Cell junctions are specialized contact sites that hold cells together and attach cells to the extracellular matrix. They are classified into three main groups: tight junctions, gap junctions, and adherens junctions. Tight junctions form continuous seals around cells to control permeability and prevent diffusion between cells. Gap junctions allow small molecules and ions to pass directly between cells to facilitate cell-cell communication. Adherens junctions, such as desmosomes and hemidesmosomes, anchor cells to other cells or the extracellular matrix. Cell adhesion molecules like cadherins, integrins, and immunoglobulin superfamily proteins mediate cell-cell and cell-matrix adhesion through homophilic or heterophilic binding interactions.

2. INTRODUCTION
 Of all the social interactions between cells in
multicellular organism, the most
fundamental are those that hold the cells
together.
 Many cells in tissues are linked to one
another & to Extracellular Matrix at
specialized contact sites called “CELL
JUNCTIONS.”

3. Classification of Cell Junctions
I. Cell-cell Junctions
Tight junctions, Gap junctions,
Desmosomes
Adhesion belt / zonula adherens
II. Cell-matrix junctions
Focal contacts / adhesion plaques
Hemidesmosomes

4. Classification as in Review of Medical
Physiology by Ganong
I. Junctions that fasten cells to one another
& to surrounding tissues
Tight junctions, Adhesion belt,
Desmosomes, Hemidesmosomes
Focal adhesions
II. Junctions that permit transfer
of ions & other molecules from
one cell to another
Gap junctions

5. Classification as in Alberts
Molecular Biology of Cell
Cell Junctions can be divided into 3 functional groups
I. Occluding Junctions :
Tight Junctions
II. Anchoring Junctions :
Desmosomes, Hemidesmosomes, Focal adhesions
& Zonula adherens
III. Communicating Junctions :
Gap Junctions

6. Tight Junctions / Zonula occludens
• Surround apical margins of
cells in epithelia
• Made up of ridges
• Degree of leakiness varies
• Composed of branching
network of sealing strands
• Claudins & Occludins
• ZO proteins

7. Adhesion Belt / Zonula Adherens
Connect actin filaments
of two interacting cells
 formed by cadherins
 network contracts with the help
of myosin motor proteins

8. Desmosomes & Hemidesmosomes

9. Gap Junctions
CONNEXONS – hexagonal array of transmembrane protein units
Regulation of Gap junction communication
• intra cellular calcium
• PH
• voltage
• extracellular
signals

10. Functions of Gap Junctions
1. In tissues containing electrically excitable
cells, coupling via gap junctions is very
usefull
2. Role in non-excitable tissues
3. Normal development of ovarian follicles
4. Role in Embryogenesis

12.  Attach cells to basal lamina & to each other
Are Transmembrane receptors with 3 domains :
intracellular, transmembrane & extracellular
Mediate both Homophilic & Heterophilic binding

13. CAMs can be divided into 4 major families
1. Cadherin superfamiliy
2. Selectins
3. Integrins
4. Immunoglobulin superfamily
Other way of classifying CAMs is,
I. Calcium-dependent
Cadherins
Selectins
Integrins
II. Calcium-independent
Ig superfamily

14. CADHERINS
 Mediate homophilic binding
 Structure contains a short transmembrane domain & a
relatively long exrecellular domain wit 4 cadherin
repeats (EC1 to EC4) each with a calcium binding
sequence.
 Interact with Catenins
Types :
N-cadherin
P-cadherin
E-cadherin

15. SELECTINS
 Calcium dependent carbohydrate binding proteins
 Mediate initial attachment of leukocytes to the
endothelium on the blood vessel wall during rolling
step of Phagocytosis
 Structure includes a NH2-terminal C-type Calcium
binding lectin domain, a membrane spanning domain
& a short cytoplasmic domain
 Mediate heterophilic binding
Types:
L-selectin
P-selectin
E-selectin

16. INTEGRINS Are principally involved in cell-matrix adhesion
 Also act as regulatory receptors that can initiate
intracellular signal pathways
 Composed of hetero dimers consisting of 2 non-
covalently associated subunits α and β
 Both subunits are membrane glycoproteins with a
large extracellular domain, a single transmembrane
domain & a short cytoplasmic domain
Types :
β 1 integrins – VLA proteins
β 2 integrins – Leu CAMs
β 3 integrins – cytoadhesions
β 4 integrins

17. IMMUNOGLOBULIN SUPERFAMILY
 Mediate many different functions including acting as
receptors for growth factors & mediating cell-cell &
cell-matrix interactions
 Structure is characterised by repeated domains similar
to those found in immunoglobulins.
 Mediate both heterophilic & homophilic binding
Types :
ICAMs – Intracellular CAMs
VCAMs – Vascular CAMs
PECAMs – Platelet Endothelial CAMs
NCAMs – Neural CAMs

18. ICAMs
• ICAM-1 (CD54), ICAM-2 (CD102) are counter receptors for
leukocyte β2 integrins
• ICAM-1 is expressed on leukocytes, fibroblasts, epithelial
cells & endothelial cells
• ICAM-2 has similar distribution but is not regulated by
cytokines as ICAM-1
VCAMs
• VCAM-1 (CD106) is expressed on surface of activated
endothelium, dendritic cells, tissue macrophages & bone
marrow fibroblasts
• It interacts with leukocyte integrin α4 β1 on eosinophils,
monocytes & with α4 β7 on activated peripheral T-cells

19. PECAMs
• PECAM-1 (CD31 or endoCAM) is found on endothelial
cells, on platelets, some monocytes & neutrophils
• Is involved in homophilic adhesion
NCAMs
• Is expressed on most of the nerve cells
• Play an important role in fine tuning of adhesive
interactions during development & regeneration
“Although cadherins & Ig family members are frequently
expressed on the same cells, the adhesions mediated by
cadherins are much stronger & are responsible for holding
cells together, segregating cell collections into discrete
tissues & maintaining tissue integrity”

20.  PEMPHIGUS
Role of Gap Junctions in Embryogenesis
E-cadherin & Prostate cancer
ICAM-1 & Melanoma
VCAM-1 & Tumour Metastases

21. REFERENCES
 Review of Medical Physiology by William F.Ganong,
21st ed; 2003 by The McGraw-Hill Companies, Inc.
 Alberts Molecular Biology of Cell 5th edition
 Cell Junctions- Molecular Biology of Cell –NCBI
Bookshelf
 Cell Junctions- Biology Encyclopedia,
www.biologyreference.com/celljunctions.html#b
 en.wikipedia.org/wiki/Cell _adhesion_molecule
 Cellular Adhesion and Adhesion Molecules, Review
Article by Zerrin Seller ; Turk Journal of Biology
25(2001)1-15