A photomicrograph of a cross section of a hair bulb is shown.
What are the stages of the hair cycle, and what are the most common patterns of growth in dogs and cats?
There are three stages or cycles in hair growth.
The first is anagen, or the growing phase. During this time the hair follicle is actively producing hair. The anagen hair is characterized by a well-developed dermal papilla that is covered or ‘capped’ by the hair matrix.
The second stage is catagen, or a transition stage between active growth and rest. This hair is characterized by retraction toward the surface. The catagen hair follicle is shortened in length and smaller. The most characteristic feature of a catagen hair is the replacement of inner root sheath by trichilemmal keratinization.
During telogen, or the resting stage, the hair stops growing and is retained in the hair follicle as a dead hair. A telogen hair is characterized as a small hair, approximately one-third the length of the anagen hair. Hairs continue to grow until they reach a preset length and then enter the resting phase. The hair will remain in this state until a new hair starts to grow, pushes the old hair out, and the old hair is shed.
Hairs in dogs and cats are replaced in a mosaic pattern with peaks of hair loss/replacement in the spring and fall. The other two most common replacement patterns are seasonal and waves.