Cell_Junctions epithelium 2024- febr.ppt

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.)

 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.

 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 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

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.

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.

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.

Cell_Junctions epithelium 2024- febr.ppt

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

 Q1-which of the following is a cell adhesion
molecule
 a-integrin
 b-lysin
 c-myosin
 d-keratin

32
 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

33
 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

34
 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
 e-is called macula adherns

35
 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

36
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

37
 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

38
 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

39
 Desmosomes are made-up of integral protein
a. Integrein
b. Connexin
C. Selectin
d. Claudin.

40
 Hemidesmosomes are connections between
a. Cell to cell
b. Cell to internal organ
c. Cell to matrix
d. Cell to cell memberane

Cell_Junctions epithelium 2024- febr.ppt

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