Normal Anatomy and Physiology ofthe Skin and Hair
The structure and function of the skin have been reviewed elsewhere (Scott 1988) and will not be discussed in detail here.
Skin
The epidermis can be divided histologically into four layers: stratum corneum, stratum granulosum, stratum spino- sum, and stratum basale (Sar and Calhoun 1956).
Goat skin is thickest on the forehead and dorsal aspect of the body. As in other species, the major histocompatibility system is involved in allograft rejection if skin grafting is attempted (van Dam et al. 1978).Specialized Skin Structures
Wattles are specialized skin appendages sometimes found in the cervical region of goats. They contain a central cartilaginous core, smooth muscle, connective tissue, nerves, and blood vessels (Sar and Calhoun 1956). Wattles have no known function, but might represent rudimentary appendages (Reber et al. 2015). Subcutaneous cysts associated with the base of the wattle are discussed in Chapter 3. The presence of wattles is determined by an autosomal dominant gene with complete penetrance but variable expression regarding location (neck, ear, face), size, and number (Lush 1926; Ricordeau 1981; Reber et al. 2015). In a study of Saanen goats in France, does with wattles were approximately 13% more prolific than does without wattles (Ricordeau 1967).
The skin caudomedial to the horns of buck goats contains branched sebaceous glands that produce lipids and chemicals that contribute to the buck odor (Van Lancker et al. 2005). The glands are also present, but much smaller in female and castrated male goats (Bal and Ghoshal 1976). These glands and descenting procedures that destroy them are discussed in Chapter 18.
Hairand Shedding
Hair growth in goats resembles that in other land mammals (Shelton 1981; Scott 1988). Hair follicles are initiated prenatally by invagination of the epidermis into the dermis.
Sweat and sebaceous glands and the arrector pili muscle develop in association with the follicle. The histologic anatomy of these structures has been reviewed by Scott (1988). The hair is produced by rapidly dividing cells in the bulb at the base of the follicle. During the active phase of the growth cycle (anagen), growth from the bulb is continuous. Anagen is followed by the resting phase (telogen) and then by molting. When growth resumes, the new fiber produced by the follicle helps to push out the old fiber. In goats not specifically selected for fiber production, fibers form brush ends and growth stops at about the time of the autumn equinox, and the follicles remain dormant until late spring (Ryder 1978).Hair follicles in goats are grouped in bundles or clusters. Within each bundle are primary follicles (often a central and two laterals) and a variable number of secondary follicles. The primary follicles produce long, coarse guard hairs, while secondary follicles produce undercoat or down. In the Angora, secondary follicles have been modified to produce mohair. Goats adapted to tropical regions have little undercoat, while secondary fibers contribute to cold resistance in goats in cold climates.
The inheritance of coat color involves numerous genes. One possible interpretation of color in American goats has been proposed by Mitchell (1989). Several other papers have attempted to summarize various aspects of color inheritance in goats (Ricordeau 1981; Adalsteinsson et al. 1994).