lecture 13: epithelial tissues – an overview

Question 1

What is an epithelium and where do we see epithelial tissue?

Answer 1

• cells in metazoans must associate to form organs
• in epithelial tissues the cytoskeletons of cells are linked
  
  • epithelia are polarised tissues
• epithelial tissue often seen lining organs
• in connective tissue the extracellular matrix is the main stress-bearing component
• mechanical stresses are transmitted from cell to cell by cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites
• extracellular matrix directly bears mechanical stresses of tension and compression

Question 2

How do epithelial tissues undergo regeneration?

Answer 2

• from stem cell pools
• e.g. intestinal crypt, hair-follicle bulge, corneal limbus, mammary gland terminal end bud

Question 3

Why focus on epithelial stem cells?

Answer 3

• important roles in regenerative medicine
• e.g. utility in being able to treat burns patients

Question 4

What are the four functional classes of cell junctions found in animal tissues?

Answer 4

• anchoring junctions
  
  • actin filament attachment sites
    
    • cell-cell junctions (adherens junctions)
    • cell-matrix junctions (actin-linked cell-matrix adhesions)
  • intermediate filament attachment sites
    
    • cell-cell junctions (desmosomes)
    • cell-matrix junctions (hemidesmosomes)
• occluding junctions
  
  • tight junctions (in vertebrates)
  • septate junctions (in invertebrates)
• channel-forming junctions
  
  • gap junctions (in animals)
  • plasmodesmata (in plants)
• signal-relaying junctions
  
  • chemical synapses (in the nervous system)
  • immunological synapses (in the immune system)
  • transmembrane ligand-receptor cell-cell signalling contacts (Delta-Notch, ephrin-Eph, etc.). Anchoring, occluding, and channel-forming junctions can all have signalling functions in addition to their structural roles

Question 5

What are tight junctions?

Answer 5

• specialised junctions of epithelial/endothelial cells
• aka occluding junctions
• they facilitate transcellular transport
• block free flow of molecules from one surface of the cell down to the other
• this means that the transport of molecules e.g. glucose is regulated - can’t just diffuse down the sides of the cells, have to have particular transporters on the apical and basal membranes of the cell
• tight junctions also help define the apical and basolateral membranes of the cell → block free movement of proteins from one side of the cell membrane to the other
• i.e. tight junctions important in giving cells polarity
• most apical of the junctions

Question 6

Of what do tight junctions form barriers to diffusion?

Answer 6

• solutes: we can see this using dye solutions
• membrane proteins: this tells us that newly synthesized proteins are directed to specific regions of the cell membrane

Question 7

How are tight junctions formed?

Answer 7

• a meshwork of sealing strands of transmembrane proteins (claudin, occludin)
• focal connections

Question 8

What are anchoring junctions?

Answer 8

• allow the cytoskeleton to adhere to the ECM or other cells
• include:
  
  • adherens junctions (connect to actin filaments)
  • desmosomes, hemidesmosomes (connect to indermediate filaments)

Question 9

Of what do anchoring junctions consist?

Answer 9

• an intracellular plaque that attaches to the cytoskeleton
• transmembrane proteins that bind to adjacent proteins on other cells/ECM

Question 10

What are features of adherens junctions?

Answer 10

• cell-cell anchoring junction
• transmembrane adhesion protein = cadherin (classical cadherin)
• extracellular ligand = cadherin in neighbouring cell
• intracellular cytoskeletal attachment = actin filaments
• intracellular anchor proteins = α-catenin, β-catenin, plakoglobin (γ=catenin), p120-catenin, vinculin, α-actinin

Question 11

What are the main features of desmosomes?

Answer 11

• cell-cell anchoring junction
• transmembrane adhesion protein = cadherin (desmoglein, desmocollin)
• extracellular ligand = desmoglein and desmocollin in neighbouring cell
• intracellular cytoskeletal attachment = intermediate filaments
• intracellular anchor proteins = plakoglobin (γ-catenin), plakophilin, desmoplakin

Question 12

What are features of hemidesmosomes?

