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.

This quiz focuses on the structure and function of epithelial tissue, particularly the modifications found in the apical surface. You will learn about various features such as microvilli, stereocilia, and cilia, and their roles in absorption and epithelial cell functionality. Test your understanding of these essential components!