Part 2: A Complete Guide to Understanding A Dog's Skin, Hair and Coat

Wag Wiki, last updated 08th, Jan 2015, Srishti Das

Dog hair

Source: HDwallsource

Coming back for a second part of wondering what’s behind our dogs fluffy, silky and soft fur. The layer after the skin, is where we have the hair. Let’s now figure out the secrets behind a dogs hair.

Types of Dog Hair

Dog hairs


Unlike us, dogs have quite a few types of hair, each serving a specific function or use. The first kind is the Primary Hair Type also called as Guard Hair. This layer forms the topmost and outermost layer that covers most of the skin surface. They are thick, long and stiff. The guard hairs are regularly arranged in broad tracts that follow the contour of the body and give the animal's coat its smooth appearance. Growing one hair per primary follicle, the guard hair provides a waterproof protective top layer covering the undercoat. Guard hairs are most abundant on the back of the dog.

The Secondary Hair Type is also known as Under Hair. They are thin, short and soft and give the coat its softness and provide insulation. The third hair type is known as Sinus or tactile Hair. This hair acts majorly as a sensor. It is much thicker than topcoat or undercoat hair, they have the same structure. However, the base of the tactile hair resides in a blood filled sinus that greatly amplifies the motion of the hair and increases its sensitivity. Merkel cells, specialized sensory cells associated with the tactile hairs, detect the slightest touch. Tactile hairs are concentrated on the sides, top and bottom of the head as whiskers, eyebrows and chin hairs and are strongly associated with sight and sense of hearing. Tactile hairs shed on a regular basis, but at a much slower rate than other hair types.

Dog Hair Structure

dog hair structure


Hair as we know, is made up of dead cells. There is no difference here between our hair and a dogs hair. But the differences lie only in the similarities. Hair is made of the protein exokeratin, an extremely strong protein, ensuring skin impermeability.  The hair is divided primarily into three categories which include the cuticle which is the outermost layer of the hair. Then comes the cortex which is the thickest portion inside the cuticle and forms the intermediate area. These cells contain pigment granules, providing the hair colour. The innermost areas is called the medulla. It forms a central core of loosely packed cells. Hair can be hollow, increasing the insulation value of the coat. The total diameter of individual hair decreases as the number of hair per follicle increases. The shape of the hair is determined by the shape of the follicle. Straight follicles produce straight hair and curly follicles produce curly hair.

The structure of the hair is broadly divided into 5 parts which again are no different from the human hair:

  • Hair Shaft: above the skin surface
  • Hair Root: imbedded in the skin surface
  • Bulb: the enlarged, hollow portion at the base of the root
  • Hair Papilla: projection of dermis into center of the bulb
  • Follicle: the skin indentation protecting the root


The total diameter of individual hair decreases as the number of hairs per follicle increases. No new hair follicles are formed after birth. Follicle density decreases with age, but follicle size increases with time and growth. The hair follicle is a unique composite organ, composed of epithelial and dermal compartments interacting with each other in a surprisingly autonomous way. The follicles are also divided into many parts, whoever thought understanding a dogs hair was so simple.

  • Primary and Secondary Follicles: Both of these follicles are associated with a sebaceous gland. A sebaceous gland is a kind of sweat gland to keep skin and hair smooth and elastic. Associated with each primary follicle is a band smooth muscle. Each primary hair has fine nerve fibers that serve as tactile sensors. Secondary Follicles are called so only because they are considerably smaller in diameter.
  • Simple and Compound Follicles:  Simple follicles are individual hair follicles while the compound follicles are in clusters
  • Sebum: responsible for hair "sheen". It acts as a lubricant for the skin and prevents dehydration of the skin.
  • Glassy Membrane: separates hair follicles from the hypodermis.

Dogs usually have compound follicle patterns with usually one primary hair and anything between 2-15 secondary strings of hair. All of these strings of hair are separated into groups of three through each sebaceous glands and circular connective tissue in the dermis. The length, thickness, density and colour vary between individuals and especially between breeds. Contrarily to coat colour, genetic determination of hair growth is managed by only a few genes.

Dog Hair Growth

There has been a common misconception that hairs grow from the skin. That is however untrue. The hair grows from the deepest part of the active hair follicle. Hair growth and pattern happens in various different phases. Everything, including the length depends on the dogs genetic profile. Each cycle consists of four phases. The duration of each phase and the rate of hair growth determine the length and amount of coat on an animal.

  • Anagen or Growth Phase: The anagen phase is the first phase of new hair growth. Dogs that do not tend to shed heavily have a longer anagen phase. The amount of time the hair follicle stays in the anagen phase is genetically determined. At the end of the anagen phase, an unknown signal causes the follicle to go into the catagen phase.
  • Catagen or Regressing Phase: The catagen phase is the transition phase. The catagen phase begins when the cell creation signals to stop. Hair stops growing during this phase as the outer root sheath attaches to the hair.
  • Telogen or Rest Phase: Telogen is a rest period between the catagen and anagen phases.
  • Exogen or Shedding Phase: The final phase, exogen, is the shedding phase. This phase occurs when the hair falls out and the follicle moves back into the anagen phase. The length of this phase depends on the season.

So here was our detailed guide through understanding the hair structure of the dog. Indeed it was all theory and no play. Time you catch your dog, ruffle his hair and have some fun. In the mean time, we’ll prepare to tell you all about their sheen fluffy coat.