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SC EXSC 223 - Chapter 5 Book Notes

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Chapter 5- The skin is composed of two distinct regions:o The epidermis: the outermost protective shield of the bodyo The dermis: makes up the bulk of the skin and is a tough leathery layer composed of mostly fibrous connective tissue; vascularized- The subcutaneous tissue just deep to the skin is the hypodermiso Not a part of the skin but it shares some of its protective functionso Also called the superficial fascia because it is superficial to the tough connective tissue wrapping fascia of the skeletal muscles, consists mostlyof adipose tissueo Anchors the skin to the underlying structures (mostly muscle) but loosely enough that the skin can slide relatively freely over those structureso Acts as a shock absorber and insulator that reduces heat losso Thickens when a person gains weight- Epidermiso Keratinized stratified squamous epithelium consisting of four distinct cell types and 4 or 5 distinct layer - Cells of the epidermis:o Keratinocytes Produce keratin that give the epidermis its protective properties Tightly connected to one other via desmosomes Arise in the deepest part of the epidermis from the stratum basale Undergo continuous mitosis in response to prompting by epidermal growth factor Cells pushes upward by the production of new cells and the cell become filled with keratino Melanocytes Synthesize pigment melanin Found in the deepest layer of the epidermis As melanin is made it accumulates in membrane bound granules called melanosomes that motor proteins move along actin filaments to the ends of the melanocytes processes From there they are transferred to nearby keratinocytes Melanin granules accumulate on the superficial or sunny side of the keratinocyte nucleus forming a pigment shield that protects the nucleus from the damaging effects of UV light o Dendritic  Arise from bone marrow and migrate to the epidermis Also called langerhans cells Ingest foreign substances and are key activators of our immune system Their slender processes extend among the surrounding keratinocytes forming a continuous networko Tactile/Merkel cells Also called merkel cells are present at the epidermal-dermal junction Intimately associated with a disclike sensory nerve ending Functions as a sensory receptor for touch- Layers of the epidermiso In thick skin: palms, fingertips, and soles of the feet; the epidermis consists of 5 layers: Stratum basale Stratum spinosum Stratum granulosum Stratum lucidum (missing in thin skin) Stratum corneum- Stratum basaleo Is the deepest epidermal layer o Attached to the underlying dermis along a wavy borderlineo Consists of a single row of stem cells; continually renewing cell population- Stratum spinosumo Is several layers thicko These cells contain a weblike system of intermediate filaments, mainly tension resisting bundles of pre-keratin filamentso The keratinocytes in this layer appear to have spineso Scattered among the keratinocytes are melanin granules and dendritic cells - Stratum granulosumo Thin stratum consists of four to six layers in which keratinocyte appearance changes drastically and the process of keratinization begins: the cells fill with the protein keratino The cells flatten and their nuclei and organelles begin to disintegrate andthey accumulate two types of granules- Stratum lucidumo Visible only in thick skin, and is a thin translucent band just above the stratum granulosumo Consists of two or three rows of clear, flat, dead keratinocytes with indistinct boundaries- Stratum corneumo Consist of flattened anulceate cellso 20 to 30 cell layers thick o Keratin and thickened plasma membranes of cells in this stratum protect the skin against abrasion and penetrationo The glycolipid between its cells waterproof this layers- Dermiso The second major skin regiono Strong flexible connective tissueo Cells are typical of those found in any connective tissue proper: fibroblasts, macrophages, and occasional mast cell and WBCso Its semifluid matrix embedded with fibers, binds the entire body together like a body stockingo Has a rich supply of nerve, fibers, blood vessels, and lymphatic vesselso Has two layers: Papillary  Reticular- Papillary layero Areolar connective tissue in which fine interlacing collagen and elastic fibers form a loosely woven mat that is heavily invested with small blood vesselso The looseness of this connective tissue allows phagocytes and other defensive cells to wander freely as they patrol the area for bacteria o Dermal papillae indent the overlying epidermis Contain capillary loops and free nerve endings and touch receptors  In thick skin these papillae lie atop mounds called dermal ridges- Reticular layero Coarse, irregularly arranged, dense fibrous connective tissueo Network of blood vessels nourishes this layer and lies between this layer and the hypodermiso Matrix consists of adipose cells and thick bundles of collagen fibers that run in various planes Separations or less dense regions between these bundles form cleavage lines in the skino The collagen fibers of the dermis give skin strength and resiliency that prevent everyday scrapes from penetrating the dermiso Collagen binds water helping to keep skin hydrated and elastic fibers provide the stretch-recoil properties of skino Flexure lines are a 3rd type of skin marking that occur near joins, where the dermis is tightly secured to deeper structures - Skin coloro Three pigments contribute to skin color: melanin, carotene, and hemoglobin Melanin: colors range from reddish yellow to brownish black; only found in the deeper layers of the epidermis; prolonged sun exposure causes melanin buildup by keratinocytes stimulating melanocytes Carotene: yellow to orange pigment; tends to accumulate in the stratum corneum and in the fatty tissue of the hypodermis  Hemoglobin: gives the pinkish hue of fair skin; oxygenated pigment in red blood cells circulating through dermal capillaries - Hair and hair follicleso Main function of body hair is to sense insectso Hair on the scalp guards the head against physical trauma, heat loss, and sunlighto Eyelashes shield the eyes and nose hairs filter large particles like lint andinsects from the air we inhale- Structure of a hairo Hairs or pili are flexible strands produced by hair follicles and consist largely of dead keratinized cellso The hard keratin that dominates hair and nails has two advantages over soft keratin found in typical epidermal


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