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Chapter 5

Chapter 5 The Integumentary System

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Anatomy and Physiology
Jacqueline Carnegie

Chapter 5: The Integumentary System -the skin and its derivatives (sweat, oil glands, hairs, nails etc.) make up the integumentary system and serves several functions that are mostly protective The Skin -also called integument, meaning "covering" -composed of 2 distinct regions: 1) Epidermis -composed of epithelial cells -the outermost protective shield of the body 2) Dermis -making up the bulk of the skin -tough, leathery and composed mostly of fibrous connective tissue -vascularised -nutrients reach the epidermis by diffusing through the tissue fluid from blood vessels in the dermis -hypodermis is the subcutaneous tissue just deep to the skin -not actually part of the skin -shares some of the skin's protective functions -called superficial fascia because it is superficial to the tough connective tissue wrapping (fascia) of the skeletal muscles and is made up of mostly adipose tissue -stores fat -anchors the skin to underlying structures (i.e.: muscles) loosely enough that the skin can slide relatively freely over those structures -sliding skins protects us by ensuring that blows just glance off the body -acts as a shock absorber and an insulator that reduces heat loss from the body -hypodermis is the first part to thicken when a person gains weight -accumulates around the thighs and waist in women -accumulates around the abdomen in men The Epidermis -made up of a keratinized stratified squamous epithelium consisting of 4 distinct cell types and 4-5 distinct layers Cells of the Epidermis -epidermal cells include keratinocytes, melanocytes, epidermal dendritic cells, and tactile cells 1) Keratinocytes -makes up most epidermal cells -"keratin cells" -produce keratin (fibrous protein that helps give the epidermis its protective properties) -connected by desmosomes -arise from the deepest part of the epidermis from a cell layer called the stratum basale -these cells undergo continuous mitosis in response to prompting by epidermal growth factor -cells are pushed upward by the production of new cells beneath them and the old cells make the keratin that eventually dominate their cell content -by the time the keratinocytes reach the free surface of the skin, they are scalelike structures that consist of keratin-filled plasma membranes -keratinocytes continuously rub off; a new epidermis forms every 25-45 days -keratinocyte and keratin formation are accelerated in body areas subject to continuous friction (i.e.: palms and soles of feet) -persistent friction causes a thickening of the epidermis called a callus 2) Melanocytes -spider shaped epithelial cells -synthesize the pigment melanin (melan = black) -found in the deepest layer of the epidermis -as melanin is made, it accumulates in membrane-bound granules called melanosomes which are moved along actin filaments by motor proteins to the ends of the melanocyte's process -they are then taken up by nearby keratinocytes -melanin granules accumulate on the superficial (upper portion that receives the most sunlight) side of the keratinocyte nucleus and forms a pigment shield that protects the nucleus from UV radiation -epidermal dendritic cells (Langerhans cells) arise from bone marrow and migrate to the epidermis -they ingest foreign substances and are key activators of our immune system -tactile (Merkel) cells are present at the epidermal-dermal junction -each tactile cell is associated with a disc-like sensory nerve ending called a tactile or Merkel disc which functions as a receptor for touch Layers of the Epidermis -thick skin covers the palms, fingertips, and soles of the feet and the epidermis consists of 5 layers or strata -from deep to superficial, these layers are stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum -thin skin covers the rest of the body and the stratum lucidum appears to be absent and the other strata are thinner *ordered from deep to superficial* 1) Stratum Basale (Basal Layer) -the deepest epidermal layer -attached to the underlying dermis along a wavy borderline that looks like corrugated cardboard -consists of a single row of stem cells that represent the youngest keratinocytes -numerous mitotic nuclei can be found in this layer and reflect the rapid division of the cells -also known as stratum germinativum -each time a basal cell divides, one daughter cell is pushed into the cell layer above and begins to specialize into a mature keratinocyte -the other daughter cell remains in the basal layer and continues to produce new keratinocytes -10 - 25% of the cells in the stratum basale are melanocytes -their branching processes extend into the surrounding cells and reach into the more superficial stratum spinosum layer -tactile cells may also be seen in the stratum basale 2) Stratum Spinosum (Prickly Layer) -several cell layers thick -contain a weblike system of