Cell Junctions & The Extracellular Matrix of Animal Cells

I. Introduction

A. Functions

• cell-matrix junctions
• cell junctions.
  

B. Tissues

• connective tissue
• epithelial tissue

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II. INTERLUDE

A. Mechanism of Cell Adhesion

1. How cells recognize each other

• "Classic" Dissociation-Reaggregation Experiment:
  
• Take vertebrate embryonic cells and subject them to trypsin and mechanical disruption to form a cell suspension
• Given enough time the cells will reaggregate so that ectoderm, mesoderm and endoderm areas reform.
  
• Why does this happen?
  
• cell adhesion molecules or CAM's
• cadherin binding

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III. Cell Junctions

Overview

overview2

1. Adhesive junctions - holds cells together acting as "buttons" or "zippers" to maintain cells in fixed positions in tissues; example: desmosome; hemidesmosome anchors cells to extracellular matrix.

2. Tight junctions - close the spaces between cells to diffusion forming a dam that prevents the flow of ions, etc. through extracellular space; forms a continuous belt around cells lining body cavities

3. Gap or communicating junctions - juncture between cells which provide open (although regulated) channels to allow flow of molecules (ex. ions) from cell to cell

• What structure previously discussed in plants seems analogous to the gap junction?
  

4. More detail on desmosome type of adhesive junction:

-discovery

-shape

-plaque

-intermediate filaments

5. Gap junctions

• discovery
• shape: connexon and connexin
• channel
  
• Regulation of Gap Junctions
  

-cylinders are not continuously open

-"gate"

-junctions respond to changes in cytoplasmic calcium and proton concentrations;

-When do you think cytoplasmic calcium concentrations might be this high? Is there any survival advantage to this response mechanism?

-calcium in a "resting" cell is generally far lower than 10^-5M.

IV. Extracellular Matrix

1. What is it?

2. Some locations:

3. Functions:

-support;

-regulation of cell division, adhesion, cell motility & migration during embryonic development.

4. What are the major components of the extracellular matrix?

-collagen (fibrous protein)

-proteoglycans (glycoprotein "glue")

What is the structure found in plants analogous to the extracellular matrix?

5. More detail on collagen:

• primary fibers of animal extracellular structures
• "semi-crystalline" in structure
• act to hold cells together and to provide tensile strength and elasticity to the extracellular matrix;
• found in all animal phyla; in animals, collagen is the most abundant protein (50% of total protein by weight)
• Collagen as an industrial product
  

(Bonus:

What is the most abundant protein in plants?

What is the most abundant protein in nature?)

6. Collagen structure

• "glycine-proline-hydroxyproline" subunit
  
• left-handed helix, "polyproline helix";
  
• "triple helix";
  
• How is collagen stabilized?
  
• Collagen also has a carbohydrate portion consisting of two sugar (galactose-glucose) subunits attached to the protein (10% of total collagen weight is carbohydrate).
• overall collagen structure
  

Variations:

-there are around 25 different kinds of chains known in vertebrates

-most common types are I, II & III (90% of collagen is type I)

depending on the tissue, the fibers of collagen are arranged differently:

• Bone
• Tendons
• loose connective tissue
  
• What does this arrangement of collagen tell you about the function of each tissue?
• Again, do you see a parallel with a plant structure/function relationship we discussed earlier?
  

7. Proteoglycans

• What are they?
• Structure
  

-linear core of polypeptide to which carbohydrate chains are attached;

-core protein: 11,000 to 220,000 MW

-many different carbohydrate chains can be covalently attached along core (mostly to serine residues);

-the primary carbohydrate type: glycosaminoglycan (GAGS)

-hyaluronic acid

• What are the functions of proteoglycans?
  
• How does the collagen-proteoglycan complex link to the cell surface?
  
• surface receptors
• linker molecules
  
• Two types of receptors:
  
• fibronectin
• laminin (restricted location in basal laminae)
  
• Detail about fibronectin:
  
• -glycoprotein 500,000 Da
• -dimer connected by disulfide linkages at ends to form a "V-shaped" structure
• -each monomer has binding sites for cell surface or for the extracellular matrix;
• -the cell surface binding site contains a short amino acid sequence "RGDS" (arginine, glycine, aspartic acid, serine) which is recognized by cell surface receptors binding fibronectin;
• there are other binding sites for GAG's and collagen

Summary