Epithelium

• Four major types of tissues

  • muscle tissue
  • epithelial tissue
  • nervous tissue
  • connective tissue
• Embryonic origins

  • very early in development the embryo undergoes gastrulation, which results in tri-laminar structure
  • 3 germ layers give rise to all tissues and organs (endoderm, mesoderm, ectoderm)
• Epithelium

  • Polar
  • Avascular (lacks blood vessels)
  • Minimal intercellular space
  • Attached to basement membrane
  • Line free surfaces (exterior of body / outer surface of organs / lining of body cavities, tubes, ducts)
  • Cells are contiguous with one another (cell-cell junctions create a barrier between the free surface and the underlying connective tissue)
• Microvilli

  • Vary depending on cell type
  • Generally the number and shape of the microvilli correspond to the cell’s absorptive capacity
  • Microvilli as extensions of the plasma membrane serve to increase the cell’s surface area of the apical domain thus aiding in absorption
  • Intestinal epithelium and the epithelium of the kidney tubules are good and varying examples of microvilli
• Microvilli Structure

  • core of actin filaments
  • + end (fast growing end) is anchored to villin
  • at the apical cytoplasm the bundle of actin filaments is anchored to the terminal web (a web of actin filaments situated horizontally) — spectrin stabalizes actin
  • actin-bundling proteins (epsin, fascin, fimbrin) cross link the actin for support and to add rigidity
  • myosin I binds actin filaments to the plasma membrane of the microvillus
  • myosin II and tropomyosin are present in the terminal web and allow contractility, when it contracts it causes the microvillus to spread out like fingers
• Stereocilia

  • Long microvilli that extend from the apical surface
  • limited in distribution (epididymis, ductus deferens, sensory hair (ear))
  • stereocilia in male repro tracts facilitate absorption
  • composed of actin filaments
  • arise from apical cell protrusion, aggregate into pointy bundles
• Stereocilia (Structure)

  • actin filaments form internal bundles that are covered with plasma membrane
  • filaments are cross-linked with fimbrin
  • the + end is toward the tip of the Stereocilia and the - end is toward the base
  • unlike microvilli, Stereocilia use the protein ezrin to anchor the actin filaments to the plasma membrane
  • Stereocilia use the protein a-actinin as cross bridges between the actin filaments in the apical cytoplasm and the apical protrusion
• Stereocilia — sensory hair cells of inner ear

  • sensitive to mechanic vibration and are termed = sensory mechanoreceptors
  • uniform in diameter but vary in height
  • arranged in staircase pattern
  • cores are densely bundled actin, cross-linked with epsin
  • lack a-actinin and ezrin
• Cilia

  • present on nearly every cell
  • hair-like extensions of the PM
  • contain an axoneme (microtubule-based structure)
  • axoneme originates from the basal body (made by centriole), an apical MTOC
  • basal bodies anchor cilia to the cytoplasm
  • subcategorized to: motile, primary, nodal
• Motile Cilia

  • found in large numbers on epithelial cells (trachea, bronchi, oviducts)
  • extend from apical surface as short, fine, hair-like structures
  • at base the basal bodies appear as a thin dark band
  • 9+2 microtubule core axonemal arrangement (share with flagella)
  • depend on microtubule-associated motor protein (dyenin) for motility (hydrolyzes ATP)
  • ciliary microtubules are modified in ways that prevent depolymerization