Cell Biology (Lecture 35 - 37)

Front 1

What are tissues?

Back 1

Cooperative assemblies of cells and extracellular matrix that work together to achieve some sort of functional goals.

Front 2

What are the characteristics of epithelial tissues?

Back 2

The cells are intimately connected. It has apico-basal polarity and very little extra-cellular matrix.

Front 3

What are the characteristics of connective tissues?

Back 3

The cells have very few contacts with one another. They have no apico-basal polarity but a large extracellular matrix.

Front 4

What are the major functions of epithelia?

Back 4

1. To line external and internal body surfaces.
2. To line tubular organs that communicate with the exterior.
3. To form secretory parts of glands and their ducts.
4. To from components of sensory organs.

Front 5

What does the respiratory tract contain?

Back 5

A ciliated pseudostratified epithelium to provide an appropriate exchange site gases but also to clean itself. It contains goblet cells that secrete mucous and basal cells that are stem cells.

Front 6

What does the respiratory tract contain?

Back 6

Apical basal polarity:
The apical surface has active transport channels to bring glucose into the cell.
The basal surface has passive channels and transporter proteins to diffuse glucose into the blood.
There are tight junctions between cells to prevent the backflow of molecules.

Front 7

What is the function of tight junctions?

Back 7

To prevent the leakage of molecules across the epithelium. They separate different membrane domain of epithelium and thus are essential to maintain cell apico-basal polarity.

Front 8

What is the structure of tight junctions?

Back 8

Usually near the apical surface. Bands of plasma membrane proteins (claudin and occludin) in adjoining cells interact, forming very strong lines that are difficult to break.

Front 9

What is the function of adherens junction?

Back 9

They are not as tight as tight junctions. They link epithelial cells to each other and link with cell cytoskeleton.

Front 10

What is the structure of adherens junction?

Back 10

It involves homophilic interactions between cadherins (cell adhesion). Cadherins are anchored to the cytoskeleton (actin) through an elaborate array of adaptor molecules.

Front 11

What are cadherins activated by?

Back 11

Cadherins are activated by calcium ions which makes them rigid. In their rigid form they protrude from the cell where they are able to interact via homophilic binding with the other cadherins in other cells.

Front 12

Why are adherens junctions not as tight as tight junctions?

Back 12

In their rigid form, cadherins protrude from the cell, the membranes of the cell aren't brought as close as with tight junctions and thus many molecules can pass freely through these.

Front 13

What are desmosomes?

Back 13

Like adheren junctions, desmosomes are also anchoring junctions. Thus they are associated with the cytoskeleton and are associated with intermediate filaments.

Front 14

How do desmosomes link cells to each other?

Back 14

Desmosomes link cells to each other by homophilic interactions between cell adhesion molecules o the cadherin family (desmoglein and desmocollin).

Front 15

How do desmosomes link to the cytoskeleton (intermediate filaments)

Back 15

Desmosomes link with the cytoskeleton via adaptor proteins (plakophillin, plakoglobin and desmoplakin).

Front 16

What are hemidesmosomes?

Back 16

They are anchoring junctions that anchor the cell to the basal lamina. They are also associated with intermediate filaments.

Front 17

What is the structure?

Back 17

Integrins (transmembrane proteins) are anchored to keratin via adaptor proteins (dystonin and plectin)

Front 18

What are focal adhesions?

Back 18

They anchor the cell to the basal lamina but are also involved in cell motility. They are regulated and so can be disengaged and engaged at the right times, helping drag the cell up and forward.

Front 19

What proteins are involved in focal adhesions?

Back 19

Extracellular matrix --> integrin --> talin and vinculin --> actin filament

Front 20

What are integrins?

Back 20

Proteins that anchor the cell to the basal lamina. They are responsible for recruiting and activating certain kinases involved in essential cell survival proliferation pathways.

Front 21

How does inside-out signalling work?

Back 21

Cells regulate the activity of cell surface integrins. Integrins will not engage the extracellular matrix ligand unless activated.

