Download CELL JUNCTIONS

CELL JUNCTIONS
I-Occluding junctions
II-Anchoring junctions
III-Channel-forming junctions
IV-Signal-relaying junctions
Plasma membrane specializations,
called cell junctions, mediate between
neighbouring cells and between cells
and the basal lamina.
Cell junctions are particularly
abundant in epithelia.
Cell junctions form barriers that inhibit
the movement of water and solutes.
Microvillus
Tight junction
Adherens junctions
Desmosomes
Gap junction
Hemidesmosome
Cell-cell and cell-matrix junctions
Basal
Lamina
I-Tight junction=Zonula occludens
=Occluding junction
Tight junctions prevent diffusion of plasma
membrane proteins and glycolipids.
Tight junctions form a barrier that seals off
body cavities from the blood.
Freeze-fracture images revealed that
tight junction consist of integral
membrane proteins : Occludin and
Claudins
Endothelial cells in most regions of the brain
and spinal cord are joined by tight junctions.
The blood-brain barrier separates the blood
from the interstitial fluid.
II. ANCHORING JUNCTIONS
1.ADHERENS JUNCTIONS and
DESMOSOMES hold cells and are formed by
adhesion proteins of the cadherin family.
2.FOCAL ADHESIONS(Actin-linked cell-
matrix adhesion) and HEMIDESMOSOMES
bind cells to the extracellular matrix and are
formed by adhesion proteins of the integrin
family.
Actin filament attachment sites
1.Cell-cell junction (ADHERENS JUNCTIONS)
2.Cell-matrix junctions (FOCAL ADHESIONS)
Intermediate filament attachment sites
1.Cell-cell junctions (DESMOSOMES)
2.Cell-matrix junctions (HEMIDESMOSOMES)
Adherens junction maintains the
physical integrity of the epithelium.
These junctions form adhesion belts
between epithelial cells.
1. Cell membrane
α-catenin
2. Cell membrane
Cadherin
p120
β-catenin
Vinculin
α-Actinin
Actin filament
Outside of cell
Adherens junctions
Focal adhesions bind cells on the
extracellular matrix through integrins
that link to actin filaments.
Actin-linked cell-matrix adhesion
forms by integrins.
Focal contacts faciliate certain types of
cellular movement.
Desmosomes are button like points
between adjacent cells
The particular type of intermediate
filaments attached to the desmosomes:
keratin filaments in most epithelial cells,
desmin filaments in heart and muscle
cells
Desmosomes are found in many tissues
especially abundant
in skin,
heart,
muscle,
the neck of the uterus.
Desmosomes prevent mechanical stress
Desmosome
1. Cell membrane
2. Cell Membrane
Desmoglein
Desmoplakin
Intermediate
filaments
Plakoglobulin
Desmocollin
Plakophilin
Outside of cell
Two types of cadherins(desmoglein and desmocolin) bind via
anchor protein to intermediate filaments. Cytoplasmic plaque
(desmoplakin,plakoglobin and plakophilin) link the cadherins.
The importance of desmosome is
demonstrated by some forms of the fatal
skin autoimmune disease PEMPHİGUS.
Desmoglein 1------Pemphigus foliaceus
Desmoglein 3------Pemphigus vulgaris
Hemidesmosomes
Hemidesmosomes connect an epithelial
cell to the basal lamina.
They act as rivets to distribute tensile.
Hemidesmosomes have a dense plaque
on the cytoplasmic surface that anchors
of intermediate filaments.
Integrin (α6β4) and type XVII collagen
(also called BPAG2) attach to the basal
lamina.
A disease of hemidesmosomal
molecules: Bullous pemphigoid
autoantibodies attack type XVII collagen.
III-Channel-forming junctions
1-Gap junctions
2-Plasmodesmata (plants only)
1- Gap junctions
Most cells are in communication with
their neighbours via gap junction.
Gap junctions serve as direct
connections between adjacent cells.
Each gap junction can contain a cluster
of many thousands of connexons.
Connexins are transmembrane proteins,
six of which assemble to form a channel,
a connexon.
Gap junction channels allow to pass
ions, second messengers and metabolites.
The sharing of metabolites and ions
provides a mechanism for coordinating the
activities of cells.
The permeability of gap junctions is
regulated by the cytosolic pH or the
cytosolic concentration of free Ca2+
Gap junction communication can be
regulated by extracellular signals.
Gap junctions are usually present
low density in most adult epithelia,
but are found in large numbers during
embryogenesis.
Electrical synapses depend on gap
junction channels.
In the neocortex and thalamus and
some other parts of the brain
electrical synapses are common.
Mutations in connexin genes cause disease
Recessive mutations in the connexin-26 gene
are the most common causes of inherited
human deafness.
Mutation in the connexin-32 cause CharcotMarie-Tooth disease, which is marked by
progressive degeneration of peripheral nerves.
2-Plasmodesmata
Adjacent plant cells communicate with
each other through cytoplasmic
connections called plasmodesmata
(singular plasmodesma).
A plasmodesma contains a fine tubular
structure, the desmotubule, derived from
smooth endoplasmic reticulum.
IV-Signal-relaying junctions
Chemical synapses
(in the nervous system)
A chemical synapse
The arrival of an impulse at the terminus of a
neuron triggers synaptic vesicles to fuse
with membrane, releasing neurotransmitter.
The released neurotransmitter in the synaptic
cleft binds to a receptor on the postsynaptic
membrane to pass information.
a- Resting
Mitochondrion
Presynaptic terminal
of the cell
Transmittergated ion
channel
::
::
Neurorotransmitter
Synaptic
cleft
b- Active
chemical
synapse
::
::
::
::
:
:
Postsynaptic cell
Chemical synapses
::
KAYNAKLAR
1-Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. :
Molecular Biology of The Cell. Fifth Edition Garland Science
Taylor & Francis Group New York 2008.
2-Pollard T.D. and Earnshaw W.C. : Cell Biology. Second Edition.
Saunders Elsevier USA 2008.
3-Lodish H, Berk A, Kaiser C A, Krieger M, Scott M P, Bretscher
A, Ploegh H, Matsudaira P: Molecular Cell Biology. Sixth
Edition. W.H. Freeman and Company New York 2008.
4-Weaver R.F. Molecular Biology. Third EditionMcGraw-Hill
Higher Education, New York 2005.
5-Cooper G.M. and Hausman R.E. : The Cell: A molecular
Approach 5th Edition. ASM Press Sinauer Associates Inc.
USA 2009.
6-Epstein R.J. : Human Molecular Biology. Cambridge University
Press UK 2003.
7-Kierszenbaum A. L. and Tres L.L.: Histology and Cell Biology:
An Introduction to Pathology. Third Edition. Mosby Elsevier
USA 2012.