidermal proteins can also conjugate with exogenous haptens (small molecules that are not immunogenic), rendering them antigenic.
Merkel cells are specialized sensory cells associated with skin sensory organs—eg, whiskers and tylotrich pads. These are slow-adapting mechanoreceptors. Merkel cells can also influence cutaneous blood flow and sweat production, coordinate keratinocyte proliferation, and stimulate stem cell growth of hair follicles.
Basement Membrane Zone in Animals
The basement membrane zone serves as a site for attachment of basal epidermal cells and as a protective barrier between the epidermis and dermis. A variety of skin diseases, including several autoimmune conditions, can damage this zone. Vesicles result from damage to the basement membrane zone.
The basement membrane zone has the following important functions:
anchoring the epidermis to the dermis
maintaining a functional and proliferative epidermis
maintaining tissue architecture
participating in wound healing
acting as a physical barrier
regulating nutritional support between the epidermis and underlying connective tissue
Dermis in Animals
The dermis is a mesenchymal structure that supports, nourishes, and, to some degree, regulates the epidermis and appendages. The dermis consists of ground substance, dermal collagen fibers, and cells (fibroblasts, melanocytes, mast cells, and occasionally eosinophils, neutrophils, lymphocytes, histiocytes, and plasma cells).
Blood vessels responsible for thermoregulation, nerve plexuses associated with cutaneous sensation, and both myelinated and unmyelinated nerves are present in the dermis.
Motor nerves in the dermis are primarily adrenergic and innervate blood vessels and arrector pili muscles. Except in horses, epitrichial (formerly "apocrine") glands do not appear to be innervated.
Sensory nerves are distributed in the dermis, hair follicles, and specialized tactile structures. These nerves allow the skin to respond to the sensations of touch, pain, itch, heat, and cold.
Appendageal System in Animals
Appendageal structures grow out of (and are continuous with) the epidermis and consist of hair follicles, sebaceous and sweat glands, and specialized structures (eg, claws, hooves).
The hair follicles of horses and cattle are simple, with one hair emerging from each pore.
The hair follicles of dogs, cats, sheep, and goats are compound, with a central hair surrounded by 3–15 smaller hairs, all exiting from a common pore (see compound hair follicle image). Animals with compound hair follicles are born with simple hair follicles that develop into compound hair follicles.
Courtesy of Dr. Karen Moriello.
The growth of hair is controlled by a number of factors, including nutrition, hormones, and photoperiod. The growing stage is referred to as anagen, the resting stage (mature hair) is telogen, and the transitional stage between anagen and telogen is catagen. Animals normally shed their hair in response to changes in temperature and photoperiod; most animals shed hair in early spring and early autumn.
The size, shape, and length of hair are controlled by genetic factors but can be influenced by disease, exogenous drugs, nutritional deficiencies, and environment. Hormones have an important effect on hair growth, with thyroxine initiating it and glucocorticoids inhibiting it.
The primary functions of hair are to provide a mechanical barrier, to protect the host from UV damage, and to facilitate thermoregulation. In most species, trapping air between secondary hairs conserves heat. To enable this function, hairs must be dry and waterproof; the cold-weather coat of many animals is often longer and finer to support heat conservation.
Hair can also help cool the skin. The warm-weather coat of animals, particularly large animals, consists of shorter, thicker hairs and fewer secondary hairs. This anatomical change allows air to move easily through the coat, facilitating cooling. Hair also helps conceal or camouflage the animal.
Sebaceous glands are simple or branched alveolar, holocrine glands that secrete sebum into the hair follicles and onto the epidermal surface. They are present in large numbers near the mucocutaneous junction, interdigital spaces, dorsal neck area, rump, chin, and tail.
In some species, sebaceous glands are part of the scent-marking system. In cats, for example, they are present on the face, dorsum, and tail in high concentration; a cat marks its territory by rubbing its face on objects and depositing a layer of sebum laced with feline facial pheromones.
Sebum is a complex lipid material containing cholesterol, cholesterol esters, triglycerides, diester waxes, and fatty acids. Sebum is important to keep the skin soft and pliable and to maintain proper hydration; it gives the hair sheen and has antimicrobial properties.
Epitrichial (formerly "apocrine") and atrichial (formerly "eccrine") sweat glands are part of the thermoregulatory system. Atrichial glands are present only on the footpads. Epitrichial glands are found in areas with hair follicles but are not present on footpads or on the planum nasale.
Evaporation of sweat from the skin is the primary body-cooling mechanism for horses and primates and, to a lesser degree, sheep and goats. Pigs have limited sweat glands—not enough for sweating to be an effective cooling mechanism. Pigs thermoregulate by finding water or mud to cool themselves.
Some clinical evidence suggests that limited sweating occurs in dogs and cats and could play a minor role in cooling the body. Dogs and cats thermoregulate primarily by panting, drooling, and spreading saliva on their coats (cats). Cats also sweat through their paws, especially when excited; this sweating is most commonly evidenced as wet paw prints on surfaces—eg, examination tables.
Subcutaneous Muscles and Fat in Animals
The “twitch muscle” (panniculus carnosus) is the major subcutaneous muscle. Subcutaneous fat (panniculus adiposus) serves many functions, including insulation; storage for fluids, electrolytes, and energy; and shock absorption.
For More Information
Souci L, Denesvre C. 3D skin models in domestic animals. Vet Res. 2021;52:21.
Lopez-Ojeda W, Pandey A, Alhajj M, Oakley AM, eds. Anatomy, Skin Integument. StatPearls; updated October 17, 2022.
World Association for Veterinary Dermatology: WAVD Foundation Course in Veterinary Dermatology
Schuldenfrei MD, Pieper JB. Evaluation of hair follicle parameters using TrichoScale Pro© in healthy dogs: a pilot study. Vet Dermatol. 2020;21(3):181-e37.
Also see pet owner content regarding skin structure in dogs, cats, and horses.