intermediate filaments -filaments are mainly tension-resisting bundles of pre-keratin filamens which span their cytosol to attach to desmosomes -the keratinocytes in this layer appear to have spines, so they are called prickle cells -the spines do not exist in the living cells but are a result of tissue preparation -scattered among the keratinocytes are melanin granules and epidermal dendritic cells which are the most abundant in this layer 3) Stratum Granulosum (Granular Layer) -thin layer -consists of 3-5 cell layers where the keratinocyte appearance changes drastically and keratinzation (when the cells fill with the protein keratin) begins -the cells flatten, their nuclei and organelles begin to disintegrate and accumulate 2 types of granules -keratohyaline granules help to form keratin in the upper layers -lamellated granules contain a water-resistant glycolipid that is released into extracellular space and is a major factor in slowing water loss across the epidermis -the plasma membranes of these cells thicken as cytosol proteins bind to the inner membrane face and lipids released by the lamellated granules coat their external surfaces, allowing it to be more resistant to destruction -the stratum granulosum relies on the capillaries underlying the connective tissue (i.e.: the dermis) for nutrients -everything above the stratum granulosum is cut off from nutrients by the glycolipids that coat their external surfaces and they die 4) Stratum Lucidum (Clear Layer) -appears as a thin transluscent band above the stratum granulosum -consists of 2-3 rows of clear, flat, dead keratinocytes with distinct boundaries -keratohyaline granules cling to the keratin filaments in the cells and cause them to aggregate in parallel formations -visible only as thick skin 5) Stratum Corneum (Horny Layer) -outermost layer -broad zone that is 20-30 cell layers thick -accounts for 3/4 of the epidermal thickness -keratin and the thickened plasma membranes protect the skin against abrasion and penetration -the glycolipid between its cells waterproofs this layer -stratum corneum provides an overcoat for the body -protects deeper cells from a hostile external environment, water loss, etc. -makes the body relatively insensitive to biological, chemical, and physical damage -cornified (horny) cells represent the shingle-like cell remnants of the stratum corneum -can be seen as dandruff and loose flakes that slough off dry skin -18kg of cornified cells is shed on average in a lifetime Dermis -the 2nd major skin region -made up of strong, flexible connective tissue -contains fibroblasts, macrophages, occasional mast cells, occasional white blood cells (i.e.: typical cells found in any CT proper) -has a semi-fluid matrix that binds the entire body together -richly supplied with nerve fibres -richly vascularised -major portions of hair follicles, oil glands, and sweat glands are derived from epidermal tissue but reside in the dermis -consists of 2 layers: 1) Papillary Layer -thin, superficial layer -made up of areolar connective tissue with interlacing collagen and elastic fibres that form a loose, woven mat and contain many small blood vessels -looseness allows phagocytes and other defensive cells to freely move and patrol for bacteria that may have breached the skin -the superior surface consists of peglike projections or dermal papillae that indent the overlying epidermis -dermal papillae may contain capillary loops, free nerve endings / pain receptors, touch receptors (Meissneer's corpuscles) -on the palms and soles, the papillae lie atop larger mounds called dermal ridges and cause the overlying epidermis to form epidermal ridges -as a whole, these skin ridges are known as friction ridges and increase firction and enhance the gripping ability of the hands and feet -fingerprints are created because sweat pores open along the crest of the friction ridges 2) Reticular Layer -deep to the papillary layer -accounts for 80% of the thickness of the dermis -made up of coarse, irregularly arranged, dense fibrous connective tissue -cutaneous plexus nourishes the reticular layer and lies between the reticular layer and the hypodermis -the extracellular matrix contains pockets of adipose cells and thick bundles of interlacing collagen fibres (most collagen fibres run parallel to the skin surface) -separations (less dense regions) between the bundles of collagen fibre form cleavage/ tension lines in the skin -tension lines are externally invisible; they run longitudinally / vertically in the skin of the head and limbs and in circular patterns around the neck and trunk -incisions that are parallel to cleavage lines gapes less and heals more readily compared to incisions across cleavage lines -collagen fibres of the dermis give strength to the skin, binds water to keep skin hydrated, etc. -elastic fibres provide stretch-recoil properties of skin -flexure lines reflect dermal modifications -they are dermal folds that occur at or near joints -the dermis is tightly secured to deeper