Cell junctions connect neighboring cells and classify into three main types - occluding, communicating, and anchoring junctions. Occluding junctions prevent molecules from passing between cells, like tight junctions. Communicating junctions allow transfer of substances between cells via channels, such as gap junctions. Anchoring junctions provide structural strength, exemplified by desmosomes attaching cells to each other or hemidesmosomes attaching cells to the extracellular matrix. Cell adhesion molecules like cadherins and selectins are transmembrane proteins that mediate cell-cell binding and participate in various cellular processes during development, wound healing, and immune responses.

1. Cell Connections & Junctions

2. Definition and Classification of cell junction
• Cell junction is the connection between the neighbouring cells or the
contact between the cell and extracellular matrix.
• It is also called membrane junction.
Cell junction are classified into three types
a-Occluding junction
b-Communicating junction
c-Anchoring junction.

3. Cell Adhesion Molecules (CAMs)
• Important cell surface proteins molecules
promoting cell–cell and cell–matrix interactions.
• Important for many normal biological processes -
embryonic cell migration, immune system
functions, wound healing.
• Involved in intracellular signaling pathways
(primarily for cell death/survival, secretion etc.)

4. Cell Adhesion Molecules (CAMs)
• Express 3 major domains:
–The extracellular domain allows one CAM to bind
to another on an adjacent cell.
–The transmembrane domain links the CAM to the
plasma membrane through hydrophobic forces.
–The cytoplasmic domain is directly connected to
the cytoskeleton by linker proteins.

6. Cell Adhesion Molecules (CAMs)
• Interactions between CAMs can be mediated by :
Binding of an adhesion
molecule on one cell to
the same adhesion
molecule on a second
cell
Cadherin - cadherin
An adhesion molecule
on one cell type binds
to a different type of
cell adhesion molecule
on a second cell
Selectins – mucins
The linker molecule in
most cases is Laminin,
a family of large cross
shaped molecules with
multiple receptor
domains.

7. • These cell adhesion molecules can be divided
into 4 major families
– The cadherin superfamily
– The selectins
– The immunoglobulin superfamily and
– The integrins

8. The Cadherin superfamily
• Cadherins are the most prevalent CAMs in vertebrates.
• 125 kD transmembrane glycoproteins - mediate
intercellular adhesion in epithelial and endothelial cells
by Ca2+ dependent homophilic adhesion.
• Primarily link epithelial and muscle cells to their
neighbors
– Form desmosomes and adherens junctions
• Play critical role during development (cell sorting).
• Do not interact with extracellular matrix.

9. The Cadherin superfamily
• Contain a short transmembrane domain
and a relatively long extracellular domain
containing four cadherin repeats (EC1-
EC4), each of which contains calcium
binding sequences
• Cadherins interact with specific
cytoplasmic proteins, e.g., catenins (α, β
and γ), as a means of being linked to the
actin cytoskeleton.
• The binding of cadherins to the catenins
is crucial for cadherin function.

10. The Selectins
• Structural features of selectins
include:
– NH2-terminal C-type Ca2+
dependent lectin like binding
domain, which determines the
ability of each selectin to bind to
specific carbohydrate lingands.
– an epidermal growth factor-like
region.
– a number of repeat sequences.
– a membrane-spanning region and
– a short cytoplasmic region

11. Immunoglobulin Superfamily
Molecules
• Consists of more than 25 molecules.
• Important ones being:
– Intracellular adhesion molecule 1(ICAM1; CD54)
– Intercellular adhesion molecule 2 (ICAM2),
– Vascular cell adhesion molecule1 (VCAM1; CD106),
– Platelet endothelial cell adhesion molecule 1 (PECAM 1;
CD31) and
– the mucosal addressin cell adhesion molecule 1
(MAdCAM1).

