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Cell Connections & Junctions
by Dr. Vani Gupta
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.
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.)
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.
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.
 These cell adhesion molecules can be divided into 4
major families
 The cadherin superfamily
 The selectins
 The immunoglobulin superfamily and
 The integrins
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.
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.
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
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).
The integrins
 Fifteen different α and eight different β
subunits give rise to over twenty
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
Types of cell junction in animal tissue
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.
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.
Adhering Junctions
 Desmosome- Connects intermediate filament of one
cell with other cells.
 Claudin
 Hemidesmosome
 Desmoplakin is essential for normal desmosomal
adhesion.
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
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).
• 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.
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
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.
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.
Desmosomes
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 multiadhesive proteins such as laminin, resident within the
extracellular matrix, thereby forming one of many
potential adhesions between cell and matrix.
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.
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.
various cell junctions found in a vertebrate epithelial cell, classified
according to their primary functions
MCQs
Q1-which of the following is a cell adhesion
molecule
 a-integrin
 b-lysin
 c-myosin
 d-keratin
Q2-desmosomes differ from tight junction because
a-allow molecules to pass in the intercellular
space
b-are non-communicating
c-are present in plants
d-lack proteins
 Q3-the Cell Junctions allowing exchange of
cytoplasmic molecules between two cells are called:
a. Gap Junctions
b. Tight Junctions
c. Anchoring Junctions
d. Focal Junctions
Q.4- Desmosome has the following
characters except:
a- is a disk like attachment between cells
b- is located only between epithelial cells
c- is specialized for adhesion
d-is called macula adherns
Q.5- Gap junctions
a- permit the passage of large proteins from cell
to cell
b- form part of the classical junctional complex
c- exist only between epithelial cells
d- are areas of low resistance for nerve
stimulation
Q-6 Which of the following apply to intercellular junctions?
a) The three major adhesive junctions of animal cells are
adherens junctions, desmosomes and hemidesmosomes.
b) Desmosomes and hemidesmosomes connect epithelial
cells to their basement membrane and adjacent cells
respectively.
c) Gap junctions and plasmodesmata are homologous
structures.
d) The junctional complexes of gastrointestinal
enterocytes ensure that nutrients are only absorbed
through the spaces between the cells, which prevents them
absorbing potentially harmful substances.
ANS-a
Q-7 tight junction
a-are essential for metabolic coupling
b-dont occur in vertebrates
c-have the closest approach of two plasma
membranes of any junction
d-surround connexions
Q-8 hemi desmosome differs from spot desmosome
because hemi desmosome
a-connect cell to cell
b-connect extracellular matrix to extracellular matrix
c-connect cell to extracellular matrix
d-having tonofilament ,made up of intermediate
filament
Q-9.Desmosomes are made-up of integral protein
a. Integrein
b. Connexin
c. Selectin
d. Claudin.
Q-10-Hemidesmosomes are connections between
a. Cell to cell
b. Cell to internal organ
c. Cell to matrix
d. Cell to cell memberane