Lesson 4

• Epithelium


  Tissue in which cells are bound tightly together and form a sheetlike or tubular structure with little extracellular material between the cells
• Epithelial cells


  • Have an apical side facing the sheet's free surface and a basal side facing a basement membrane and underlying connective tissue
  • Exhibit continuous renewal
  • For absorption or transcytosis, pinocytosis of material at the apical side and exocytosis at the basolateral side
• Basement Membrane


  • Two parts: a basal lamina and a more fibrous reticular lamina
  • Basal lamina: thin meshwork of type IV collagen and laminin produced by the epithelial cells
  • Reticular lamina: contains type III collagen and anchoring fibrils of VII collagen
• Tight or occluding junctions


  Formed by interacting transmembrane proteins such as claudin and occludin; linear arrangements surround the apical ends of the cells and prevent paracellular passage of substances
• Adherent or anchoring junctions


  Formed by interacting proteins of the cadherin family, points of strong attachment
• Adherent junctions


  Form zonula adherens that encircle epithelial cells, spot-like attachment sites called desmosomes or maculae adherens
• Skin / Integument / Cutaneous layer


  Largest single organ of the body, typically accounting for 15% to 20% of total body weight and, in adults presenting 1.5 to 2 m2 of surface to the external environment
• Functions of the skin

  • Protection
  • Sensory
  • Thermoregulatory
  • Metabolic
  • Sexual Signalling
• Thick Skin

  Covers the palms and soles, 400 to 1400 μm (1.4 mm) thick
• Thin Skin


  Covers the whole body except the palms and the soles, varies from 75 to 150 μm thick
• Layers of the Epidermis

  • Stratum basale / Stratum Germinativum / Basal layer
  • Stratum spinosum / Spinous layer / Prickle Layer
  • Stratum granulosum / Granular layer
  • Stratum lucidum / Clear Layer
  • Stratum corneum / Horny layer
• Stratum basale


  Also called stratum germinativum, attached to basement membrane by hemidesmosomes, forms epidermal ridges down into dermis, dermis has dermal papillae up into epidermis, epidermis is avascular, ridges and papillae increase surface area for diffusion between dermis and epidermis, contains basal or germinative cells
• Stratum spinosum


  The result of stem cell division
• Stratum granulosum


  Cells make a lot of keratin, a protein that provides water resistance and the foundation for hair and nails
• Stratum lucidum


  Cells are densely packed into a highly keratinized layer, only found in thick skin
• Layers of the Dermis

  • Papillary Layer
  • Reticular layer
• Accessory Structures of the Skin

  • Cutaneous Sensory Receptors
  • Hair
  • Nails
  • Glands
• Cutaneous Sensory Receptors

  • Free Nerve Endings: detects pain and temperature extremes
  • Basal Merkel Cells: mechanical receptor that responds to pressure and touch
  • Ruffini's corpuscles: respond to sustain pressure, and stretch
  • Meissner/ Tacticle corpuscles: detects light touch, vibration, and changes in texture
  • Lamellated/ Pacinian corpuscles: detects pressure, firm touch, deep pressure, high frequency vibrations
• Hair


  Filamentous keratinized structure that forms within epidermal invaginations which covers practically the whole body except the lips, palms, soles, distal dorsal parts of the fingers and toes, prepuce and glans of penis, labia minora, and inner surface of labia majora, clitoris, and nipple
• Types of Hair

  • Lanugo Hair
  • Vellus Hair
  • Terminal Hair
• Hair Structure


  • Cuticle
  • Cortex
  • Medulla
• Nails


  Hard plates of keratin on the dorsal surface of each distal phalanx
• Glands

  • Sebaceous glands
  • Sweat Glands
  • Modified sweat glands
  • Mammary glands
  • Ceruminous glands
• Skin Repair


  Occurs because stem cells of epithelium and connective tissue undergo cell division, replacing lost or damaged tissue, time depends on extent of damage, four steps: scab formation, tissue granulation, scab removal, scar formation
• The integumentary system provides mechanical protection against environmental hazards, forms the external surface of the body and provides protection from dehydration, environmental chemicals, and external forces
• The integument (skin) is separated and insulated from the rest of the body by the hypodermis layer, but it is interconnected with the rest of the body by an extensive circulatory network of blood and lymphatic vessels
• Although the protective mechanical functions of the skin can be discussed independently, its physiological activities are always closely integrated with the rest of the body
• Thomas Alva Edison: 'There is no substitute for hard work.'