Ch 20 Epithelial Sheets and Cell Junctions Textbook Info (Exam 4)

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55 Terms

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Many tissues contain

epithelial cells embedded in the extracellular matrix populated with other cell types/connective tissue

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All ducts, lumens, and tubes are lined with

epithelial cells

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Epithelium/epithelia are

sheets of cells covering an external surface or lining of an internal body cavity

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Some epithelia act mainly as

a protective barrier

  • some have complex biochemical functions

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Epithelial sheets are

polarized and rest on a basal lamina

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An epithelial sheet has two faces

  • the apical surface

  • basal surface

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The apical surfae of an epithelial sheet is

free and exposed to the air or to a bodily fluid

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The basal surface of an epithelial sheet is

attached to a thin, tough sheet of extracellular matric called the basal lamina

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Laminin provides

adhesive sites for integrin molecules in the basal plasma membranes of epithelial cells

  • serves as a linking role

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The basal lamina supports

a sheet of epithelial cells

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Epithelial cells are

polarized

  • apical (top) and basal (bottom) surfaces have different structures and functions

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In the small intenstine, the epithelium contains two main cell types

  • absorptive cells (polarized)

  • goblet cells (polarized)

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Absorptive cells

take in nutrients apically from the gut and pass them to underlying tissues basally

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Goblet cells

secrete mucus from their apical side

  • polarized in a way that supports mucus secretion

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Cell polarity in absorptive and goblet cells relies on

junctions between neighboring cells and the basal lamina

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Tight junctions make an epithelium

leakproof and separate its apical and basolateral surfaces

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Epithelial cell junctions have different functions,

some seal cells tightly to prevent leakage, others provide mechanical support, and some allow communication between cells

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Tight junctions are very important in

vertebrates because it forms a barrier that prevents water-soluble molecules from passing between cells

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Without tight junctions,

functions like nutrient absorption would not work properly

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What are the main functions of epithelial cell junctions?

  • prevent leakage between cells (tight seal)

  • provide strong mechanical attachments

  • allow cytosolic exchange between cells

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What type of junction provides a barrier function in vertebrate epithelial sheets?

tight junctions

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What proteins form tight junctions?

claudins and occludins

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How do tight junctions prevent leakage fo water-soluble molecules?

they seal neighboring cells together so that molecules cannot pass between them

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What would happen without tight junctions in absorptive epithelia like the gut?

absorptive pumping would be ineffective, and extracellular fluid composition would equalize on both sides of the epithelium

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How do tight junctions help maintain epithelial cell polarity?

  • they prevent the diffusion of membrane proteins between the apical and basolateral surfaces

  • they serve as sites where protein complexes assemble to organize apical-basal polarity inside the cell

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What are the main types of cell junctions that provide mechanical strength in epithelia?

  • adherens junctions

  • desmosomes

  • hemidesmosomes

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What do adherens junctions and desmosomes connect?

they connect one epithelial cell to another

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What do hemidesmosomes connect?

they attach epithelial cells to the basal lamina?

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How do mechanical junctions provide strength to epithelial sheets?

by linking junction to cytoskeletal filaments, forming a continuous network across cells

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What family of transmembrane proteins are adherens junctions and desmosomes built around?

cadherins

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What kind of binding do cadherins exhibit?

homophilic binding

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Homophilic binding is

cadherin-to-identical-cadherin

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What ion is required for cadherin-mediated adhesion?

calcium (Ca2+)

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To what cytoskeletal filaments are cadherins linked at adherens junctions?

actin filaments

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What structure do adherens junctions often form in epithelial cells?

a continuous adhesion belt near the apical end of the cell, just below tight junctions

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What is the function of the actin-myosin network in adherens junctions?

it allows contraction of the epithelial sheet, enabling it to change shape

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How can contraction of adherens junctions shape epithelial sheets during development?

  • 1 axis contraction → sheet rolls into a tube

  • multiple axes contraction → sheet forms a cup that can pinch off into a sphere

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What embryonic structures are formed by epithelial sheet movements?

the neural tube (brain and spinal cord) and the lens vesicle (lens of the eye)

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What type of cytoskeletal filaments are connected at desmosomes?

keratin filaments

  • a type of intermediate filament

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What type of cadherins are involved in desmosomes?

a different set of cadherins specific to desmosomes (not the same as those in adherens junctions)

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How do desmosomes provide strength to epithelial sheets?

by linking keratin filaments between neighboring cells, forming a strong, interconnected network

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What type of epithelium especially relies on desmosomes for strength?

tough, exposed epithelia such as the epidermis of the skin

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Why do epidermal cells need to be anchored to underlying connective tissue?

to prevent separation and blistering

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What proteins mediate the attachment of epithelial cells to the basal lamina?

integrins in the basal plasma membrane

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What are hemidesmosomes?

structures that anchor epithelial cells to the basal lamina

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What do integrins bind to on the outside of the cell?

laminin in the basal lamina

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What do integrins bind to on the inside of the cell?

keratin filaments via linker proteins

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What is the main function of gap junctions?

to allow direct transfer of ions and small molecules between the cytoplasms of adjacent cells, enabling electrical and metabolic coupling

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What protein complexes form gap junction channels?

connexons

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How are connexons arranged between cells?

in adjacent cells theyre aligned end-to-end to form water-filled channels across the membranes

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Are gap junctions able to open and close?

yes because they respond to extracellular and intracellular signals

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What structure in plant cells is functionally similar to gap junctions?

plasmodesmata

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How do plasmodesmata differ structurally from gap junctions?

theyre lined with plasma membrane and connect the cytoplasm of adjacent cells through the cell wall

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What types of molecules can pass through the plasmodesmata?

small molecules, some proteins, and regulatory RNAs

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Why is regulated movement through plasmodesmata important in plants?

it helps control development by transporting transcription regulators and regulatory RNAS between cells