Cell Surface Specializations: Cilia and Microvilli (Lecture 6.4)✅

Overview of Cell Surface Specializations
  • Focus on contributions of the cytoskeleton to the ultra-structure of surface specializations in epithelial cells.
  • Key concepts: cytoskeleton components, functions, and specific examples (cilia and microvilli).
Cytoskeleton Components

  • Microfilaments (Actin)

    • Found at the periphery of cells, providing structure and integrity to the plasma membrane.
    • Concentrated in motile cells (e.g., neutrophils) for shape changes and movement.
    • Responsible for cytokinesis during cell division.
    • Present in muscle cells (cardiac, smooth, skeletal) in combination with myosin, forming sarcomeres for contraction.

  • Microtubules

    • Present in cilia, flagella, and during mitosis (mitotic spindle formation).
    • Key role in cell division and structure.

  • Intermediate Filaments

    • Found in most cells, prominent around the nucleus and particularly in skin cells.
Cilia

  • Structure and Location

    • Composed of microtubules arranged in a 9+2 microtubule doublet configuration.
    • Located abundantly in trachea (tracheal epithelium) and oviduct (fallopian tubes).

  • Function

    • Involved in movement; cilia sweep mucus and trapped particles out of the airway (mucociliary escalator).
    • In the oviduct, they help transport the egg towards the uterus.

  • Microscopic Views

    • Electron microscopy shows cilia as finger-like projections, typically 10 microns in length.
    • Cilia are embedded in the plasma membrane, creating increased surface area for movement.
Microvilli

  • Structure and Function

    • Composed of actin microfilaments, microvilli increase the surface area for absorption in epithelial cells.
    • Found in high quantities in the small intestine (for nutrient absorption) and proximal convoluted tubules of the kidney (for reabsorption of plasma ultrafiltrate).

  • Microscopic Views

    • Electron microscopy shows microvilli as tiny projections, about 1 micron in length, increasing surface area to volume ratio.
    • Light microscopy shows microvilli as a brush border or fuzzy thickening due to carbohydrate molecules at their tips.
Key Locations for Ciliated and Microvilli Epithelial Cells

  • Ciliated Epithelial Cells:

    • Trachea: traps and removes pathogens via mucus.
    • Oviduct: assists in moving the egg.

  • Ciliated Epithelial Cells Features:

    • Goblet cells: Produce mucus in epithelial layers (e.g., trachea).
    • Pseudostratified epithelium: All nuclei contact the basement membrane, yet appear stratified.
Importance of Microvilli
  • Critical for reabsorption processes in the small intestine and kidneys.
  • Microvilli greatly enhance the cells' efficiency in absorbing nutrients and substances back into the body.
  • Destruction or dysfunction of these structures can lead to significant impairment in absorption and debris clearance.