CELL JUNCTION
3 types Intercellular junctions in animals
1. Desmosomes - Hold cells of tissue together .
2. Tight Junctions - Prevent leakage and occur where adjacent cell
membranes are fused
3. Gap Junctions - Small channels allowing passage of macromolecules
between adjacent cells
Intercellular junctions in animals
1-Tight junctions (zonula occludens)
These junctions are the principal seal against the passage of extracellular
materials between the cells.
-They are well developed in epithelia ,which are highly impermeable
e.g.urinary bladder.
apparent in leaky epithelium e.g. kidney proximal tubules.
1
-Tight junction may be patchy or band-like e.g endothelia cells except
those of the brain.
Tight Junction
1- No space between adjacent membranes
2- Acts as seal – prevent flow of fluids between cells can’t pass from 1 side of
cell layer to the other between cells
3- Abundant in intestinal epithelial cells, ducts and cavities of organs – liver,
pancreas and bladder
Tight Junction
2
Not entire area of contact – forms a series of ridges
Major protein – claudin and occludin
Claudin is more important – large family – different ones
expressed in different cells types
Number of ridges corresponds to tightness of junction
Tight junctions
Tight junctions seal off the extracellular space
Cell Junctions
3
Desmosomes (macula adherens)
-Desmosomes are numerous in stratified epithelia. They can be seen by
the light microscope as intercellular bridges.
-Electron micrographs show that this is not true but there is a space
separating the two adjacent cells.
This space is filled with sialic acid-rich mucoprotein which serves as
strong adhesive anchoring
the two membranes appear as a dense intermediate line in the electron
micrographs.
4
Desmosomes are the main junctions for binding cells together and are
scattered along cell margins
The surface of each plasma membrane has an area called the circular
plaque
The plaques of adjacent cells are joined by linker proteins called
cadherins
Desmosomes
Tight junctions
5
Tight junctions are typically located in the apical region of most
epithelial cells
2-Gap junction (communicating junction or nexus)
-In this junction there is a narrow extra cellular gap between the
adjacent cells (2nm).
-There are channels interconnecting the cytoplasm of one cell to that of
its neighbour, allowing the passage of materials like ions. Ca++ ions are
essential for the regulation of the activity of the pores.
-These junctions are present in adjacent smooth and cardiac muscle cells
and also between neuronal cells of lower vertebrate
Gap junctions let small molecules move directly between neighboring
cells
At gap junctions the adjacent plasma membranes are very close and the
cells are connected by hollow cylinders of protein (connexons)
Ions, simple sugars, and other small molecules pass through these
cylinders from one cell to the next
Gap junctions
1- Point of contact between 2 adjacent cells
2- Can move ions and small molecules across – electrical and chemical
communication
3- Joined by tightly packed, hollow cylinders called connexons – 1 to
1000s of connexons
6
Each has 6 subunits of connexin – many different types found in
different cells
Invertebrates have innexins
4- Abundant in cells that need rapid communication between cells
Gap junctions
Gap junctions or nexus is a spot-like junction that can occur anywhere along
the lateral membranes of adjacent cells
