mod 2

• Epithelial tissues line all external and internal surfaces of the body
• Epithelial tissues are composed of closely aggregated polyhedral cells and a thin layer of extracellular matrix (ECM)
• Functions of epithelial tissues include covering, lining, and protecting surfaces, absorption, and secretion
• Embryonic origin of epithelial cells:

  • Ectoderm: covers the external surface of the body, skin, and cornea of the eyes
  • Endoderm: lines the digestive tract, liver, gall bladder, pancreas, respiratory tract, urinary bladder, urethra (except mouth & anus)
  • Mesoderm: lines the heart, blood, lymphatic vessels, serous cavities, urinary system (except bladder), male & female reproductive system
• Characteristics of epithelial cells:

  • Shapes and dimensions are quite variable
  • Indistinguishable by light microscopy
  • Avascular
  • Show polarity
  • Exhibit continuous renewal
• Basement membrane:

  • Thin extracellular sheet where the basal surface of all epithelia rests
  • Functions include attaching epithelia to connective tissue, regulating substances passing from connective tissue into epithelia, providing a guide during tissue regeneration, and compartmentalizing epithelial cells
• Basal lamina:

  • External laminae
  • Nearest the epithelial cells
  • Consists of Type IV collagen, laminin, nidogen, and perlecan
• Reticular lamina:

  • Below the basal lamina
  • Contains Type III collagen and Type VII collagen
• Intercellular adhesion & junctions:

  • Mechanically bind cells together
  • Form a permeability barrier
  • Provide a mechanism for intercellular communication
  • Types include tight or occluding junctions, adherent or anchoring junctions, and gap junctions
• Specializations of the apical cell surface:

  • Microvilli: small membrane projections specialized for absorption
  • Stereocilia: increase surface area for absorption, motion-detecting function in the male reproductive system
  • Cilia: long, motile structures with internal microtubules, primary cilium enriched with receptors for detection, motile cilia abundant in many epithelia
• Types of epithelia:

  • Covering (lining) epithelia
  • Secretory (glandular) epithelia
• Secretory epithelia & glands:

  • Formed by cells specialized in fluid secretion
  • May synthesize, store, and secrete proteins, lipids, or complexes of carbohydrate and proteins
• Mechanism of exocrine gland secretion:

  • Merocrine secretion: involves exocytosis from membrane-bound vesicles or secretory granules, classified as serous or mucous
  • Holocrine secretion: cells accumulate product continuously and release it with cell debris
  • Apocrine secretion: product accumulates at the cells' apical ends and is extruded with small amounts of cytoplasm and cell membrane
• Epithelial tissues line all external and internal surfaces of the body
• Epithelial tissues are composed of closely aggregated polyhedral cells and a thin layer of ECM
• Functions of epithelial tissues include covering, lining, protecting surfaces, absorption, and secretion
• Embryonic origin of epithelial cells:

  • Ectoderm: covers the external surface of the body, skin, cornea of the eyes
  • Endoderm: lines digestive tract, liver, gall bladder, pancreas, respiratory tract, urinary bladder, urethra (except mouth & anus)
  • Mesoderm: lines the heart, blood, lymphatic vessels & serous cavities, urinary system (except bladder), male & female reproductive system
• Characteristic features of epithelial cells:

  • Shapes and dimensions are quite variable
  • Indistinguishable by light microscopy
  • Avascular
  • Show polarity
  • Exhibit continuous renewal
• Basement membrane:

  • Thin extracellular sheet where the basal surface of all epithelia rests
  • Functions include attaching epithelia to connective tissue, regulating substances passing from connective tissue into epithelia, providing a guide during tissue regeneration, and compartmentalizing epithelial cells
• Basal lamina:

  • External laminae
  • Nearest the epithelial cells
  • Consists of Type IV collagen, laminin, nidogen, and perlecan
• Reticular lamina:

  • Below the basal lamina
  • Contains Type III collagen and Type VII collagen
• Intercellular adhesion & junctions:

  • Mechanically bind cells together
  • Form a permeability barrier
  • Provide a mechanism for intercellular communication
  • Types include tight or occluding junctions, adherent or anchoring junctions, and gap junctions
• Specializations of the apical cell surface:

  • Microvilli: small membrane projections specialized for absorption
  • Stereocilia: increase surface area for absorption, motion-detecting function in the male reproductive system
  • Cilia: long, motile structures with internal microtubules, primary cilium enriched with receptors for detection, motile cilia abundant in many epithelia
• Types of epithelia:

  • Covering (lining) epithelia
  • Secretory (glandular) epithelia
• Secretory epithelia & glands:

  • Formed by cells specialized in fluid secretion
  • May synthesize, store, and secrete proteins, lipids, or complexes of carbohydrate and proteins
• Mechanism of exocrine gland secretion:

  • Merocrine secretion: involves exocytosis from membrane-bound vesicles or secretory granules, classified as serous or mucous
  • Holocrine secretion: cells accumulate product continuously and release it along with cell debris
  • Apocrine secretion: product accumulates at the cells' apical ends and is extruded with small amounts of cytoplasm and cell membrane