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Home , Cell junction

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 . Cell junctions are particularly

abundant in epithelia.

Cell junctions form barriers that inhibit

the movement of water and solutes.

Tight junction

Adherens junctions

Desmosomes

Gap junction

Hemidesmosome

Basal Lamina Cell-cell and cell-matrix junctions I-=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 : and

Endothelial cells in most regions of the 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 family.

2.FOCAL ADHESIONS(-linked cell- matrix adhesion) and bind cells to the and are formed by adhesion proteins of the 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 .

These junctions form adhesion belts

between epithelial cells. 1. 2. Cell membrane

α- Cadherin p120 β-catenin

Vinculin

α-Actinin

Actin filament

Outside of cell Adherens junctions

Focal adhesions bind cells on the extracellular matrix through 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:

filaments in most epithelial cells, filaments in heart and muscle cells Desmosomes are found in many tissues especially abundant in , heart, muscle, the neck of the uterus.

Desmosomes prevent mechanical stress

1. Cell membrane 2. Cell Membrane Intermediate filaments

Plakoglobulin Desmocollin

Plakophilin

Outside of cell Two types of (desmoglein and desmocolin) bind via anchor protein to intermediate filaments. Cytoplasmic plaque (desmoplakin, 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 ( only)

1- Gap junctions

Most cells are in communication with their neighbours via .

Gap junctions serve as direct connections between adjacent cells.

Each gap junction can contain a cluster of many thousands of .

Connexins are transmembrane proteins, six of which assemble to form a channel, a .

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 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 Charcot- Marie-Tooth disease, which is marked by progressive degeneration of peripheral nerves.

2-Plasmodesmata Adjacent cells communicate with each other through cytoplasmic connections called plasmodesmata (singular ).

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 b- Active chemical Mitochondrion synapse

Presynaptic terminal

:: Neurorotransmitter of the cell :: :: Synaptic :: :: cleft :: : :: : Transmitter- gated ion Postsynaptic cell channel

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.