The dermis contributes most to the thickness of the skin. The haired skin is thickest over the dorsal surface of the body and lateral aspect of the limbs and thinnest on the ventral aspect of the trunk and medial limbs. In areas that lack hair such as the paw pads, nasal planum, and lips, the epidermis is thickest to protect from surface trauma. The epidermis is thinnest in well-protected sites such as the ventral abdomen and inguinal areas. It is important to keep these differences in mind when taking skin samples for histologic evaluation. For example, if an endocrinopathy or negative energy balance, and thus atrophic dermatopathy, is suspected, the ventral abdomen and inguinal regions are not the preferred sampling sites since the skin is normally quite thin in these areas.

Most of the mammalian species discussed in this book are covered by hair. Exceptions include hairless breeds of dogs (i.e. Mexican hairless dog, Chinese crested dogs), cats (i.e. Sphynx cat), and small mammals such as the nude mouse. Microscopically, these hairless animals have hair follicles, but their hair follicles are abnormal and are not able to produce normal hairs. Hairs that do form are usually fragile and break off as soon as they enter the infundibulum.

Hair serves a number of functions, including protection, thermal insulation, social communication, and sensory perception. Arrangement and type of hair follicles vary with species, breed, individual, and body region. However, in general, hair follicle density is greatest over the dorsolateral aspect of the body and least on the ventral aspect. Hair follicles are formed by a downward invagination of the surface ectoderm and are essentially an infolding of the epidermis. Hair follicles are classified as primary or secondary and simple or compound. Dogs, cats, rabbits, chinchillas, sheep, goats, camelids, and ferrets have compound hair follicles; this gives them their dense coats. Cattle, pig, horses, mice, and rats have simple hair follicles. The pilosebaceous unit of a compound hair follicle is composed of one or more primary hair follicles surrounded by multiple secondary hair follicles. Primary hair follicles have a larger diameter, are embedded more deeply in the dermis or subcutis, and are associated with sebaceous and epitrichial sweat glands and an arrector pili muscle. These follicles produce larger hairs, called guard hairs. Secondary hair follicles are smaller in diameter, are more superficial in the dermis, and may be accompanied by a sebaceous gland but lack a sweat gland and arrector pili muscle. Secondary hair follicles produce finer hairs, referred to as undercoat hairs. Plush coated arctic dog breeds, such as Huskys, have more secondary hairs than breeds with coarser coats such as Labrador retrievers and Boxer dogs. Each hair of the compound follicle has its own papilla, but at the level of the sebaceous gland opening, the follicles unite to exit from a single external follicular orifice.

Vibrissae (or whiskers) are specialized sensory hairs found in all mammals except monotremes and humans. They are thicker and stiffer than the other (pelagic) hairs and provide information about shapes, locations, and textures of objects in the environment. Most vibrissae are located on the face, but they can also be found on the legs of cats and rodents. Especially in dogs, the follicles of the vibrissae are prominent and may be misinterpreted as lesions (Figure 4.1).

Mammalian sweat glands include eccrine and apocrine glands. Apocrine glands produce a lipid and protein rich fluid and are found throughout the body. Apocrine glands play an important role in thermoregulation in horses and ruminants. Eccrine glands are found in hairless or poorly haired areas of the body (footpad, nasal planum) and produce a watery secretion.

Sebaceous glands are found throughout the haired skin and are concentrated in areas used for scent marking. These glands secrete a lipid-rich fluid (sebum) that has multiple functions including lubrication, waterproofing, slowing the growth of bacteria, and communicating with other members of the species (pheromones).

Mammary glands are modified apocrine glands composed of tubuloalveolar units grouped together into lobules, separated by connective tissue. The milk flows from the glandular tissue through lactiferous ducts to the lactiferous sinus, which extends into the teat and continues as the papillary duct to the orifice, which is surrounded by smooth muscle. The numbers and location of mammary glands in domestic mammals is listed in Table 4.1. In most species, each gland has one teat, and each teat has one orifice; however, in horses, each teat has two openings or orifices (Figure 4.2). The size of mammary glands in females varies significantly based on reproductive status. Males of all domestic species have at least rudimentary mammary glands; however, male rats and mice and most male horses do not have nipples.

In all mammals, the specialized covering of the distal phalanges protects the underlying tissue. These structures also have species-specific functions such as shock absorption, climbing, digging, and fighting. Nails, claws, and hooves are specializations of the epidermis, dermis, and subcutaneous tissue. The epidermis forms the durable outer layer of hard keratin (horn), which is formed by a specialized germinal area of the dermis (coronary band or cuticle). This epidermal region grows toward the free edge, where it is worn away by friction. The dermis forms ridges (few in dogs and cats, several hundred in horses) that interdigitate with and prevent detachment of the epidermal layer. The subcutaneous tissue forms the digital pads, which, in hoofstock, are also covered by a thick protective layer of hard keratin. The structure of a horse hoof is illustrated in Figure 4.3.

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