Integumentary System

• Skin is the body's largest organ and is the first line of defense against pathogens.
• The skin is 15-20% of the body's surface area and is the largest organ.
• Skin is also known as integument or cutaneous covering or layer.
• The functions of the skin are protection, sensory, thermoregulation, metabolic, and sexual signaling.
• The skin protects the body against UV light because of the presence of melanin.
• The skin also protects us from mechanical, chemical, and thermal insults, prevents dehydration, and acts as a physical barrier.
• The skin is the largest sensory organ and because of the receptors, the skin helps in the body's interactions with physical objects.
• The receptors of the skin are called Merkel cells. They are sensitive to touch and vibration.
• Meissner corpuscles detect fine touch and pressure.
• Pacinian corpuscles detect deep pressure and vibrations.
• Ruffini endings sense stretch and temperature changes.
• Free nerve endings respond to pain, temperature, itch, tickle, and other stimuli.
• Melanocytes produce melanin, which determines skin color.
• The body is the major organ of thermoregulation and is responsible for maintaining a constant internal temperature.
• Function as Thermoregulation:

  It has insulating components like hairs and adipose tissues. It also accelerates heat loss through sweat production and the presence of superficial blood vessels in our skin (dense superficial microvasculature).
• Metabolic function:

  • Cells of the skin synthesize vitamin D3 (calcium metabolism, and proper bone formation)
  • Local action of UV light on vitamin’s precursor
  • Sweat removes excess electrolytes
  • subcutaneous tissue stores energy as fat
• Function as Sexual Signaling:

  • Visual indicators of health involved in attraction between sexes
  • Pigmentation
  • Hair
  • Sex pheromones secreted by apocrine glands
• 2 main layers of the skin: epidermis and dermis.
• Epidermis:

  • 0.07 to 0.12 mm
  • surface epithelium
  • The germinal basal layer of mitosis (stratum basale)
  • Production of keratin
  • Rate of mitosis = rate of desquamation (15-30 days, 25 to 50 days)
• Stratum Basale:

  • Germinal layer
  • Constant supply of new keratinocytes
  • Mitotic figures (active production of cells)
  • Cells: cuboidal or low columnar in a single layer separated from Dermis -> basement membrane-bound by -> hemidesmosomes
  • Nucleus: Large
  • Cytoplasm: basophilic
• Stratum Spinosum:

  • The thickest layer of the epidermis
  • Cells: Polyhedral cells with central nuclei
  • Synthesis of keratin and proteins
  • Some cells may still divide
  • SB + SS = Stratum germinativum
  • Tonofibrils are keratin filaments that converge at desmosomes, holding cells together
  • Epidermis of thick skin has thicker stratum spinosum
• Stratum Granulosum:

  • Layer of dead keratinocytes and keratin filaments
  • 3 to 5 layers of flattened cells
  • Undergoing terminal differentiation of
  • keratinization
  • Cytoplasm-filled intensely basophilic masses -> keratohyaline granules
• Stratum lucidum:

  • Found only in thick skin
  • A thin translucent layer of flattened eosinophilic keratinocytes
  • Held by desmosomes
  • Organelles & nuclei are absent
  • Cytoplasm contains packed keratin filaments in an electron-dense matrix
• Label the following:

  A) stratum corneum

  B) stratum lucidum

  C) stratum spinosum

  D) stratum basale

  E) dermis

  
• Stratum Corneum:

  • 15 to 20 layers of squamous keratinized cells
  • Birefringent filamentous keratins
  • End of keratinization happens, squames (or flat cells) are fully keratinized or cornified
  • Continuously shed at the epidermal surface
  • Deeper layers contain cornified cells with desmosomal junctions, orderly patterns of intracellular keratin
  • Most superficial layer sloughs off or “desquamates”
  • Desquamation: preceded by disruption of desmosomes and internal structure of cells
• Structural changes in keratinization involve the following:

  • Aggregation and arrangement of filaments
  • Formation of keratohyaline granules
  • Loss of organelles as a result of accumulation of granules

  A) desquamated superficial cells

  B) keratinized cells

  C) desquamating superficial cells

  D) keratinized epithelium

  E) stratified squamous epithelium

  
• Label the following:

  A) stratum corneum

  B) stratum granulosum

  C) stratum spinosum

  D) stratum basale

  E) dermis

  
• Label the following:

  A) palms and soles

  B) 4

  C) 5

  D) x

  E) ✓

  F) ✓

  G) ✓

  H) ✓

  I) x

  J) x

  
• 4 types of cells in the Epidermis:

  • Keratinocytes
  • Melanocytes
  • Langerhans cells
  • Merkel cells
• Keratinocytes are the principal cells or parenchyma of the skin. It synthesizes keratin, a family of polypeptides.
• Melanocytes

  • Synthesis and release of the brown pigment “melanin”
  • Located scattered infrequently in the basal layer & junction of the dermis
  • Round cells with pale-staining cytoplasm
  • Contain tyrosinase for melanin synthesis
• Langerhans Cells:

  • Are dendritic cells present in all layers of the epidermis, especially stratum spinosum
  • 2 to 8% of epidermal cells
  • Antigen-presenting cells
  • Intercepts microbial invaders in adaptive immune response
  • Antigen-presenting cells for contact allergic and other cell-mediated response.
• Merkel cells:

  • Epithelial tactile cells, also known as low-threshold mechanoreceptors
  • Senses gentle touch which is abundant in fingertips and bases of hair follicles
• The dermis supports the epidermis and binds to the hypodermis (not part of the skin).
• The dermis is 0.3-4mm thick, and the thickest is found in the upper back. It is an avascular epidermis, which is why nutrients only diffuse in it.
• Dermis does not have its own blood supply.
• Dermis:

  • Thermoregulation
  • 0.3 – 4 mm
  • Two zones: Papillary & Reticular layer
• The dermis contains elastin, an important constituent of both zones that appears black against red-stained collagen, is long and thick in reticular, and follows the course of collagen fibers. They appear very fine, scanty, and scarcely stained in the papillary layer.
• The papillary dermis is a loose connective tissue (Types I and III)

  • More cellular:
  • Fibroblasts
  • Mast cells
  • Macrophages
  • Leukocytes
  • Each papilla is made of very fine interlacing

  collagen fibers with few elastic fibers
• Label:

  A) Meissner's corpuscle

  B) capillary loop