CHAPTER 5LECTURE OUTLINEI) INTRODUCTION1) The skin and its accessory structures make up the integumentary system.2) The integumentary system functions to guard the body’s physical and biochemical integrity, maintain a constant body temperature, and provide sensory information about the surroundingenvironment.3) Dermatology is the medical specialty that deals with diagnosing and treating skin disorders.II) STRUCTURE OF THE SKINA) The skin consists of different tissues that are joined together to perform specific functions and is the largest organ of the body. Structurally the skin consists of two parts (Figure 5.1).1) The superficial portion of the skin is the epidermis and is composed of epitethelial tissue.2) The deeper layer of the skin is the dermis and is primarily composed of connective tissue.3) Deep to the dermis is the subcutaneous layer or hypodermis.(a) It is not a part of the skin.(b) It consists of areolar and adipose tissue.(c) It serves as a fat storage area, an area for blood vessel passage, and an area of pressure nerve endings.III) EPIDERMIS1) The epidermis is composed of stratified squamous epithelium and contains four principal types of cells: keratinocytes, melanocytes, Langerhans cells, and Merkel cells (Figure 5.2).(a) Keratinocytes produce the protein keratin, which helps protect the skin and underlying tissue from heat, microbes, and chemicals, and lamellar granules, which release a waterproof sealant (Figure 5.2a).(b) Melanocytes produce the pigment melanin which contributes to skin color and absorbs damaging ultraviolet (UV) light (Figure 5.2b).(c) Langerhans cells participate in immune responses (Figure 5.2c).(d) Merkel cells contact a sensory structure called a tactile (Merkel) disc and function in the sensation of touch (Figure 5.2d).B) Layers of the Epidermis(a) There are four or five layers of the epidermis, depending upon the degree of friction and mechanical pressure applied to the skin. From deepest to most superficial the layers of the epidermis are stratum basale (stratum germinativum), stratum spinosum, stratum granulosum, stratum lucidum (only in palms and soles), and stratum corneum (Figures 5.3 a and b; table 5.1).(b) The stratum basale is the deepest layer of the epidermis and contains some stem cells capable of undergoing cell division to form new cells.(1) Keratinocytes with a cytoskeleton of tonofilaments are found in this layer.(2) This layer is sometimes called the stratum germinativum to indicate its role in the formation of new cells. (3) When the germinal portion of the epidermis is destroyed, new skin cannot regenerate with a skin graft.C) The stratum spinosum provides strength and flexibility to the skin.(a) The stratum granulosum marks the transition between the deeper, metabolically active strata and the dead cells of the more superficial strata. This layer also shows the formation of a water repellent sealant between the cells.(1) This layer consists of keratinocytes that are undergoing apoptosis.(2) This layer is characterized by the presence of keratohyalin which converts tonofilaments into keratin.(b) The stratum lucidum is present only in the fingers, palms, and soles.(c) The stratum corneum is the most superficial layer and consists of dead cells.(1) Lamellar granules in this layer make it water-repellent.(2) Constant exposure to friction will cause this layer to increase in depth with the formation of a callus, an abnormal thickening of the epidermis.Clinical Connection: Psoriasis is a chronic skin disorder characterized by a more rapid division and movement of keratinocytes through the epidermal strata D) Keratinization and Growth of the Epidermis1) Keratinization, replacement of cell contents with the protein keratin, occurs as cells move to the skin surface over 2-4 weeks.2) Epidermal growth factor and other hormone-like proteins play a role in epidermal growth.3) Table 5.1 presents a summary of the features of the epidermal strata.IV) DERMISA) The dermis is composed of connective tissue containing collagen and elastic fibers and has two regions (Figure 5.1).1) The papillary layer is areolar connective tissue containing fine elastic fibers, dermal papillae, corpuscles of touch (Meissner’s corpuscles), and free nerve endings for sensations of heat, cold, pain, tickle, and itch.2) The reticular layer is the deeper part of the dermis consisting of dense, irregular connective tissue containing bundles of collagen fibers and some elastic fibers.(a) Spaces between the fibers may contain adipose cells, hair follicles, sebaceous glands, andsudoriferous glands.(b) The collagen and elastic fibers provide strength, extensibility (ability to stretch), and elasticity (ability to return to original shape after stretching) to skin.B) Epidermal ridges increase friction for better grasping ability and provide the basis for fingerprintsand footprints. The ridges typically reflect contours of the underlying dermis.C) Table 5.2 presents a comparison of the structural features of the papillary and reticular regions of the dermis.D) Clinical Connection: Tension Lines and Surgery. Making surgical incisions along lines of cleavage reduces skin scarsV) STRUCTURE AND BASIS OF SKIN COLORA) Melanin, hemoglobin, and carotene are three pigments that impart a wide variety of colors to skinB) Melanocytes synthesize melanin from the amino acid tyrosine in the presence of an enzyme called tyrosinase1) Melanin: causes the skin’s color to vary from pale yellow to reddish-brown to black. (a) pheomelanin (fe¯-o¯-MEL-a-nin) (yellow to red) (b) eumelanin (u¯-MEL-a-nin) (brown to black)2) Freckles: accumulation of melainin in patches in some people who are genetically predisposedC) Overexposure to the sun can lead to photodamage of the skin and photosensitivity in individuals who are taking certain medications.D) Clinical Connection: Albinism and Vitiligo: Albinism is the inherited inability of an individual to produce melanin. Vitiligo is the complete or partial loss of melanocytes from patches of the skin resulting in irregular white spots.E) Clinical Connection: The color of skin and mucous membranes can provide clues for diagnosing certain problems, such as cyanosis, jaundice, and erythema F) Tattoos and skin piercings have medical implicationsVI) ACCESSORY STRUCTURES OF THE SKINA) General Characteristics1) Accessory structures of the skin develop from the embryonic epidermis and include hair, glands, and nails.B) Hair1) Hairs, or pili,