Answer 12

• cell-matrix anchoring junction
• transmembrane adhesion protein: integrin α6β4, type XVII collagen (BP180)
• extracellular ligand: extracellular matrix proteins
• intracellular cytoskeletal attachment: intermediate filaments
• intracellular anchor proteins: talin, vinculin, α-actinin, filamin, paxillin, focal adhesion kinase (FAK) plectin, dystonin (BP230)

Question 13

What do adherens junctions do?

Answer 13

• form a continuous belt below the tight junctions, the zona adherens, in epithelial cells that bring actin filaments into alignment
• cadherins form the transmembrane linkages

Question 14

What is one of the functions of adherens junctions?

Answer 14

• myosin motors can cause contraction of bundles of actin filaments in adhesion belts – this results in cells to narrow at the apex and epithelia to form tubues
• sheet of epithelial cells
• invagination of epitehlial sheet caused by an organized tightening of adhesion belts in selected regions of cell sheet
• epithelial tube pinches off from overlying sheet of cells
• epithelial tube
• e.g. neural tube

Question 15

How do cadherins bind?

Answer 15

• cadherins bind in a homophilic manner and binding is dependent upon calcium concentration
• e-cadherin is most common form of cadherin found in epithelial cells
• calcium binds to the flexible hinge regions to make them stiff therefore allowing binding

Question 16

What happens during the process of compaction?

Answer 16

• cells of the early mouse embryo stick together weakly
• at the 8-cell stage they begin to express E-cadherin
• as a result strongly adhere to one another
• roughly the same point at which totipotency is lost

Question 17

How can expression of cadherins cause cell sorting?

Answer 17

• homophilic adhesion and differential expression of classical cadherins can cause cell sorting and is important in tissue formation
• can be seen in both two different types of cadherin being expressed and different levels of the same cadherin being expressed
• the latter may cause different tissue layers
• loss of a specific cadherin will disrupt formation of particular epithelial tissues → e.g. loss of N-cadherin will prevent correct formation of the neural tube

Question 18

What was an experiment demonstrating the sorting out and reconstruction of spatial relationships in aggregates of embryonic amphibian cells?

Answer 18

• take epidermis + mesoderm → sort out so mesoderm is on inside and epidermis is on outside
• mesoderm + endoderm → sort
• all three → something a lot like a normal embryo
• differential adhesion starts to form the tissue layers seen in the early embryo

Question 19

What are some members of the cadherin superfamily?

Answer 19

• classical cadherins
  
  • e-cadherin → many epithelia, adherens junctions, -/- = death at blastocyst stage; embryos fail to undergo compaction
  • n-cadherin → neurons, heart, k/o die from heart defects
  • p-cadherin → placenta; k/o abnormal mammary gland development
  • ve-cadherin → endothelial cells; abnormal vascular development (apoptosis of endothelial cells)
• nonclassical cadherins
  
  • desmocollin → skin, desmosomes, blistering of skin
  • desmoglein → skin, desmosomes, blistering skin disease due to loss of keratinocytes-cell adhesion
  • t-cadherin
  • cadherin 23
  • fat (in drosphila)
  • fat1 (in mammals)
  • alpha, beta, and gamma protocadherins
  • flamingo

Question 20

What do desmosomes do?

Answer 20

• desmosomes “spot-weld” cells together to distribute tensile forces
• through desmosomes the intermediate filaments of adjacent cells are connected to form a continuous network of great strength

Question 21

How do desmosomes link to the intracellular filaments?

Answer 21

• the sides of cytokeratin filaments interact with the cytoplasmic plaque that is attached to an adjacent cell via cadherin interactions
• non-classical cadherins, desmoglein and desmocollin, transmembrane proteins that form homophilic interactions with proteins from adjacent cell
• intracellular plaque links transmembrane proteins to intermediate filaments
• intracellular plaque = desmoplakin, plakoglobin, plakophilin

Question 22

What is the function of hemidesmosomes?

