Skin

Hermeline_Diffomedia

Broadly speaking, the skin is the boundary between the body and the external environment. It consists of two structures – the skin in the strict sense, which is a keratinised organ, and its associated structures (hair and glands).

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The skin is a keratinised structure

© Diffomédia/Royal Canin

The skin is split into three layers:

• The epidermis itself consists of several layers:

- a basal layer of dividing cells and cells producing melanin (the pigment that gives the skin its particular colour); a very thick clear layer (two or three cells deep) on the nose and footpads, produced by the cell divisions in the basal layer and macrophages (cells that digest cell debris and pathogens); a granulated layer of flattened cells; a horny layer of very flat cells without a nucleus, which contain a lot of keratin; and an outer layer of flaking cells.

• The dermis is a thick layer – 1.3 mm on the back and up to 2.5 mm on the footpads – separated from the epidermis by a basement membrane. It contains the elastic fibres and collagen, which give the skin its toughness and pliability.

• The hypodermis is the lowest layer, which has a high number of adipocytes, cells that store fat.

Only the dermis and the hypodermis are vascularised and innervated (contain nerves), which enables them to receive information from both the outside and the inside.

The functions of the skin

Duhayer_Royal Canin

• The skin first and foremost has a barrier function. It ensures that substances, such as water, ions and macromolecules, are retained in the body. It also ensures that a number of substances including water and bacteria cannot enter the body. When the cells of the epidermis become swollen with water some substances can pass through into the body. This mechanism is exploited in the use of wet dressings. The skin barrier is also mechanical, which means it protects the body from infrared and ultraviolet radiation (outer layers, and hair and pigmentation respectively) and biological agents.

© Diffomédia/Royal Canin

• The skin also has an exchange function. The sweat produced by the apocrine and exocrine glands (the latter are only located in the nose and footpads) seems only to cool down the skin locally in dogs. It is worthwhile remembering that dogs only sweat from their nose and the skin between their toes. The sebum produced by the sebaceous glands in the hair follicles provides protection by destroying bacteria. The skin can also absorb drugs and toxins such as alcohol, as well as fat-soluble vitamins, sex hormones and other substances. Thermal exchanges are also possible, with the transfer of calories in the wake of a change in temperature.

• Metabolic function: The adipocytes in the hypodermis store fat and the skin also plays a minor role in the production of vitamin D3 when ultraviolet rays strike the outer layers of the epidermis.

• Sensory function: The nerve endings in the dermis and hypodermis enable the skin to pass on information about temperature, pressure, pain or contact with other objects.

Associated structures

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Hair follicles are made up of a sheath of cells and connective tissue, a sebaceous gland and an arrector pili muscle, which is responsible for making the hair stand on end.

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There are various associated structures:

• Hair and hair follicles, which are made up of the sheath of cells and connective tissue which surrounds the root of a hair, a sebaceous gland and an arrector pili muscle, which is responsible for making the hair stand on end.

• The sweat glands. These are the apocrine glands, located deep in the dermis throughout the body, which play a role in the production of pheromones and have a duct opening under the sebaceous gland, and the exocrine glands, which are located in the nose and in the skin between the toes, which pour the sweat they produce directly onto the skin, with a duct opening onto the epidermis independently of the hair follicle.

• Other glands, including the anal glands and the violet or supracaudal glands on the upper surface of the tail, both used for scent marking.

Hair structure

In dogs, the hair follicles are arranged in groups consisting of one primary hair surrounded by thinner, shorter secondary hairs.

Hair density depends on the age and the breed. The softer the hair the denser it is. A German Shepherd, for instance, has 100-300 follicle groups per square centimetre, whereas a breed with a softer coat can have 400-600 follicle groups per square centimetre. The number of follicle groups is established at birth, although young dogs only have down hair which is why they are so soft to the touch. During growth, the angle of hair to skin decreases to about 45°, that of adult dogs.

Hair colour is genetically determined by the dominance of one colour over one or more other colours. This explains the palette of coat colours and the specific markings on some breeds, as explained above.

There are two specific periods of moulting, which produces a winter and a summer coat. This is explained by the three stages of hair follicle activity.

© Diffomédia/Royal Canin

During the anagen phase the hair grows upwards and the hair follicle drives itself down into the dermis. This phase lasts an average 4 months in most dogs, although it takes 18 months in an Afghan Hound.

In the catagen phase growth stops and the follicle recedes.

In the telogen phase the follicle shortens all the way down to the sebaceous gland and the base of the hair contracts into a cone before dropping out. Another hair then starts its anagen phase, taking over the same cavity.

The hairs do not all fall out at the same time, of course. Moulting starts at the rear and progresses forwards. The winter coat is much thicker than the summer coat, to protect the dog from the lower temperatures.

These changes to the dog’s coat do not occur randomly. The main trigger of moulting seems to be the photoperiod (the duration of daylight). As the days grow longer, spring moulting is triggered, whereas autumn moulting is triggered as the nights draw in. Changes in temperature only affect hair density and the speed of regeneration. They have no major impact on moulting.

Although the hairs are replaced the coat colour remains the same, except for some greying on the muzzle of ageing dogs. It is important to clean a dog’s coat regularly to safeguard it against skin disease.

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