The dermal adnexa, or appendages, are accessory structures that include hair and feather follicles as well as secretory glands.
Hair and feather follicles
Hair and feathers serve as an adjunct barrier to the skin, providing a thermal barrier, physical barrier, and a photo-protective barrier. Hair and feathers are produced from follicles. Follicles are tubular structures formed by invaginations of epidermis into the dermis. Follicles serve to generate, attach, and support hair or feather shafts, and also contribute to epidermal re-epithelialization and repair following injury.
Hair follicles have three distinct zones; from deep to superficial they are: bulb, isthmus, and infundibulum. Hair growth begins at the deepest portion of the follicle, the bulb. A specialized structure, the dermal papilla, lies adjacent to the bulb and induces growth of the hair follicle and hair production. Hairs/feathers are essentially rolled cylinders of keratinocytes. On section, hair/feathers have three distinct zones; from outermost to innermost, they are: cuticle, cortex, and medulla.
Hair and feather growth occurs in a cyclical pattern of proliferation progressing through distinct stages of development which include anagen (growth phase), catagen, and telogen (resting phase). Terminally, the hair is exfoliated (shed). Growth is influenced by numerous factors that include anatomic location, hormones, daylight, ambient temperature, nutrition, and various growth factors.
Hair shafts are categorized into primary and secondary hairs. Primary hairs are larger and also referred to as “guard hairs”, whereas secondary hairs are smaller, and comprise the “undercoat”. Species such as horses only have primary hairs; dogs and cats have both primary and secondary hairs. This, in part, accounts for the different texture of their coats. Further, hair follicles are categorized as simple and compound. These categories are based on the number of hair shafts that exit a single infundibulum. Horses and cattle only have simple follicles (one hair shaft emerges from one infundibulum), whereas dogs and cats have both simple follicles interspersed with compound follicles (multiple hair shafts emerge from one infundibulum). In sheep, wool is produced as compound follicles.
Arrector pili muscle
The arrector pili muscle is a band-like bundle of smooth muscle that attaches to the hair follicle and dermis. The primary role of the arrector pili muscle is pilo-erection (hair erection). Pilo-erection serves to establish a layer of air between the surface of the hair/fur and the skin, serving as insulation. In addition, pilo-erection has a major role in animal behavior for domestic species such as the cat and dog.
Tactile hairs (vibrissae, sinus hairs, whiskers)
Tactile hairs are modified hair follicles surrounded by a blood-filled sinus and highly innervated. Because of this arrangement, motion of the hair transmits through the blood-filled sinus to the nerve, transducing physical stimulus to sensation.
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