Dr Mahmood S Choudhery, PhD
Assistant Professor
Advance Center of Research in Biomedical Sciences
Incharge Tissue Engineering Laboratory, Mayo Hospital
Lahore, Pakistan
King Edward Medical University, Lahore, Pakistan
Learning objective
How cells attach to neighboring cells
 How cells communicate with other
cells and extracellular matrix (ECM)
 Types of different junctions
 Role of ECM in cell signaling

They need to perform
specific functions
more effectively
Permits the organism
as a whole to move,
metabolize,
reproduce, and carry
out other essential
activities.
Integration of Cells into Tissues
Involves 2 processes
1. Cell Recognition
2. Cell Adhesion
Cell Adhesion……………
Cells can adhere to each other by two ways
1. Cell-cell
adhesion
Cell Adhesion Molecules (CAMs)
2.
Cell-matrix adhesion
Adhesion Receptors
Cell Adhesion Molecules (CAMs)………..
1. Are proteins (glycoproteins)
2. Involved in binding with other cells or with ECM
3. Typically transmembrane proteins
4. Cluster to form specialized cell junctions
Cell Adhesion Molecules………..
4 Types
 Cadherins
 Immunoglobulins
 Integrins
 Selectins
TYPES OF INTERACTIONS
Between adhesion molecules
 Homophilic interactions
 Heterophilic interactions
Between cells
 Homotypic interactions
 Heterotypic interactions
What are Cis and Trans
interactions?
SPECIALIZED CELL JUNCTIONS
Between cells
 Tight junctions
 Adhesive/ Anchoring junctions
 Gap junctions
 Between
cells and matrix
 Hemidesmosomes
 Focal Junctions
2. Cell junctions - types based on function
 Adhering
1.
2.
3.
4.
junctions:
Tight junctions
Adhesive/ Anchoring junctions
Hemidesmosomes
Focal Junctions
 Communicating
1.
junctions:
Gap junctions
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1. Tight Junctions
 Only found in epithelial cells
 Composed of 3 transmembrane protein families
 Prevent substances from moving through intercellular
spaces
2. Adhesive/ Anchoring junctions
2 types
a)
b)
Desmosomes
Adherens Junctions
Hold cells tightly together
 Connect a cell’s cytoskeleton to another cell

1. Form links between cells
2. Connect intermediate filaments of adjacent cells
3. Composed of cadherins
b) Adherens Junctions
Under the cell
membrane,
contractile fibers of
microfilaments
connect to cell
membrane proteins
called cadherins


Homophilic pairing of E- cadherins
Adapter proteins (plakoglobin and α and βcatenins)
link cadherins to the belt of actin filaments.
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Gap junctions allow cells to exchange electrical
and/or chemical signals
Composed of proteins that form channels that allow
small molecules to pass.
Subunits of these channels are connexins that are
assembled together to make connexons. The
connexons from 2 cells join together to make a gap
junction.
This channel is big enough to allow small molecules such as inorganic ions, and
other small water soluble molecules (smaller than 1000kDa) to pass between the
cells. However the channel is too small for proteins, nucleic acids or sugars to pass
through.
Animals
Liver
Plants
Plasma membrane
(edge view)
Plasma membrane
(edge view)
Cell wall
(edge view)
Root
Liver cells
Middle lamella
(edge view)
Root cells
Gap Junctions:
pairs of channels
connect insides of
adjacent cells
desmosome
Gap Junctions
Plasmodesmata
connect insides
of adjacent cells
Plasmodesmata
 Connections between cell and ECM
 Composed of Integrins
EXTRACELLULAR MATRIX
What is ExtraCellular Matric (ECM)?
•Secreted by surrounding cells
Two types of Extracellular Matrix
1. Basal lamina
2. Interstitial Matrix
ECM COMPONENTS
1. Proteoglycans
2. Structural Proteins (Collagens and Elastins)
3. Adhesive Glycoproteins (Fibronectins and
Laminins)
ECM components…………….
1. Proteoglycans: (Protein-polysaccharide
complexes)
Composed of a core protein and GAGs
Have net negative charge
Trap and store growth factors
ECM components…………….
2. Structural Glycoproteins (Collagens and
Elastins)
Collagens are the main proteins of ECM
Elastin give elasticity to tissues
3. Adhesive Glycoproteins (Fibronectins and
Laminins)
Fibronectin connect cells with collagen fibers
in the ECM and integrin receptors on cell
membrane
Laminin found in the basal lamina
Role of Extracellular Matrix in Cell
Signaling
Provides space where different ligands
and growth factors are released
 Act as a reservoir for various growth
factors
 Different proteins of ECM act as ligand for
cell receptors

How Extracellular Matrix Components
Mediate Cell Signaling?
Integrins are transmembrane
receptors for extracellular
matrix (ECM) proteins such as
collagens, fibronectin, laminins
and vitronectin.
The Extracellular Matrix Participates in Signaling
lntegrin adhesion receptor mediated signaling pathways that control diverse cell functions.
Recommended Books
• Molecular Cell Biology By Lodish 7th
• Molecular Biology of The Cell by Bruce Alberts