12. The integrins
• Fifteen different α and eight different β
subunits give rise to over twently different
heterodimeric combinations at cell surfaces.
• Bind epithelial and muscle cells to laminin in
the basal lamina
• Allow platelets to stick to exposed collagen
in a damaged blood vessel
• Allow fibroblasts and white blood cells to
adhere to fibronectin and collagen as they
move

13. Types of cell junction in animal tissue

14. Occluding Junction
• A cell-cell junction that seals cells together in an epithelium in a way that
prevents even small molecules from leaking from one side of the sheet to the
other.
• Tight Junction
Tight Junction- occluding junctions / zonulae occludens - zonula
occludens), are the closely associated areas of two cells whose
membranes join together forming a virtually impermeable barrier to fluid.
 A type of junctional complex present only in vertebrates.
 Consist of linear array of several integral proteins.
 Junctional proteins occludins and claudins & members of IG suprfamily
are transmembrane proteins.

15. Function of Tight Junction
• Strength and stability
• Selective permeable for ions.
• Fencing function
• Maintance of cell polarity
• Blood-brain barrier
• Cludin -16 in Thick Junctions of Ascending Loop of
henle.
• Cludin- 15 Permability of cations / anions.

16. Adhering Junctions
• Desmosome- Connects intermediate filament
of one cell with other cells.
• Claudin
• Hemidesmosome
• Desmoplakin is essential for normal
desmosomal adhesion.

17. Communicating Junction
• Cell junction which permit the intercellular exchange
of substance are called communicating junction, these
junction permit the movement of ions and molecules
from one cell to another cell.
a- Gap junction
b- Chemical synapse

18. Gap Junction
• Gap junctions are clusters of intercellular channels that
allow direct diffusion of ions and small molecules
between adjacent cells.
• At gap junctions, the intercellular space narrows from
25 nm to 3 nm.
• gap junctions were first discovered in myocardium and
nerve because of their properties of electrical
transmission between adjacent cells (Weidmann 1952;
Furshpan and Potter 1957).

19. • Low resistance intercellular junction that allows passage
of ions and smaller molecules between the cells.
• It present in heart, basal part of epithelial cell of intestinal
mucosa, etc
• Junctional unit-Connexons- 6 connexins
• Connexon of one cell have allignment with connexon of
other cells.

20. Gap Junction
• Electron microscopy of gap junctions joining adjacent hepatocytes in the mouse. The
gap junction (GJ) is seen as an area of close plasma membrane apposition

21. Function of gap junction-
 channel passage the substance have molecular
weight less than 1000.
 Exchange of chemical messenger between cells
 Rapid propagation of action potential from one cell
to another cell.

22. Desmosomes
• Also known as macula adherens is a cell structure specialized for cell-
to-cell adhesion.
• Are molecular complexes of cell adhesion proteins and linking proteins
that attach the cell surface adhesion proteins to intracellular keratin
cytoskeletal filaments.
• The cell adhesion proteins of the desmosome, desmoglein and
desmocollin, are members of the cadherin family.
• On the cytoplasmic side of the plasma membrane, there are two
dense structures called the Outer Dense Plaque (ODP) and the Inner
Dense Plaque (IDP).
– The Outer Dense Plaque is where the cytoplasmic domains of the cadherins
attach to desmoplakin via plakoglobin and plakophillin.
– The Inner Dense Plaque is where desmoplakin attaches to the intermediate
filaments of the cell.

23. Desmosomes

24. Hemidesmosomes
• Hemidesmosomes look like half-desmosomes that
attach cells to the underlying basal lamina.
• Rather than using desmogleins, hemidesmosomes use
desmopenetrin cell adhesion proteins,which are
members of Integrin family.
• The integrin molecule attach to one of many multi-
adhesive proteins such as laminin, resident within the
extracellular matrix, thereby forming one of many
potential adhesions between cell and matrix.

25. Chemical synapse
• Chemical synapse is the junction between a nerve fibre and a
muscle fiber or between two nerve fibre ,through which
signals transmitted by the release of chemical transmitter.

28. Anchoring junction.
• Anchoring junction are the junction ,which provides strength
to the cell by acting like mechanical attachment.
• These junction provide firm structural attachment between two
cells or between a cell and extracellular matrix
• Anchoring junction are responsible for structural integrity of
the tissue.

29. various cell junctions found in a vertebrate epithelial cell, classified
according to their primary functions

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