Beyond the cell

Question 1

What are cell surface markers?

Answer 1

• they are a unique set of cell surface molecules, they are the part of the cell that other cells read. [markers]
• Some markers are glycolipids.
• The markers differentiate organs and tissues in a vertebrate body.
• On red blood cells the markers identify A,B,O blood types.
• Some markers are membrane protein.
• immune system “self” markers proteins
• Using MHC Proteins, Major histocompatibility complex
• Immunoglobulin family of cell surface markers, these are antibodies.

Question 2

Intercellular adhesion

Answer 2

• Cell junctions - connections between cells.
• Most cells are in physical contact with other cells at all times e.g heart, lungs, or gut.
• The nature of the connection determines what tissue is like, and the function of the tissue is dependant on how individual cells are arranged in the tissue.
• What are the three types of cell junctions/ connections? = Tight junctions, Anchoring junctions and Communication junctions.

Question 3

Tight Junctions

Answer 3

Tight Junctions

• Connect adjacent cells to prevent small molecules from leaking.
• Cell act as wall within an organ. Molecules sequestered [locked in?] within a region e.g: cells lining digestive tract
• Partition plasma membranes of lining cells together
• Nutrient transport proteins must stay in proper orientation to function.

Question 4

Anchoring Junctions

Answer 4

• Mechanically attach the cytoskeleton of a cell to the cytoskeleton of other cells or the ECM.
• These junctions are commonest in tissue subject to mechanical stress eg muscle and skin.
• Desmosomes connect the cytoskeletons of adjacent cells.
• Hemidesmosomes anchor epithelial cells to basement membrane.
• There are Proteins called cadherins (single pass glycoproteins) create this link between cells. Attachment proteins link the cytoplasmic end of cadherins to intermediate filaments in cytoskeleton. Proteins tethered in this way stronger than free floating protein in membrane.

Question 5

Cadherin and actin filaments

Answer 5

• Cadherin also connect actin filaments, but this is a weaker connection than intermediate filament connections.
• Occurs in different tissue or same tissue at different times
• During development, migration of cells (neurons) associated with changes in expression of cadherins. Expression may provide migrating cells with roadmap to destination.

Question 6

Integrin mediated links

Answer 6

• Anchoring Junctions called Adherens junctions.
• Intergrin mediated links connects actin filaments of one cell with those of neighbouring cell. The linking proteins called integrins
• It is a transmembrane protein with two subunits that extend to out of the plasma membrane, together they bind to protein of extra cellular matrix.

Question 7

Communicating Junctions

Answer 7

Communicating junctions are called Gap junctions.

• Cells communicate with adjacent cells via communicating junctions.
• Chemical signals pass directly form one cell to adjacent one.
• Communicating junctions permit small molecules and ions to pass from one to other, Communicating junctions are called Gap junctions.
• They are composed of connexons which are a complex of six transmembrane proteins arranged in a circle to create a channel.
• Gap junctions form when two connexons align and the gap not large enough to allow proteins through.
• The gaps are Dynamic meaning they can open or close to variety of factors like e.g Ca++ and H+ ions.
• When a cell is damaged calcium flows in and this closes gap junction to protect other cells from damage.

Question 8

The extracellular matrix

Answer 8

• Tissue is not only composed of cells
• Cells interact with extracellular materials that are critical for tissue structure and function
• Variety of forms of extracellular matrix: e.g
  i) Bone consists of rigid matrix that contain tiny number of interspersed cells.
  ii) Cartilage constructed of matrix materials – proteoglycans - more flexible
  iii) Connective tissue around glands, blood vessels and skin more gelatinous with numerous fibroblasts
  vi) Epithelial cells secret specialised ECM called Basal Lamina.

Question 9

• The ECM determines the shape and mechanical properties of organs and tissues.
• The ECM has diverse functions but consists of 3 classes of molecules:

Answer 9

1. Structural proteins such as collagens and elastins which give strength and flexibility
2. Protein polysaccharide complexes called proteoglycans which provide the matrix in which the structural proteins are embedded
3. Adhesive glycoproteins such as fibronectins and laminins which allow cells to attach to matrix.
   - Collagen structure

Question 10

What are cadherins?

Answer 10

Cadherins are a group of proteins that help cells stick together. They are the main components of certain types of junctions between cells. These connections help define how a cell will be integrated into a structure, like a layer of skin or an organ.