Canine and Feline Skin

STRUCTURE

The skin is divided into three layers: epidermis, dermis and hypodermis.  The epidermis, the outermost layer of the skin, is composed of keratinocytes (scales), melanocytes (pigment producing cells) and Langerhans cells (immune surveillance cells).

Keratinocytes are aligned in layers in the epidermis.  The deepest one, the stratum basale, is formed by a single row of germinative keratinocytes (actively growing cells) and melanocytes.  The next layer, stratum spinosum, differs from stratum basale by the presence of intercellular junctions and Langerhans.   The stratum granulosum is next, followed by the stratum corneum, the outermost epidermis layer, is composed of keratinocytes in their maximal differentiation stage (scale) interspersed in a lipid (oily) matrix.

The dermis, the layer under the epidermis, is composed of a conjunctive matrix where reticular, elastic and collagen fibers are found.  The dermis is made up of a cellular structure composed of fibroblasts, mast cells, histiocytes and epidermal appendages (hair, nails, sebaceous and sweat glands), arrector pili muscles and blood and lymph vessels.

The hypodermis or subcutaneous tissue provides support and cushioning against physical trauma.  It is composed of a loose connective tissue and elastic fibers interspersed by adipocytes (fat cells).

FUNCTION

The hair follicles exhibit activity cycles that result in hair formation.  Anagen is a period of active growth when a new hair is being formed.  Catagen is when the hair growth stops and degenerative changes occur in the base of the follicle.  Telogen represents a period of follicle inactivity, when the hair is shed so that a new one will start to grow.  Hair cycle activity, in some dog breeds, is strongly related to temperature variation and photoperiodism (day length), leading to decreased hair density in the warmer months, which helps reduce heat in these animals.  Short or long hair depends on the length of the hair cycle.

The skin is the largest organ of the body with many different functions such as ornamentation, thermoregulation, immune protection, sensory perception, vitamin D production and as a protective barrier between the animal and the environment.  Besides all of these important functions and the diseases that affect it directly, the skin can also reflect pathologic processes stemming from internal organs.  Due to these characteristics, dermatologic problems are among the most commonly seen disorders in veterinary hospitals.