Overview
The skin (cutis) covers the whole body surface and is continuous with the mucous membranes at the natural openings: lips, nostrils, eyelids, anus and external genitalia. It is the largest organ of the dog, about 12% of body weight in the adult. It works as a physical, chemical and microbiological barrier, takes part in thermoregulation and in the perception of touch, temperature, pain and itch, synthesises vitamin D and stores water, electrolytes and fat.
Together with the hair, skin glands, pads and claws it forms the common integument (integumentum commune).
Layers
- Epidermis: keratinised stratified squamous epithelium without blood vessels. From deep to superficial it has the basal, spinous, granular, clear (only in the nasal planum and pads) and cornified layers. Besides keratinocytes it contains melanocytes (pigment), Langerhans cells (antigen presentation) and Merkel cells (touch). In the haired skin of the dog it is thin; it becomes much thicker where the skin is hairless and bears friction.
- Dermis (dermis or corium): connective tissue of collagen and elastic fibres that gives strength and elasticity. It holds the vessels and nerves of the skin, the hair follicles, the arrector pili muscles and the sebaceous and sweat glands.
- Hypodermis (tela subcutanea): loose connective tissue with a variable amount of fat. It binds the skin to the fascia and muscles and lets it move. It is loosest over the dorsal neck and the lateral thoracic wall.
Hair and glands
- Hair follicles: in the dog they are compound: one primary (guard) hair and several secondary (wool) hairs leave through the same opening. Hair grows in cycles of anagen (growth), catagen (transition) and telogen (rest).
- Tactile hairs (pili tactiles): long whiskers with a venous sinus around the follicle and a rich nerve supply, on the lips, chin, cheeks and above the eyes.
- Sebaceous glands: open into the follicles; their secretion lubricates the hair and is part of the surface film of the skin.
- Sweat glands: apocrine (epitrichial) glands open into the follicles all over the haired skin; eccrine (atrichial) glands are found only in the pads. That is why dogs sweat little and lose heat mainly by panting.
- Specialised glands: circumanal glands, the tail gland (dorsal, at the base of the tail) and the mammary glands, which are modified skin glands (usually five pairs).
Specialised regions
- Nasal planum (planum nasale): hairless, heavily keratinised skin around the nostrils. Its surface forms a groove pattern unique to each dog. It has no glands: its moisture comes from nasal secretion and licking.
- Pads (tori): thickened skin with a papillated epidermis and a hypodermis rich in fat and elastic fibres that cushions weight bearing. There are digital pads (tori digitales), one metacarpal or metatarsal pad (torus metacarpeus/metatarseus) and, on the thoracic limb only, a carpal pad (torus carpeus) that does not touch the ground at a walk.
- Other regions: the skin of the ear pinna is thin and attached to the cartilage; scrotal skin is thin and sparsely haired; the skin of the dorsal neck and trunk is thick and mobile.
Clinical relevance
- Dermatology: itching and skin lesions are among the most common reasons for consultation. Frequent causes are atopic dermatitis, flea allergy dermatitis, bacterial pyoderma, demodicosis, dermatophytosis and otitis externa. The basic tests are skin scraping, cytology, trichogram, culture and biopsy. Mast cell tumour is the most common malignant skin tumour of the dog.
- Hydration: a skin fold that is slow to return suggests dehydration, although age and body condition affect the result.
- Routes of administration: subcutaneous injections go into the loose skin of the dorsal neck or the lateral thoracic wall; intradermal allergy tests are done on the clipped lateral thoracic wall. Intramuscular injections go into muscles such as the quadriceps femoris or the lumbar epaxial muscles; on the caudal thigh the sciatic nerve must be avoided.