structures and the skin cannot glide easily to accommodate joint movement in such regions, so the dermis folds and deep skin creases form -e.g.: found on palms of hands, on the wrists, fingers, soles, toes, etc. -striae are dermal tears and are indicated by white scars (i.e.: during pregnancy) -blisters are formed during short-term but acute trauma that causes the separation of the epidermal and dermal layers by a fluid-filled pocket Skin Color -3 pigments contribute to skin color: 1) Melanin -the only pigment made in the skin -made up of a polymer of tyrosine amino acid -has 2 forms that range from yellow to tan to reddish-brown to black -synthesis depends on the tyrosinase enzyme in melanocytes -passes from the melanocytes to the basal keratinocytes -melanin pigment is found only in the deeper layers of the epidermis since melanosomes are broken down by lysosomes -all humans have the same relative number of melanocytes -skin color reflects the relative kind and amount of melanin made and retained -melanocytes of black and brown skinned people produce more and darker melanosomes than fair-skinned individuals -keratinocytes of dark skinned people retain the melanosomes longer -freckles and moles (pigmented nevi) are local accumulates of melanin -when we are exposed to sunlight, melanocytes are stimulated to greater activity by chemicals that are secreted by the surrounding keratinocytes -the melanin helps protect DNA of skin cells from UV radiation by absorbing the rays and dissipating the energy as head -the initial signal for speeding up melanin synthesis seems to be a faster rate of repair of photo-damaged DNA -this response causes darkening of the skin / a tan except in very dark people -excessive sun exposure eventually damages the skin by causing clumping of elastic fibres, resulting in leathery skin, a temporarily depressed immune system, and may alter the DNA of skin cells and lead to skin cancer -UV also destroys the body's folic acid stores which are necessary for DNA synthesis; in pregnant women, this may impair the development of the embryo's nervous system 2) Carotene -a yellow to orange pigment -found in carrots -deposits in keratinocytes; tends to accumulate in the stratum corneum and in fatty tissue of the hypodermis -can be converted to vitamin A in the body which is essential for normal vision and epidermal health 3) Hemoglobin -gives skin a pinkish hue; the pink hue of fair skin reflects the crimson color of the oxygenated pigment (i.e.: hemoglobin) in the red blood cells that are circulating through the dermal capillaries cyanosis- when the skin appears blue because hemoglobin is poorly oxygenated -in dark skinned individuals, cyanosis is apparent in their mucous membranes and nail beds erythema- redness; may indicate blushing, fever, hypertension, inflammation, or allergy pallor- blanching; pale skin during fear, anger, emotional stress; may signify anemia or low blood pressure jaundice- yellow cast; abnormal yellow skin that signifies a liver disorder; yellow bile pigments (bilirubin) accumulates in the blood and is deposited in the body tissues bronzing- metallic appearance of the skin black and blue marks- bruises; when blood escapes from the circulation and clots beneath the skin; clotted masses are called hematomas Appendages of the Skin -include the nails, sweat glands, sebaceous (oil) glands, hair follicles, and hair -the formation of any skin appendages first involves the formation of an epithelial bud which is stimulated by a reduced production of cell adhesion factor (cadherin) -when cell-to-cell attractions are broken, the cells can move about and rearrange themselves, allowing an epithelial bud to form Sweat (Sudoriferous) Glands -distributed over the entire skin surface except the nipples and parts of the external genitalia -up to 3 million per person -2 types of sweat glands: 1) Eccrine Sweat Glands -also known as merocrine sweat glands -the most numerous sweat glands -abundant on the palms, soles of feet, and forehead -involves a simple, coiled, tubular gland -the secretory part lies coiled in the dermis, and the duct extends to open in a funnel-shaped pore at the skin surface -eccrine secretion is sweat -sweat is a hypotonic filtrate of the blood that passes through the secretory cells of the sweat glands and is released by exocytosis -99% water with some salts (i.e: sodium chloride), vitamin C, antibodies, microbe-killing peptide (i.e: dermcidin) and some metabolic wastes (i.e.: urea, uric acid, ammonia) -composition also depends on heredity and diet -sweat is acidic with a pH between 4 to 6 -sweating is controlled by the sympathetic division of the ANS -role is to prevent the body from overheating -heat-induced sweating begins on the forehead and then spreads inferiorly over the remainder of the body -emotionally-induced sweating (cold sweats) are brought on by fright, embarrassment, nervousness, etc. and begin on the palms, soles, and axil
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