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Cell Junctions

Oral Histology II
by

Fady Raafat

on 16 October 2012

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Transcript of Cell Junctions

photo (cc) Malte Sörensen @ flickr Cell Junctions Presented By :

- Fady Raafat
- Fady Adel
- Omar Hosny
- Mohamed Usama
- Mahmoud Ahmed
Tight junctions are multiprotein complexes that mediate cell-cell adhesion and regulate transportation through the extra-cellular matrix. And it is a unique feature of epithelial cells as tight junctions are found in no other cell type.


Because tight junctions encircle the cell and attach it tightly to its neighbors, these junctions act as a barrier preventing molecules from diffusing across an epithelial sheet between adjacent cells.

Tight junctions are a major regulator of permeability, expressing different levels of "tightness" based on location and chemical stimuli. The 'looser' the connection, the greater the variety of molecules that can get across the epithelial sheet. Tran membrane proteins of the tight junctions are responsive to glycoprotein immunoglobin G (IgG) from cells and to surface antigens on bacteria. Binding of these molecules act as chemical stimuli regulating the tightness of the connections. Functions of Tight Junctions :
1) Barrier and fence function:

- Mediates transport of molecules between cells, deemed the barrier function.
- Helps to distinguish the apical and basolateral domains of the cell
by acting as intracellular barriers to macromolecules and proteins between these two regions.

2) Regulation of cell polarity:

- Increases permeability to calcium and chlorine ions.
- regulates epithelial proliferation. ANY Questions ?! Fady Raafat
Fady Adel
Omar Hosny
Mohamed Usama
Mahmoud Ahmed We Appreciate Your Attention - Tight Junctions are located at the uppermost portion of the lateral plasma membrane.
- They are found in tissues that are involved in polarized secretions, absorption functions, and maintaining barriers between blood and interstitial fluids,
like: intestinal, renal, mammary, and airway epithelium.

The composition of tight junctions:

Tight junctions are composed of three protein families that are:

1) Occludin proteins consist of 4 domains- 2 intracellular and 2 extracellular.

2) Claudins proteins that compose the major structural and functional elements of tight junctions. They also consist of 4 domains- 2 intracellular and 2 extracellular

3) JAMs have only a single Transmembrane domain, unlike occludin and claudins. Tight Junctions : Locations of Tight Junctions : Cell junctions can be classified into three functional groups :

1. Occluding junctions seal cells together in an epithelium in a way that prevents even small molecules from leaking from one side of the sheet to the other.
2. Anchoring junctions mechanically attach cells (and their cytoskeletons) to their neighbors or to the extracellular matrix.
3. Communicating junctions mediate the passage of chemical or electrical signals from one interacting cell to its partner.

OCCLUDING JUNCTIONS

1. Tight junctions (vertebrates only)
2. Septate junctions (invertebrates mainly) ANCHORING JUNCTIONS

Actin filament attachment sites
1. cell-cell junctions (adherens junctions)
2. cell-matrix junctions (focal adhesions)
Intermediate filament attachment sites
1. cell-cell junctions (desmosomes)
2. cell-matrix junctions (hemidesmosomes) COMMUNICATING JUNCTIONS

1. Gap junctions
2. Chemical synapses
3. Plasmodesmata (plants only) Functional Classification of Cell Junctions Gap junctions are probably the most common type of join between two cells, and are found in almost all animal tissues. Each junction allows small, water soluble molecules to move directly between the cytoplasms of the two cells in contact, which means that both cells share metabolites and even electrical properties.

These types of junctions are made from proteins that completely cross the plasma membrane of one cell, and then make contact with an identical protein that crosses the plasma membrane of the neighbor cell. A small group of these proteins come together to form a channel or connexon through the membrane. - Tight junctions in the oral cavity :
Tight junctions are found in the epithelium of salivary glands and because primary acinar secretion and its modification in the ducts vary depending on the gland type, it is clear that TJ structure and function must be different between serous, mucous, and mixed acini, as well as between intercalated, striated, and excretory ducts.TJs in salivary epithelia provide a barrier between the extracellular compartments and the lumen that is critical to normal acini functions, including the maintenance of cell polarity and normal transepithelial ion gradients. - Epithelial cells differentiate during the 8-cell embryonic.
- Begins with a signal cascade to form occludin proteins and claudins proteins.
- These transmembrane proteins protrude from the cell from one cell wall to the adjacent one
and they remain attached to thedead cells. Formation of cell junctions : Gap junctions The Anchoring junctions Definition:
Junctions that mechanically attach cells and their cytoskeletons to their neighbours and to the extracellular matrix - Cells within tissues and organs must be anchored to one another and attached to components of the extracellular matrix. Types of Anchoring Junctions
Three types of anchoring junctions are observed, and differ from one another in the cytoskeletal protein anchor as well as the transmembrane linker protein that extends through the membrane Desmosomes can be visualized as rivets through the plasma membrane of adjacent cells Desmosomes: - Desmosomes is found in the oral cavity stratified squamous epithelium Communicating Junctions :

Hemdismosomes is found in the oral cavity . Hemidesmosomes : A structure representing half of a desmosome, found on the basal surface of some epithelial cells, forming the site of attachment between the basal surface of the cell and the basement membrane. These types of junctions are made from proteins that completely cross the plasma membrane of one cell, and then make contact with an identical protein that crosses the plasma membrane of the neighbor cell. A small group of these proteins come together to form a channel or connexon through the membrane. Water soluble materials can move through the membrane using this channel, and then pass directly into a similar channel, or connexon, in the opposite membrane of the adjacent cell.

Since it is the protein connexons that are forming the junction, the membranes of the two cells remain separated by a slight gap of about 3 nm (hence the name "gap" junction!).

The united connexons cluster together in the membranes of the cells in numbers that average about 2-300 channels per cluster.

The proteins in these connexon channels and gap junction clusters are all the same and are probably located in the plasma membrane as individual subunits. When needed they come together (by moving laterally in the plasma membrane), form a channel, and connect one cell to the next. An adherens junction is defined as a cell junction whose cytoplasmic face is linked to the actin cytoskeleton
The adherens junctions appears as:

1)Bands encircling the cell (zonula Adherens)
2)Spots of attachement to the extracellular matrix (adhesion plaques) 3) Adherens Junctions : - Adherens juctions is also found in the oral cavity.
- Adherens junctions are thought to participate infolding and bending of epithelial cell sheets. Outline : 1) Introduction : Cell Jucations.
- Classification
- Types
2) Tight Junctions.
- Functions
- Locations
3) Communication Junctions
4) Anchoring Junctions OCCLUDING JUNCTIONS 1. Tight Junctions (vertebrates only)
2. Septate junctions (invertebrates mainly)
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