20 Introduction to alopecia
HAIR FOLLICLE ANATOMY AND PHYSIOLOGY
Hair, a characteristic of mammals, serves several important functions including thermal insulation, a barrier to physical, chemical, thermal and microbial insults, photoprotection and as a visual stimulus for sexual attraction in some cases. Hair follicles also serve as a site for re-epithelialization during wound healing.
The hair follicle unit consists of the hair follicle itself, the sebaceous gland, sweat gland and an arrector pili muscle (Fig. 20.1). Anatomically the hair follicle is divided into three segments: the infundibulum, the isthmus and the hair bulb (see Chapter 13, Fig. 13.5). The hair follicle consists of inner and outer tubes, the inner and outer root sheaths. The inner root sheath and the hair shaft itself arise from and are produced by a swelling at the base of the hair follicle called the hair bulb.
Figure 20.1 The hair follicle and sweat glands. Free sebaceous glands and atrichial glands open directly to the skin surface; pilosebaceous and epitrichial glands open into the hair follicle lumen.
Carnivores (dogs, cats, etc.) have compound follicles, which means that hairs produced by multiple hair follicles exit from the same infundibulum at the skin surface. Some of these hairs are large-diameter ‘primary’ hair shafts and the remainder are small ‘secondary’ undercoat hairs.
The hair growth cycle
Hair growth occurs in a cyclical manner comprising an active growth phase, a transitional phase and a resting, involutory phase.
The active growth phase is termed anagen, and is followed by a transitional period, catagen, and a resting phase, telogen (Fig. 20.2a–c). The duration of each phase varies according to age, body region, breed and sex, as well as intrinsic, extrinsic and external factors. Breed has a dramatic effect on the hair cycle, with some dogs having an anagen- or a telogen-dominated hair cycle. Breeds such as poodles and Bichon Frise have anagen-dominated cycles. Essentially, this is where hair follicles are in anagen for long periods of time. These are breeds that require hair cuts. Plush-coated breeds such as the chow-chow, Malamutes and Pomeranians have telogen-dominated hair cycles. This is where the hair follicle spends long periods of time in telogen with a retained telogen hair shaft. Presumably, evolutionarily speaking this is advantageous in a cold climate when hair growth requires protein and energy.
Figure 20.2 (a) Anagen hair – new hair being formed below a telogen.
(b) Catagen phase – transitional period between the anagen and telogen hair.
Hair in dogs is replaced in a mosaic pattern, which means that over the same area of skin there will be hair follicles at all three stages of the hair cycle at the same time, in contrast to mice, where the hair cycle occurs in waves starting at the head. In dogs and cats there are peaks of hair growth in spring and autumn; however, this pattern is further influenced by the photoperiod, temperature and nutritional status.
Factors controlling the hair growth cycle
The control of hair growth is complex and still relatively poorly understood. There are many factors controlling hair growth that can be summarized as intrinsic, extrinsic and external factors (mainly identified from research into the murine hair growth cycle).
Intrinsic factors are produced by epithelial and mesenchymal cells in and around the hair follicle and include:
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