SURFACE SPECIALIZATIONS & CELL POLARITY

What are the three morphologic domains of an epithelial cell?

Apical domain (free surface)

Lateral domain (adjoining cells)

Basal domain (attached to basement membrane)

What determines the properties of each epithelial domain?

Specific lipids and integral membrane proteins characteristic of each domain.

What cytoskeletal structures maintain apico-basal polarity?

Microtubules: oriented apico-basal, mediate vesicle transport

Microfilaments (actin): form microvilli, junctional complexes, and basal focal contacts; interact with cadherins (via catenins) and integrins.

What is the functional importance of cell polarity in epithelia?

Enables directional transport of materials across and along the cell surface — crucial for absorption, secretion, and barrier regulation.

What are the main types of intercellular junctions in epithelial cells?

Zonula occludens (tight junction)

Zonula adherens (adhering junction)

Desmosomes (macula adherens)

Gap junctions

What are the key proteins of the zonula occludens (tight junction)?

Claudins,

occludins,

and junctional adhesion molecules (JAMs), which regulate paracellular permeability and maintain polarity.

What determines the permeability of tight junctions?

The number and complexity of strands and the combination of specific claudins forming aqueous channels for paracellular passage.

Which proteins link tight junctions to the cytoskeleton?

ZO-1, ZO-2, and ZO-3 proteins connect occludins/claudins to actin filaments.

What are the components and function of gap junctions?

Composed of connexons (each made of 6 connexin subunits)

→forming intercellular channels.

Allow direct cytoplasmic communication between adjacent cells.

Name one disease associated with connexin mutations.

Mutation of connexin-26 (Cx26) causes congenital deafness due to altered K⁺ circulation in the cochlear sensory epithelium.

What are anchoring junctions and their key functions?

Zonula adherens: links actin filaments between cells via cadherins.

Desmosomes: link intermediate filaments (keratin) via desmogleins/desmocollins.

Provide mechanical stability and resistance to stress.

What are the structural surface modifications of epithelial cells?

Apical pole: cilia, stereocilia, microvilli

Basolateral pole: lateral infoldings, basal labyrinth, junctions, basal lamina

What are microvilli and their function?

Finger-like cytoplasmic projections containing actin filaments; increase surface area for absorption (e.g., intestinal brush border, kidney tubule brush border).

What is the structural organization of microvilli?

Each contains 20–30 parallel actin filaments cross-linked by

fimbrin,

fascin,

and espin.

Anchored into the terminal web (actin + spectrin).

Myosin I links actin filaments to the membrane.

How does the terminal web contribute to microvillus movement?

Contains myosin II and tropomyosin, enabling contractility that changes microvillus spacing to adjust absorption.

What are stereocilia and where are they found?

Very long,

immotile microvilli found in the epididymis and inner ear; function in absorption and mechanosensation.

What is a key structural difference between microvilli and stereocilia?

Stereocilia lack villin at their tips and are anchored by ezrin and
\alpha -actinin; microvilli have villin and myosin I.

What are cilia and their main types?

Cytoplasmic projections with a 9+2 microtubule axoneme.

Types: motile, primary (nonmotile sensory), and nodal (embryonic).

Describe the ultrastructure of motile cilia.

• Axoneme: 9 peripheral doublets + 2 central microtubules

• Dynein arms (ATPase activity) → sliding motion

• Nexin links & radial spokes → restore position

• Anchored by basal bodies (modified centrioles)

How does ciliary movement occur?

Dynein-driven sliding of microtubule doublets produces bending — an effective stroke and a slower recovery stroke, coordinated in a metachronal rhythm.

What is the function of basal bodies?

Act as microtubule-organizing centers (MTOC) anchoring cilia and synchronizing their movement.

What is primary ciliary dyskinesia (Kartagener
’s syndrome)?

Autosomal recessive disorder with absent dynein arms → immotile cilia.

Causes chronic respiratory infections, infertility, and increased ectopic pregnancies.

What are lateral and basal infoldings?

Plasma membrane invaginations that increase surface area for transport; prominent in epithelia involved in fluid/electrolyte transport (intestine, gallbladder).

What is the basal labyrinth?

Basal membrane infoldings with parallel mitochondria providing ATP for active transport (e.g., Na⁺/K⁺-ATPase); appears striated under the microscope