types and explanation of cell junctions

1. CELL BIOLOGY,GENETICS &EVALUTION-4401
BS-CHEMISTRY SEMESTER 1
PRESENTED BY:- ALI RAZA TARIQ
ROLL NO:-BZ486811
PRESENTED TO:-Ms Madiha Khalid

2. CELL JUNCTIONS
INTRO & FUNCTION
Cells are the small parts of human body.They are organized to form tissues.
They are linked together by cell junctions.
Cell junctions are a class of cellular structures consisting of multi protein
complexes that provide contact or adhesion between neighboring cells or
between a cell and the extracellular matrix in animals.
They are important because they maintain tissue integrity, act as barriers to
permeability and allow intercellular transport.
They have three general types of junctions.The types of junction have each
functions in human body.

3. HISTORY & GENERALTYPES
1963 A mess of nomenclature is sorted out by Marilyn Farquhar and George
Palade, who use superb microscopy to define three of the four major types of cell–
cell junctions in the polarized epithelial cells of vertebrates.
1967 Jean-Paul Revel and Morris Karnovsky unite the fields of adhesion and
intercellular current transfer around a distinct, structural correlate called the gap
junction.
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 cytoskletons) 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.

4. TIGHT JUNCTIONS
:Tight junctions are areas where the membranes of two adjacent cells join
together to form a barrier.
Tight junctions bind cells together, prevent molecules from passing in
between the cells, and also help to maintain the polarity of cells.
Located mainly in blood brain barrier, blood vessels, intestines, nephrons
and skin.
Tight junctions keep the correct proteins on the correct sides of the cell in
order for these functions to occur.

5. ANCHORING JUNCTIONS
Anchoring junctions are cell junctions that are firmed to one another and
attached to components of the extracellular matrix.
They are important in keeping the cells together and structural cohesion of
tissues.
They are commonly found in tissues that are prone to constant mechanical
stress, e.g. skin and heart.

6. CLASSIFICATION OF ANCHORCHING
JUNCTIONS
Basicaly divides in cell to cell junctions & cell-matrix junctions;
Adhesive junctions: Adhesive junctions link cells together into tissues,
therby enabling the cells to function as a unit. All junctions in this category
anchor the cytoskeleton to the cell surface. It has two types;
1. Adherens junctions: Adherens junctions connect bundles of actin filaments
from cell to cell.Play role in epithelial cells.
2. Desmosomes: are button-like points of strong adhesion between adjacent
cells in a tissue.They enable cells to function as a unit and to resist stress.
Cell-matrix junction includes the most important as;
1. Hemidesmosomes: are multi protein complexes that facilitate the stable
adhesion of basal epithelial cells to the underlying basement membrane.

7. COMMUNICATION JUNCTIONS
Communicating junctions, also known as gap junctions allow for direct
chemical communication between adjacent cellular cytoplasm through
diffusion without contact with the extracellular fluid.
This is possible because gap-junction protein known as Connexin (are
structurally related transmembrane proteins that assemble to form
vertebrate gap junctions).
Retinal and skin cells are also dependent on gap junctions in cell
differentiation and proliferation.
Gap junctions are especially important during embryonic development, a
time when neighboring cells must communicate with each other in order for
them to develop in the right place at the right time. If gap junctions are
blocked, embryos cannot develop normally.

8. BYSTANDER EFFECT
Role of gap junctions in disease: When a cell starts to die from disease or
injury, it sends out signals through its gap junctions.
These signals can cause nearby cells to die even if they are not diseased or
injured.This is called the “bystander effect.
Gap junctions are being researched as a method of therapeutic drug
delivery.

9. IMPORTANTTYPES OF GAP JUNCTIONS
CHEMICAL JUNCTIONS
Occurs in humans and animals
 Chemical neural
connections permit neurons to
create circuits inside the
central apprehensive framework.
 Chemical neural
connections are organic intersections through
which neurons' signals can be sent to each
other and to non-neuronal cells such as those
in muscles or organs.
PLASMODESMATA
occurs in plants only
 Plasmodesmata are narrow channels that act
as intercellular cytoplasmic bridges to
facilitate communication and transport of
materials between plant cells.
 Plasmodesmata are located in narrow areas
of cell walls.
 They are specialized channels that allow for
intercellular movement of water.

10. CONCLUSION
Cell junctions are present in many cell types, Some contains one
junction type such as gap junctions and other contain many unction
type such as tight junctions .Plays an important role in normal
biological and pathophysiological process such as cancer and
epithelial layer function. Usually studied by traditional cell biological
techniques such as electron microscopy.

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