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The Skin

M.S. Amarendhra Kumar

Department of Medical Education, Tufts University Schools of Medicine and Dental Medicine, Boston, MA, USA

The integument comprises the skin and its appendages (referred to as the adnexa), including structures such as hair, glands, digital pads, and claws [1-3]. The adnexal structures are of epidermal origin; they are continuous with the epidermal layer of the skin, supported by the underlying connective tissue.

Skin: The skin (cutis) is one of the body's largest and most important organs, for it forms a protective layer against the external environment and plays a crucial role in homeostasis. It is composed of three layers [4], the epidermis, dermis, and hypodermis (or subcutis), all firmly attached to each other. Important differences between cats and dogs exist (Table 1.1) and effect the healing properties and treatment options available when managing injuries. Skin transmits various stimuli from the external environment to the central nervous system (CNS). The nerve fibers carrying these stimuli penetrate the tissues (muscles and fascia) underlying the hypodermis and travel to the CNS, often within the fascial planes that ultimately merge with the periosteum of the appendicular and axial skeletal elements (Figure 1.1). Fascial planes form distinct compartments for individual muscles in many regions of the body. The skin's vascular components travel by similar routes and are responsible for maintaining body temperature within physiologic limits and regulating systemic blood pressure.

Epidermis: The epidermis of the skin is avascular and serves as the outermost protective layer of the body (Figure 1.2). It minimizes trans-epidermal water loss, prevents invasion by infectious agents and other harmful substances, absorbs ultraviolet radiation by the melanocytes, and aids in Vitamin D biosynthesis. The basic structure of the epidermis is similar in all domesticated mammals with some minor regional and species differences. The thickness of the epidermis is inversely proportional to the density of the hair coat. In dogs and cats, since most of the skin surface is covered with hair, the epidermis is relatively thin. In the dog, the epidermis consists of two to three layers of living cells increasing to 10 layers [5-7] while in the cat, the epidermis is slightly thinner. The average time for epidermal turnover is 22?days in carnivores regardless of the thickness of the epidermis [8, 9]. Dermal papillae are small fingerlike extensions into the epidermis, surrounded by rete ridges of the epidermis. These two structures interlock with each other, anchoring the epidermis. Epidermal rete ridges are absent in most of the skin in carnivore skin due to the dense haircoat [10, 11]. The hair follicles extend into the dermis, firmly anchoring the epidermis. In sparsely haired regions such as the scrotum, inguinal, and axillary areas, the epidermis is slightly thicker and epidermal rete ridges may be observed. The term glabrous skin is applied to areas devoid of hairs, such as the nasal plane, lips, and genitals as well as parts of the limb extremities such as digital pads. These regions may have several layers of living keratinocytes, prominent basement membranes, and form epidermal rete ridges [10].

The epidermal layer rests upon a meshwork of extracellular fibers (dermal-epidermal junction) upon which the keratinocytes rest, called the basement membrane (or basal lamina), which is acellular and avascular (Figure 1.2). If the basement membrane is disrupted, as with a skin wound, other cells (such as activated fibroblasts and neutrophils) will pass through it from beneath to participate in healing processes, forming scars, extending capillary loops, and developing granulation beds. Otherwise, the basal lamina remains impassable. Beneath it is the dermis, the vascularized second layer of the skin.

Nonkeratinocytes: Several cell types are contained in these two major layers of the skin (the epidermis and dermis). The most common cells within the epidermis are keratinocytes, making up 85% of all epidermal cells. The nonkeratinocytes account for approximately 15% of the epidermis and include the melanocytes, tactile epithelioid cells (Merkel cells), and intraepidermal macrophages (or Langerhans cells). Melanocytes are derived from neural crest cells. Dendritic cells (DCs) are a heterogeneous group of antigen-presenting leukocytes with a common origin that play an important role in the activation of the immune system. These cells have potent antigen-presenting capabilities with characteristic dendritic morphology. Three main cutaneus DC populations have been described: intraepidermal Langerhans cells (LCs), dermal myeloid DCs, and dermal plasmacytoid DCs (pDCs). The intraepidermal macrophages (Langerhans cells) are interspersed among the much more numerous keratinocytes (Figure 1.3) and act as antigen-presenting cells [13]. The LCs are one type of antigen-presenting DCs involved in cutaneus hypersensitivity reactions. They are capable of inducing antiviral-specific immune responses in vivo [14]. The LCs survey the epithelium constantly for pathogens and migrate to the lymph nodes where they present microbial antigens to T-cells. This results in developing tolerance and maintaining tissue homeostasis [15]. Langerhans cells in the skin are continuously replenished from circulating bone marrow precursors [16, 17]. There is steady-state migration of LCs to skin-draining lymph nodes, perhaps to induce and maintain tolerance to cutaneus antigens. Their number in the epidermis is small compared to keratinocytes, and they are largely present in the upper stratum spinosum.

Figure 1.1 (a) Lateral approach to the shaft of the femur is shown. The fascia lata is split along the cranial margin of the biceps femoris m. (b) Cross-sectional view of left thigh. Note the location of nerves and blood vessels between the fascial planes. 1, Femoral artery and vein; 2, Rectus femoris m; 3, Fascia lata; 4, Vastus lateralis m; 5, Deeper lamina of fascia lata which runs between the vastus lateralis and biceps femoris muscles and reaches the shaft of the femur. The biceps femoris and vastus lateralis are therefore separated along this plane to reach the femur shaft; 6, Superficial lamina of fascia lata; 7, Biceps femoris; 8, Lateral shaft of femur; 9, Sciatic nerve.

Table 1.1 Summary of skin differences between dogs and cats.

Figure 1.2 Layers of epidermis. (a) Basement membrane. (b) Stratum basale. (c) Stratum spinosum. (d) Stratum granulosum. (e) Stratum lucidum. (f) Stratum corneum. (g) Cells of stratum basale are anchored to the basement membrane by hemidesmosomes. 1, Melanocyte; 2, Keratin filaments (tonofilaments); 3, Plate; 4, Bullous pemphigoid antigen-1; 5, Plasma membrane; 6, Bullous pemphigoid antigen-2; 7, Integrin; 8, Anchoring filaments; 9, Basal lamina.

Tactile epithelioid cells (Merkel cells) are in the stratum basale of hairless and hairy skin and are numerous in the nasal plane of carnivores. These cells, in association with sensory nerve endings, function as epidermal mechanoreceptors (Figure 1.3) that transmit tactile sensations (touch) through cutaneus nerves [18]. Merkel cells are neurosecretory cells thought to be derived from neural crest cells [19, 20]. However, recent studies in mice and humans indicate they may be...