Skin Intro PDF

CH APTER 17 The Integumenta Monika M. Welle and Keith E. Linder Key Readings Index Structure, 1095 Function, 1107 Dysfunction/Responses to Injury, 1108 Types of Clinical Skin Lesions, 1108 Types of Histopathologic Skin Lesions, 1127 Regeneration and Repair, 1146 Portals of Entry/Pathways of Spread,...

CH APTER 17 The Integumenta Monika M. Welle and Keith E. Linder Key Readings Index Structure, 1095 Function, 1107 Dysfunction/Responses to Injury, 1108 Types of Clinical Skin Lesions, 1108 Types of Histopathologic Skin Lesions, 1127 Regeneration and Repair, 1146 Portals of Entry/Pathways of Spread, 1146 Defense Mechanisms/Barrier Systems, 1148 Diseases Affecting Multiple Species of Domestic Animals, 1149 Diseases of Horses, 1219 Structure The skin is the largest organ of the body. It is composed of three layers that are stacked on top of one another to create a sheetlike organ that covers the entire body. Starting externally, these layers are the (1) epidermis, (2) dermis, and (3) subcutis (also known as panniculus or hypodermis) (Figs. 17.1 and 17.2). Specialized appendages called adnexa exist within these layers and consist of hair follicles, sebaceous glands, and sweat glands. Structures such as claws or hooves that develop from these layers are present as distal extensions of the digits of the limbs formed over phalanges of the skeleton (see Figs. 17.10 and 17.61). Depending on the body location, skin is haired or hairless. Haired skin is thickest over the dorsum and lateral limbs and thinnest over the pinnae, axillary, and inguinal regions, the ventrum, and the medial regions of the limbs. The skin of larger animals is generally thicker than the skin of smaller animals. Nonhaired skin of the nasal planum and pawpads has the thickest epidermis on the body (Fig. 17.3). The dermis is much thicker than the epidermis and is crucial to maintain structural support, to resist tensile forces, and to maintain the shape of the skin. The subcutis is the thickest layer of the skin. It occurs within the trunk, neck, and proximal limbs, and it is composed of layers of adipose tissue (panniculus adiposus), skeletal muscle (panniculus carnosus, cutaneous trunci), and fibrous tissue (panniculus fibrosus, also known as superficial fascia). Fibrous septa of the subcutis connects dermal collagen bundles with the deep fascia. The deep fascia consists of connective tissue that attaches to the surface of underlying skeletal muscle and bone. The subcutis of the head, legs, and tail tapers to form a thin, fibrous connective tissue layer or becomes absent in the lips, eyelids, external ear canal, and anus. Blood and lymphatic vessels as well as nerves are present in all three layers of the skin to varying amounts, except for the epidermis, which is devoid of all vessels. aFor a glossary of abbreviations and terms used in this chapter, see E-Glossary 17.1. Diseases of Ruminants (Cattle, Sheep, and Goats), 1223 Diseases of Pigs, 1229 Diseases of Dogs, 1233 Diseases of Cats, 1256 Epidermis The epidermis is a stratified keratinizing epithelium composed mostly of keratinocytes (85%) that is, based on cell morphology, divided into four layers: the innermost stratum basale, the stratum spinosum, the stratum granulosum, and the outermost stratum corneum (see Figs. 17.1 and 17.3). Keratinocytes continuously proliferate in the basal epidermal layer, migrate and differentiate to form the upper epidermal layers, and then shed (desquamate) from the skin surface. This differentiation process takes about 1 month for a basal cell to complete. The epidermis also contains resident Langerhans cells, migrating T lymphocytes, and occasional neuroendocrine Merkel cells (