Front 22

How does outside-in signalling work?

Back 22

The extracellular matrix binding recruits intracellular proteins which recruit tyrosine kinases. These tyrosine kinases activate enzymes involved in vital cell survival and proliferation pathways - without activation of these enzymes the cell will apoptose. Thus without binding to the basal lamina, epithelial cells can't grow and thus can't float off and invade other tissues.

Front 23

What are gap junctions?

Back 23

They are channels between the membranes of two cells that allow small molecules to pass from cell to cell.

Front 24

What is the structure of gap junctions?

Back 24

Hexamers of connexin proteins create a structure called connexon. A connexon on one cell then forms a continuous channel with a connexon on the next cell, forming a gap junction.

Front 25

What are the types of connexon?

Back 25

Same type of connexin - homomeric
Different - heteromeric

Front 26

What are the different types of connexon channels?

Back 26

Two connexons with the same connexins: homotypic
Different connexins: heterotypic

Front 27

What is the extracellular matrix?

Back 27

The ECM is the space between cells composed of a complex array of molecules. It is composed of tough, fibrous proteins embedded in a polysaccharide gel-like material (ground substance).

Front 28

What proteins are in the ECM?

Back 28

Fibrous/structural - collagen and elastin
Adhesive - laminin

Front 29

What is the structure of collagen molecules?

Back 29

Collagen alpha chains (left handed) are composed of a series of Gly-X-Y triplets. Collagen molecules consist of three alpha chains arranged in a super helix.

Front 30

What is the structure of fibrils?

Back 30

Many collagen molecules assemble together via covalent cross links to form fibrils. Fibrils assemble outside the cell.

Front 31

What is the structure of a fibre?

Back 31

Many collagen fibrils pack together to form collagen fibres.

Front 32

Do all collagens form fibres?

Back 32

No, only collagens I, II, III and V will form fibrils. Collagen IV forms a sheet. Collagens VI, IX and XIII decorate fibrillar collagens and mediate fibril interactions - they are characterised by the presence of a non-helical domain which interrupts the triple chain and makes them flexible.

Front 33

How are the fibrils synthesised?

Back 33

The pro-alpha chains are synthesised and aggregate into precollagen molecules is secreted outside the cell by exocytosis. The propeptide is cleaved and the collagen can now assemble. Covalent bonds between lysine residues crosslink the collagene molecules to form fibrils.

Front 34

How does crosslinking of fibrils affect tensile strength?

Back 34

More crosslinking leads to more tensile strength - highest level of tensile strength occurs in tendon collagen.

Front 35

What is the function of elastin?

Back 35

Elastin is elastic and provides elasticity to tissues.

Front 36

What is the structure of elastin?

Back 36

Elastin forms a covalently cross-linked network of elastin molecules. Elastin associates with fribrillin to form fibres.

Front 37

What is fibronectin?

Back 37

The insoluble form forms fibres when associated with integrins.

Front 38

What is laminin?

Back 38

It is common in basement membrane where it interacts with collagens. It also interacts with integrins on the cell surface. It contains 3 chains held together by disulphide bonds that forms a trimer.

Front 39

What is the basal lamina/basement membrane?

Back 39

It is found in epithelial tissue and it what epithelial cells bind to. It is synthesised by the epithelium. It is not inert and contains growth factors.

Front 40

What are proteoglycans?

Back 40

They serve complimentary function to collagens. They form the gel-like 'ground' substance that is able to resist compressive forces.

Front 41

What is the structure of proteoglycans?

Back 41

They contain a large protein core linked to negatively charged chain of polysaccharides (glycosaminoglycans). The GAG chains are linked to a core protein via linker tetrasaccharides.

Front 42

What is the function of proteoglycans?

Back 42

The negative charge of the GAG repeats allow the proteoglycan to attract cations. Thus is can form gels due to water following the ion concentration. This osmotic pressure gives resistance to compression which helps to balance the tensile strength of collagen.