How does the dog's coat gets renewed?

Dog’s coat has got several functions:

• thermal insulation,
• protection against injuries, UV sunlight,
• social role for the dog (that raises its hackles when there is a conflict), and for the owner, very sensitive to the aspect and the beauty of his dog.

Any alteration of the coat should be taken as an alarming sign, but this sign is not specific at all. The hair is told to be “ the health’s mirror ”. Dull or brittle hair can be caused by a dietary unbalance, but also by various diseases from digestive, hepatic, renal, thyroidic, immunologic, or parasitic origin… These alterations appear rather late in the course of the disease, because hair grow is rather slow. It is the same for the dietary supplements that aim at improving the aspect of the coat: 4 weeks at leats are necessary before their effects become obvious. Let’s study some data about anatomy and physiology of the hair.

Structure of a primary hair follicle

In dogs, follicles are compound ones.

• There is 1 central follicle that produces primary hair or guard hair.
• There are 2 lateral ones, that produce 5 to 25 secondary hair each, also called underhair.

Cyclic activity of the hair follicle

The cellular renewing starts in the hair bulb.
The hair matrix cells are actively dividing and differentiating along their vertical migration.
The cells from the exterior part of the hair contain pigments, responsible for the hair pigmentation.
30 % of the daily proteic requirement of an adult dog would be used for the renewal of the skin and the hair.

There are 3 steps in the cyclic activity of the hair follicle:

• a growth phase
• an intermediate phase
• a resting phase.

The duration of the growth phase is determined genetically, and varies according to the hair length, i.e according to the breeds and the individuals. The longer the coat, the slower the growth is.

During the intermediate phase, the matrix cells stop multiplying. Pigments synthesis stops and the upper part of the hair becomes white.

During the resting phase,. the hair is still there, but only anchored by a keratin internal root sheath. It will fall when the follicle comes back to the growth phase; then, a new hair will push the old one out.

In Dog as in Man, the follicle activity is independant from its neighbours one, so the coat renewal takes place imperceptibly. However, the proportions of the hair in the different phases vary considerably, from one season to another.

At spring (May-June), and at fall (November-December), 90 % of the hairs are in growth phase. When a new hair push an old one out, it is shedding time. At fall, shedding gives birth to a longer, thicker coat. On the contray, at spring, lots of follicles get atrophied after hair fall.

The determinism of the follicular cycle

Of course, health and beauty of the hair rely on the balance of the diet: proteins (rich in sulfurated amino-acids), essential fatty acids, cupper, B vitamins (…) intake must be regular and sufficient. But other elements are also very important.

More than temperature fluctuations, the duration of daylight has got much influence: probably through secretion of melatonine by the hypothalamus.

Other hormonal factors can interfere:

• thyroidic hormones and growth hormone stimulate the activity of the hair follicles, whereas corticoids and sexual hormones slow it down ;
• when the prolactine level stays high in the blood (hormone produced by lactating bitches), the coat looks like the summer one, rather thin and sparse.

At last, any stress is likely to put all the follicles in resting phase. Then, abnormal shedding is observed 2 – 3 months after the stressful event (disease, anaesthesia, drug administration), when the follicles start to be active again.

Donated by the Borg Cardona and Co. Ltd. visit Borg Cardona website