The horse’s largest and most visible organ is the skin. On average around a centimetre thick, it is made up of the epidermis – the outer dried cellular layer, and the dermis – the live portion of the skin.
The dermis contains the nerve endings, hair follicles, blood vessels and sweat glands.
Hair grows from follicles that originate deep in the dermis layer and pass through the epidermis to the skin surface.
Hair itself is a protein called Keratin, the same protein that makes up the hoof. As it requires energy to replenish itself, poor nutrition has a significant effect on both the quantity and quality of hair growth.
There are essentially two layers of hair that make up the coat; the primary coat known as the outer or “guard” coat, and a secondary one known as the inner. Here are approximately 800-1,200 primary and 1,200-2,000 secondary hairs per square inch of skin. Although hair itself is lifeless, it is moisturised by the sebaceous gland connected to the hair follicle, which produces a lubricating oil called sebum. Hair life has two phases; the active or anagen phase when it is growing, and the resting or telegen when it has grown.
In cold weather the hair grows longer for added insulation and the hair follicles then move into a resting phase.
As springtime approaches they are stimulated into activity and new hairs are produced, dislodging the old ones. The horse’s coat is thereby renewed annually, with the exception of the mane and tail, which grows all year.