4.2- anaphy

• Skin


  The body's largest organ
• Skin


  • Serves as a protective shield against heat, light, injury, and infection
  • Regulates body temperature
  • Stores water and fat
  • Is a sensory organ
  • Prevents water loss
  • Prevents entry of bacteria
  • Acts as a barrier between the organism and its environment
  • Helps to make vitamin D when exposed to the sun
• The skin comprises 15% of the total adult body weight
• The skin's thickness ranges from 0.5mm at its thinnest part (eyelids) to 4mm at its thickest part (palms of the hands and soles of the feet)
• Epidermis


  Composed of keratinized stratified squamous epithelium
• Principal types of cells in the epidermis

  • Keratinocytes
  • Melanocytes
  • Langerhans cells
  • Merkel cells
• Keratinocytes

  Arranged in four or five layers and produce the protein keratin
• Keratin


  A tough, fibrous protein that helps protect the skin and underlying tissues from heat, microbes, and chemicals
• Keratinocytes


  Also produce lamellar granules, which release a water-repellent sealant that decreases water entry and loss and inhibits the entry of foreign materials
• Melanocytes

  Develop from the ectoderm of a developing embryo and produce the pigment melanin
• Melanin


  A yellow-red or brown-black pigment that contributes to skin color and absorbs damaging ultraviolet (UV) light
• Melanocytes are particularly susceptible to damage by UV light
• Langerhans cells


  Arise from red bone marrow and migrate to the epidermis, where they constitute a small fraction of the epidermal cells. Their role in the immune response is to help other cells of the immune system recognize an invading microbe and destroy it
• Merkel cells


  The least numerous of the epidermal cells. They are located in the deepest layer of the epidermis, where they contact the flattened process of a sensory neuron (nerve cell), a structure called a Merkel (tactile) disc
• Layers of thin skin

  • Stratum basale
  • Stratum spinosum
  • Stratum granulosum
  • Thin stratum corneum
• Layers of thick skin

  • Stratum basale
  • Stratum spinosum
  • Stratum granulosum
  • Stratum lucidum
  • Thick stratum corneum
• Dermis


  The second, deeper part of the skin, composed of a strong connective tissue containing collagen and elastic fibers
• Cells present in the dermis

  • Fibroblasts
  • Macrophages
  • Adipocytes
• Blood vessels, nerves, glands, and hair follicles (epithelial invaginations of the epidermis) are embedded in the dermal layer
• Regions of the dermis

  • Papillary region
  • Reticular region
• Papillary region


  Makes up about one-fifth of the thickness of the total layer. Consists of areolar connective tissue containing thin collagen and fine elastic fibers. Some contain capillary loops and tactile receptors called Meissner corpuscles or corpuscles of touch
• Reticular region


  Attached to the subcutaneous layer, consists of dense irregular connective tissue containing fibroblasts, bundles of collagen, and some coarse elastic fibers. The collagen fibers interlace in a netlike manner, providing the skin with strength, extensibility, and elasticity
• Extreme stretching may produce small tears in the dermis, causing striae (stretch marks), visible as red or silvery white streaks on the skin surface
• Hypodermis


  Deep to the dermis, but not part of the skin, consisting of areolar and adipose tissues. Fibers that extend from the dermis anchor the skin to the subcutaneous layer. Serves as a storage depot for fat and contains large blood vessels that supply the skin and pacinian corpuscles sensitive to pressure
• Functions of the integumentary system

  • Thermoregulation
  • Storage of blood
  • Protection
  • Cutaneous sensations
  • Excretion and absorption
  • Synthesis of vitamin D
• Thermoregulation


  The skin contributes by liberating sweat at its surface and by adjusting the flow of blood in the dermis
• In response to high environmental temperature or heat produced by exercise, sweat production from exocrine sweat glands increases. In response to low environmental temperature, production of sweat is decreased, which helps conserve heat
• Blood reservoir


  The dermis houses an extensive network of blood vessels that carry 8–10% of the total blood flow in a resting adult
• Protection


  • Keratin protects underlying tissues from microbes, abrasion, heat, and chemicals
  • Tightly interlocked keratinocytes resist invasion by microbes
  • Lipids released by lamellar granules inhibit evaporation of water from the skin surface and retard entry of water
  • Sebum from sebaceous glands keeps skin and hairs from drying out and contains bactericidal chemicals
  • Acidic pH of perspiration retards the growth of some microbes
  • Melanin helps shield against the damaging effects of ultraviolet light
  • Epidermal Langerhans cells and dermal macrophages carry out protective functions that are immunological in nature
• Cutaneous sensations


  Sensations that arise in the skin, including tactile sensations, thermal sensations, and pain. There is a wide variety of nerve endings and receptors distributed throughout the skin
• Excretion and absorption


  The skin normally has a small role in excretion, the elimination of substances from the body, and absorption, the passage of materials from the external environment into body cells. About 400 mL of water evaporates through it daily, and a sedentary person loses an additional 200 mL per day as sweat
• The absorption of water-soluble substances through the skin is negligible, but certain lipid-soluble materials do penetrate the skin
• The dermis also contains nerve endings called Pacinian corpuscles that are sensitive to pressure
• Epidermal Langerhans cells


  Alert the immune system to the presence of potentially harmful microbial invaders by recognizing and processing them
• Macrophages in the dermis

  Phagocytize bacteria and viruses that manage to bypass the Langerhans cells of the epidermis
• Accessory structures of the skin

  • Hair
  • Nail
  • Sweat glands
  • Sebaceous glands
• Hair


  A keratinous filament growing out of the epidermis, serving for protection, sensory input, thermoregulation, and communication. Hair grows and is eventually shed and replaced by new hair
• Hair growth phases

  • Anagen
  • Catagen
  • Telogen
• Nail


  A specialized structure of the epidermis that is found at the tips of our fingers and toes
• Sweat glands

  • Merocrine glands
  • Eccrine sweat glands
  • Apocrine sweat glands