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Questions and Answers
What is the primary function of occluding junctions in epithelial tissue?
What is the primary function of occluding junctions in epithelial tissue?
Which type of junction is primarily responsible for providing mechanical support by attaching cells to other cells or their extracellular matrix?
Which type of junction is primarily responsible for providing mechanical support by attaching cells to other cells or their extracellular matrix?
Which statement best describes the role of epithelial tissues?
Which statement best describes the role of epithelial tissues?
What types of connections are required between epithelial cells to maintain their organized structure?
What types of connections are required between epithelial cells to maintain their organized structure?
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What type of adhesion is primarily involved when cells adhere directly to each other?
What type of adhesion is primarily involved when cells adhere directly to each other?
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What type of cytoskeleton do adherens junctions primarily connect to?
What type of cytoskeleton do adherens junctions primarily connect to?
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What is the primary function of the zonula adherens?
What is the primary function of the zonula adherens?
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Which of the following statements best describes desmosomes?
Which of the following statements best describes desmosomes?
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What role do myosin motor proteins play in adherens junctions?
What role do myosin motor proteins play in adherens junctions?
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Where are intercalated disks primarily found and what is their function?
Where are intercalated disks primarily found and what is their function?
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What is the primary function of occluding junctions in epithelial cells?
What is the primary function of occluding junctions in epithelial cells?
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Which statement best describes the structure of epithelial cells?
Which statement best describes the structure of epithelial cells?
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How do tight junctions contribute to glucose absorption in the gut?
How do tight junctions contribute to glucose absorption in the gut?
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Which of the following is true about the role of anchoring junctions?
Which of the following is true about the role of anchoring junctions?
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What distinguishes epithelial cells from other cell types in vertebrates?
What distinguishes epithelial cells from other cell types in vertebrates?
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What is a primary function of gap junctions in electrically excitable tissues?
What is a primary function of gap junctions in electrically excitable tissues?
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Which statement accurately describes the structure of gap junctions?
Which statement accurately describes the structure of gap junctions?
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What consequence occurs from antibodies attacking desmosomal cadherin proteins in emphigus?
What consequence occurs from antibodies attacking desmosomal cadherin proteins in emphigus?
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Which of the following correctly distinguishes gap junctions from tight junctions?
Which of the following correctly distinguishes gap junctions from tight junctions?
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What role do connexons play in the function of gap junctions?
What role do connexons play in the function of gap junctions?
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What is the primary mode of assembly and disassembly of gap junctions?
What is the primary mode of assembly and disassembly of gap junctions?
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What type of subunits are primarily found in gap junctions in vertebrates?
What type of subunits are primarily found in gap junctions in vertebrates?
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Which cells rely on gap junctions to synchronize contractions in the heart and smooth muscle?
Which cells rely on gap junctions to synchronize contractions in the heart and smooth muscle?
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Study Notes
Animal Tissue Composition
- Epithelial cells form a layer that delimits a structure, like the gut lumen
- Epithelial cells have tight junctions to prevent leakage
- Cells can adhere directly to each other or via extracellular materials
- Epithelial cells form an organized, multicellular structure
Cell Junctions
- Three types of cell junctions exist
- Occluding junctions: seal cells tightly, form an impermeable barrier
- Anchoring junctions: attach cells/cytoskeleton to other cells or extracellular matrix, provide mechanical support
- Communicating junctions: allow chemical/electrical information exchange between cells
- Junctions include tight junctions, adhesion belts, desmosomes, gap junctions, etc.
Junctions and Epithelial Function
- Over 60% of vertebrate cell types are epithelial
- Epithelial cells are anchored to underlying tissues via basal lamina, but free at apical surface
- Epithelial cells act as selective barriers, preventing fluids from passing readily
- Occluding junctions (tight junctions) are crucial in vertebrates for this barrier function
Occluding Junctions
- Seal gaps between epithelia cells to create an impermeable or selectively permeable barrier
- Tight junctions are a type of occluding junction in vertebrates
- Septae junctions are a type of occluding junction in invertebrates
Absorption of Glucose from the Gut
- Specialized absorptive epithelial cells in the gut express Na+/glucose symporter on their apical side and glucose carrier protein on their basalateral side
- Tight junctions are critical to this process
- The tight junctions help partition transporters and prevent uncontrolled fluid flow
Anchoring Junctions
- Cell-cell adhesions and cell-matrix adhesions transmit stresses
- Tethered to cytoskeletal filaments within cells
- Types include:
- Actin-based junctions (adherens junctions & actin-linked cell-matrix adhesions)
- Intermediate filament-based junctions (desmosomes & hemidesmosomes)
Adherens Junctions
- Important in shaping multicellular structures and coordinating cell activities
- Forms a belt around epithelial cells near apical surface
- Actin filaments link via cadherins, forming a network
- Myosin motor proteins can make the junction contractile and allow cell reshaping
- Important in heart muscle (intercalated discs)
Desmosome Junctions
- Link to intermediate filaments, not actin like adherens junctions
- Provide mechanical strength in vertebrates
- Absent in Drosophila, but found in mature vertebrate epithelia
- Key proteins include desmoplakin, plakoglobin, plakophilin, etc. linked to intermediate filaments
- Desmosomes form regions that stick cells together
Channel-Forming Junctions
- Create connections between cytoplasm of different cells
- Gap junctions are present in animals, and plasmodesmata are present in plants
- These junctions allow small molecules to pass between adjacent cells
Gap Junctions and Plasmodesmata
- Gap junctions in contrast with tight junctions, facilitate passage of small molecules between cells
- Gap junctions are found in animals; plasmodesmata in plants
- Allow exchange of small molecules between cells
Functions of Gap Junctions
- Important for rapid spread of action potentials
- Crucial in escape responses in fish and insects
- Essential in vertebrate heart muscle and smooth muscle synchronization
- Allow coordinated signals, like between liver cells and nerve signals
Structure of a Gap Junction
- Composed of connexins; homomeric or heteromeric channel complexes
- Connexins are present in vertebrates (mainly); innexins are found in other organisms (like Drosophila and Caenorhabditis)
- Gap junctions can rapidly assemble/disassemble
Mutations in Connexin Genes
- Connexin 26 mutations can cause death of cells in the organ of Corti (leads to deafness)
- Other connexin mutations can cause cataracts and demyelinating disease
Signal-Relaying Junctions
- Allow signals to be relayed between cells across plasma membranes
- Similar in principle to Channel-forming junctions but with more complex structures
- Include specialized proteins for signal transduction and anchorage
- Examples include chemical synapses in nervous system, immunological synapses
Summary of Cell Junction Types
- Tight junctions seal gaps between cells.
- Adherens junctions connect actin filaments amongst cells.
- Desmosomes connect intermediate filaments of adjacent cells.
- Gap junctions allow direct passage of ions and small molecules between cells.
Proteins in Junction Complexes
- Proteins like E-cadherin, JAM, occludin, claudin are found in junction complexes.
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Description
This quiz covers the composition and functions of animal epithelial tissues, including the types of cell junctions. Learn about how epithelial cells form layers, their roles as selective barriers, and the various junction types that enable cellular communication and structural integrity. Test your knowledge on these fundamental biological concepts.
