Epithelial Tissue Part 2: Apical Surface Modifications

Epithelial Tissue Part 2: Apical Surface Modifications

• Epithelial cells exhibit polarity, with three main domains: apical, lateral, basal, and basolateral.
• The apical domain faces the lumen or external environment.
• The lateral domain lies between adjacent cells.
• The basal domain faces the basement membrane.
• The basolateral domain encompasses both the lateral and basal domains.

Apical Domain Modifications

• Modifications of the apical domain include microvilli, stereocilia, and cilia.

Microvilli

• Microvilli are small, finger-like projections, approximately 1 μm in length, visible under electron microscopy (EM).
• They increase the surface area for absorption.
• The number and shape of microvilli correlate to the absorptive capacity of the tissue.
• Microvilli contain bundles of parallel actin filaments.
• Proteins such as villin and fimbrin hold these actin filaments together in bundles.
• Myosin I and calmodulin connect the actin filament bundle to the plasma membrane.
• Actin filaments in microvilli extend into the apical cytoplasm, interacting with a horizontal network of actin filaments.
• The terminal web consists of actin filaments, spectrin, and myosin II.
• The terminal web may result in decreased diameter of intestinal absorbing cells, increasing the intermicrovillar space.

Stereocilia

• Stereocilia are extremely long, immobile processes extending from the apical cell surface.
• Similar to microvilli, they are supported by actin filaments.
• Stereocilia are found in sensory hair cells of the inner ear.
• Stereocilia facilitate fluid absorption and serve as sensory mechanoreceptors, rather than absorptive structures. Stereocilia aggregate into pointed bundles.

Cilia

• Cilia are hair-like extensions of the apical plasma membrane.
• Cilia contain microtubules or the axoneme.
• The microtubules originate from the basal body, a microtubule organizing center (MTOC) derived from centrioles.
• Cilia are classified as motile or primary.
• Motile cilia actively beat, while primary cilia do not actively move but may bend with fluid flow.
• Microtubules are arranged as a 9+2 array—nine peripheral doublets surrounding two central microtubules.

Motile Cilia

• Motile cilia are present in the respiratory tract and oviduct form a muco-ciliary unit.
• They help to transport ova and fluid as well as clear away mucous and particulate material.

Ciliary Movement

• Ciliary movement is based on the sliding of doublet tubules relative to one another.
• Each doublet contains a pair of arms (dynein).
• Dynein arms extend from the A tubule to temporarily bridge with the B microtubules.
• ATP hydrolysis causes these bridges to slide, causing the cilia to bend.
• Nexin protein connects the doublet tubules and returns the cilium to its straight position.

Disorders Affecting the Muco-ciliary Unit

• Primary Ciliary Dyskinesia (PCD) or Immotile Cilia Syndrome (ICS) is an autosomal recessive disorder that affects ciliary function.
• Kartagener syndrome is a specific form of PCD characterized by situs inversus, chronic sinusitis, and bronchiectasis.

Primary Cilia and Nodal Cilia

• Primary cilia are non-motile and function as signal receptors.
• Nodal cilia are found in the embryonic disc and function in signaling, establishing left-right asymmetry.