Epithelial Tissue Part 2: Apical Surfaces

Study Notes

Epithelial Tissue Part 2: Apical Surface Modifications

Epithelial cells exhibit polarity, having three domains: apical, lateral, basal, and basolateral.
The apical domain faces the lumen or external environment.
The lateral domain is between adjacent cells.
The basal domain faces the basement membrane.
The basolateral domain is the lateral and basal domains together.

Apical Domain Modifications

Microvilli: Small finger-like projections (approximately 1 µm in length), visible in electron microscopy (EM). They increase the surface area for absorption. Number and shape correlate with absorptive capacity.
- Contain bundles of parallel actin filaments.
- Filaments are held together by villin and fimbrin cross-linking proteins.
- Lateral arms in microvilli contain myosin I and calmodulin, linking the actin bundle to the plasma membrane.
- Actin filaments extend down into the apical cytoplasm and interact with a horizontal network of actin filaments. This network is called the terminal web.
- The terminal web is made of actin filaments, spectrin, and myosin II. The organization of the web influences the diameter of the apex thus influencing the overall spread and space between the microvilli.
Stereocilia: Extremely long, immobile processes extending from the apical cell surface. They are supported by actin filaments.
- Found in sensory hair cells within the inner ear.
- Facilitate fluid absorption.
- Aggregate into bundles (similar to wet paintbrushes).
- Have an internal filamentous structure composed of actin.
- Act as sensory mechanoreceptors rather than absorptive structures.
Cilia: Hair-like extensions of the apical plasma membrane.
- Contain microtubules (or axoneme).
- Microtubules originate in the basal body (microtubule organizing center), derived from centrioles.
- Classified as motile (actively beat) or primary (no active movement, bend with fluid flow).
- Motile cilia have a 9+2 arrangement, i.e., nine peripheral microtubule doublets surrounding two central microtubules.
- Dynein arms and radial spokes within the axoneme facilitate bending motion.
  - Dynein "arms" are microtubule-associated proteins.
  - Radial spokes run from peripheral doublets to central microtubules.
  - Nexin-link proteins connect microtubule doublets.
- Ciliary movement is based on the sliding of doublet tubules.

Ciliary Movement

Ciliary movement depends on sliding movements of doublet tubules.
Each doublet has a pair of arms that contain Dynein (microtubule-associated protein) and ATPase.
- Dynein arms extend from the A microtubule to the B microtubule of the adjacent doublet forming temporary connections (cross-bridges).
- ATP hydrolysis causes the sliding movement, bending the cilia.
- Nexin proteins maintain the structure between the doublets, returning the cilia to a straight position.

Disorders Affecting the Muco-Ciliary Unit

Primary Ciliary Dyskinesia (PCD) or Immotile Cilia Syndrome (ICS): An autosomal recessive disorder characterized by abnormal ciliary motion and impaired muco-ciliary clearance.
- Recurrent respiratory infections, sinusitis, otitis media, and male infertility can arise from PCD.
- In ~50% of patients, ICS is associated with situs inversus (mirror-image arrangement of internal organs).
Kartagener Syndrome: A subset of PCD, characterized by a triad of situs inversus, chronic sinusitis, and bronchiectasis.