Answer 22

• in contrast to desmosomes, hemidesmosomes attach to the ECM (via integrins) and to the ends of cytokeratin filaments
• they are chemically and functionally distinct from desmosomes

Question 23

How do hemidesmosomes link to cytokeratin filaments?

Answer 23

• utilise a specialised integrin (α6β4) to link cytokeratin filaments via plectin and dystonin anchor proteins to extracellular laminin
• link to ends of intermediate filaments
• defects → cotton wool babies

Question 24

Apically to basally, what cell junctions are found?

Answer 24

• occluding junction
• cell-cell anchoring hunctions
  
  • adherens
  • desmosomes
• channel-forming junctions
• cell-matrix anchoring junctions

Question 25

What is the ECM?

Answer 25

• the ECM is an organised meshwork of proteins and polysaccharides
• structural proteins include collagen and elastin
• cellular adhesion to the ECM depends upon:
  
  • adhesive ligands: laminin, fibronectin
  • anti-adhesive ligands: tenascin, chondroitin sulfate proteoglycan

Question 26

What is the basal lamina?

Answer 26

• the basal lamina is a specialised ECM that underlies all epithelia
• also found surrounding muscle and the glomeruli of the kidney

Question 27

What is a primary component of the basal lamina?

Answer 27

• laminin
• adhesive ligand
• trimeric molecule - three seperate chains
• crucifix shaped molecule
• binds to integrins
• meshwork → self-assembly
• binds to other components of ECM

Question 28

What is a model of the basal lamina?

Answer 28

• grid meshwork type thing link to each other and the plasma membrane of the cell

Question 29

What molecules regulate formation of apico-basal polarity?

Answer 29

• Par3, Par6: scaffold proteins that bind to each other and to aPKC (atypical protein kinase C)
• these proteins associate with the tight junctions and serve as binding sites for Cdc42 and Rac – organisers of the actin cytoskeleton
• Crumbs complex in apical side
• scribble complex in basal side

Question 30

What does epithelial cell polarity involve?

Answer 30

• actin polarization
• regulated protein trafficking to apical, lateral, and basal membranes
• regulated secretion and absorption
• regulated ion flow
• signalling via integrin attachment to the ECM – this can influence cell proliferation

Question 31

What is EMT?

Answer 31

• epithelial to mesenchymal transition
• can also have MET
• when epithelial cells receive specific signals they can break the adhesions between cells, loosen the grip on the ECM without undergoing apoptosis, released from basement membrane
• e.g. development of neural crest cells, gastrulation
• important in development of tissues and organs:
  
  • cutaneous structures, limb, gut organs, foregut and respiratory associated organs, kidney, tooth

Question 32

What does EMT generate?

Answer 32

• cells with properties of stem cells
• driving expression of Snail or Twist (mesenchymal inducers) in human mammary epithelial cells causes them to take on stem cell characteristics
• intestinal epithelial stem cells express mesenchymal markers
• e.g. highly upregulated expression of fibronectin (mesenchymal cell marker)
• formation of mammospheres
• self-renewing spheres
• don’t entirely understand why yet
• pseudomesenchymal state
• if these proteins have a role in maintenance/proliferation and movement of stem cells, might understand why tumours are so dangerous

Question 33

review points:

Answer 33

• what is the difference between epithelial and connective tissues?
• what are characteristics of epithelial tissues?
• what is an occluding junction (tight junction)?
• what are anchoring junctions (desmosomes, hemidesmosomes, adherens junctions)?
• what is the basal lamina?
• what regulates polarity of epithelial cells?
• what is the epithelial to mesenchymal transition (EMT)?
• why is it suggested that epithelial stem cells have mesenchymal properties?