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1. CELL JUNCTIONS
Cell junction is the connection between neighbouring
cells or the contact between the cell and the
extracellular matrix.
Connection between two cells is called Intercellular
junction like tight junction, gap junction, adherence
junction and desmosome.
Contact between the cell and extracellular matrix are
focal adherence and hemidesmosome.
Cell junctions are classified into 3 types:
1.Occluding junction
2.Communicating junction
3.Anchoring junction
OCCLUDING JUNCTIONS
Cell junctions which prevent intercellular exchange of
substances are called occluding junctions. These
junctions prevent the movement of ions and molecules
from one cell to another cell. Tight junction belongs to
this category.

2. Tight Junction
It is the region where the cell membranes of the
adjacent cells fuse together firmly.
It is also called zona occludens.
Structure:
Tight junction is made up of two halves. One half is
from one cell and another half is from the other cell.
Both half fuse together very tightly.
Proteins of tight junctions are Integral proteins and
cytoplasmic plaque proteins.
Functions:
It holds neighbouring cells and tissues firmly.
It forms a selective barrier for small molecules and total
barrier for large molecules.
It prevents lateral movements of proteins and lipids.
Thus acts as a fence.
It maintains cell polarity by keeping proteins in apical
region of the cell.
Tight junction in brain capillaries form BLOOD
BRAIN BARRIER which prevents entrance of many
substances from capillary blood to brain tissues. Only
lipid soluble substances like drugs and steroid

3. hormones can pass through the blood brain brain
barrier.
COMMUNICATING JUNCTIONS
Cell junctions which permit the intercellular exchange
of substances are called communicating junctions.
These junctions permit the movement of ions and
molecules from one cell to another cell.
Gap junction and Chemical Synapse are the
communicating junctions.
Gap Junction
It is the intercellular junction that allows passage of
ions and smaller molecules between the cells.
It is also known as nexus.
It is present in heart.
Structure:
Membranes of two adjacent cells lie very close to each
other and intercellular space is reduced from the usual
size of 2.5 to 3 nm.

4. Cytoplasmof two cells is connected by channels so the
molecules move from one cell to another cell without
having contact with extracellular fluid.
Each channel consists of two halves. Each half belongs
to one of the adjacent cells.
Each half is surrounded by 6 subunits of proteins which
are called connexions.
Functions:
Diameter of the channel in the gap junction is about 1.5
to 3 nm so the channel permits the passage of glucose,
amino acids, ions and other substances, which have a
molecular weight less than 1000.
It helps in the exchange of chemical messengers
between the cells.
It helps in rapid propagation of action potential from
one cell to another cell.
The diameter of channels is regulated by the
intracellular calcium ions, pH of the cell, electrical
potential, hormones and neurotransmitters.
Chemical synapse:
It is a junction between a nerve fiber and a muscle fiber
or between two nerve fibers through which the signals
are transmitted by the release of chemical transmitters.

5. ANCHORING JUNCTIONS
They are the strength providing junctions.
They provide firm structural attachment between two
cells or between a cell and extracellular matrix.
They are present in the heart muscles and epidermis of
skin, which are subjected to severe mechanical stress.
The attachment is provided by either ACTIN
FILAMENTS or the INTERMEDIATE FILAMENTS.
They are further classified as below:
1.Actin filament attachment:
Adherens junction (cell to cell)
Focal adhesion (cell to Matrix)
2.Intermediate filament attachment:
Desmosome (cell to cell)
Hemidesmosome (cell to matrix)
Adherens Junction
It is the cell to cell junction.
It connects the actin filaments of one cell to another
cell.

6. Adherens junction is provides strong mechanical
attachments of the adjacent cells.
It is present in the intercalated disks between branches
of cardiac muscles. During contractionand relaxation of
heart, the cardiac muscle fibers are held together tightly
by means of this junction.
Focal Adhesion:
It is the cell to matrix junction which connects the actin
filaments of the cell to extracellular matrix.
Desmosome:
It is the cell to cell junction of intermediate filaments.
It is also known as macula adherens.
These junctions function like tight junctions.
Hemidesmosomes:
It is the cell to matrix junction.
It connects the intermediate filaments of the cell to the
extracellular matrix.

7. This type of junction is like half desmosome.
CELL ADHESION MOLCULES:
They are the protein molecules present on the cell
surface.
They are responsible for binding of cells to their
neighbour cells or to basal lamina.
They are responsible for structural organization of
tissues.
Types of cell adhesion molecules:
1.Cadherins: which form adherens junction and
desmosome.
2.Integrins: which form focal adhesion and
hemidesmosome.
3.IgG super family which form the cell adhesion
molecules in nervous system.
4.Selectins which act as receptors for carbohydrates
and are found in platelets and endothelial cells.