Download cell junction

Cell‐ cell
interaction
Junctions Between Cells
• In many animal tissues (e.g., connective tissue), each cell is separated from
the next by an extracellular coating or matrix.
• However, in some tissues (e.g., epithelia), the plasma membranes of adjacent
cells are pressed together.
•
Four kinds of junctions occur in vertebrates:
1.
2.
3.
4.
Tight junctions (Occluding Junctions)
Adherens junctions
Gap junctions (Communicating Junction)
Desmosomes (Anchoring Junctions)
CELL JUNCTION
• A cell junction (or intercellular bridge) is a type of structure that
exists within the tissue of some multicellular organisms
• (for example true for animals, but not plants, which possess
plasmodesmata instead).
•
Cell junctions consist of protein complexes and provide contact
between neighboring cells or between a cell and the extracellular
matrix.
• They also build up the paracellular barrier of epithelia and control
the paracellular transport.
• Cell junctions are especially abundant in epithelial tissues.
•
1.
2.
3.
4.
In vertebrates, there are three major types of cell junctions:
Adherens junctions and
Desmosomes (Anchoring Junctions)
Gap junctions (Communicating Junction)
Tight junctions (Occluding Junctions)
• Invertebrates have several other types of specific junctions,
Many plant tissues, it turns out that the plasma membrane of each
cell is continuous with that of the adjacent cells.
• The membranes contact each other through openings in the cell
wall called Plasmodesmata
DESMOSOMES= patches that hold two cells tightly together.
Tight Junctions
• Epithelia are sheets of cells that provide the interface between
masses of cells and a cavity or space (a lumen).
• The portion of the cell exposed to the lumen is called its apical
surface.
• The rest of the cell (i.e., its sides and base) make up the
basolateral surface.
• Tight junctions seal adjacent epithelial cells in a narrow band
just beneath their apical surface.
Adherens Junctions
adherens
junctions
Tight junctions seal adjacent epithelial cells in a narrow band
• Tight junctions perform two vital functions:
• They prevent the passage of molecules and ions through the space
between cells.
• So materials must actually enter the cells (by diffusion or active
transport) in order to pass through the tissue.
• This pathway provides control over what substances are allowed
through.
• They block the movement of integral membrane proteins (red and
green ovals) between the apical and basolateral surfaces of the cell.
• Thus the special functions of each surface, for example
– receptor-mediated endocytosis at the apical surface
– exocytosis at the basolateral surface
can be preserved.
• Adherens Junctions:
• Adherens junctions provide strong mechanical attachments between
adjacent cells.
• They hold cardiac muscle cells tightly together as the heart expands and
contracts.
• They hold epithelial cells together.
• Some adherens junctions are present in narrow bands connecting adjacent
cells.
• Others are present in discrete patches holding the cells together.
Gap junction
• Gap junctions are intercellular channels some 1.5–2 nm in diameter.
• These permit the free passage between the cells of ions and small
molecules (up to a molecular weight of about 1000 daltons).
• They are cylinders constructed from
6 copies of transmembrane proteins
called connexins.
• Because ions can flow through them,
gap junctions permit changes in
membrane potential to pass
from cell to cell.
Gap junction
• A gap junction or nexus is a specialized intercellular connection
between a multitude of animal cell -types.
• It co-ordinates the neighboring cells and allows sharing of
signaling information among them.
• These are specialized cell-cell junctions, It directly connects the
cytoplasm of two cells, which allows various molecules and ions
to pass freely between cells via narrow water filled channeles.
• Gap junctions are analogous to the plasmodesmata that join plant
cells.
• It is believed that like endocrine signaling
Signaling via gap junction
At gap junctions, the intercellular space is 4 nm and unit connexons in the
membrane of each cell are lined up with one another.
Cell junction
Intracellular signaling molecules
These channels allow the exchange of small intracellular signaling molecules
like Ca++, it does not allows exchange of macromolecules like proteins or
nucleic acids
Properties
•
Allows for direct electrical communication between cells, although different connexin subunits
can impart different single channel conductances, from about 30 pS to 500 pS.
•
Allows for chemical communication between cells, through the transmission of small second
messengers, such as inositol triphosphate (IP3) and calcium (Ca2+), although different connexin
subunits can impart different selectivity for particular small molecules.
•
Generally allows molecules smaller than 1,000 Daltons to pass through, although different
connexin subunits can impart different pore sizes and different charge selectivity. Large
biomolecules, for example, nucleic acid and protein, are precluded from cytoplasmic transfer
between cells.
•
Ensures that molecules and current passing through the gap junction do not leak into the
intercellular space.
•
To date, five different functions have been ascribed to gap junction protein: a) electrical and
metabolic coupling between cells b) Electrical and metabolic exchange through hemichannels c)
Tumor suppressor genes (Cx43, Cx32 and Cx36) d) Adhesive function independent of
conductive gap junction channel (neural migration in neocortex) e) Role of carboxyl-terminal in
signaling cytoplasmic pathways
• Examples:
• The action potential in heart (cardiac) muscle flows from cell to cell
through the heart providing the rhythmic contraction of the heartbeat.
• At some so-called electrical synapses in the brain, gap junctions permit
the arrival of an action potential at the synaptic terminals to be
transmitted across to the postsynaptic cell without the delay needed for
release of a neurotransmitter.
• As the time of birth approaches, gap junctions between the smooth
muscle cells of the uterus enable coordinated, powerful contractions to
begin.
• Several inherited disorders of humans such as
• certain congenital heart defects and
• have been found to be caused by mutant genes encoding connexins
• Desmosomes: (CELL CELL JUNCTIONS)
Desmosomes form links between cells, and provide a connection between
intermediate filaments of the cell cytoskeletons of adjacent cells.
• This structure gives strength to tissues.
• Hemidesmosomes are similar to desmosomes but anchor the cell to
underlying extracellular matrix.
• These are similar to desmosomes
but attach epithelial cells to the
basal lamina ("basement membrane" )
instead of to each other.
• Desmosomes
• Desmosomes are localized patches that hold two cells
tightly together.
• They are common in epithelia (e.g., the skin).
Desmosomes are attached to intermediate filaments of
keratin in the cytoplasm.
• Desmosomes are button like points of intracellular
contracts that serves as anchoring sites for intermediate
filaments and help holding adjacent cell together
The structure of a primary plasmodesma
CW=Cell wall ,CA=Callose ,PM=Plasma membrane ,ER=Endoplasmic
reticulum DM=Desmotubule , Red circles=Actin ,Purple circles and
spokes=Other unidentified proteins.
Plasmodesmata (singular: plasmodesma) are microscopic channels which
traverse the cell walls of plant cells and some algal cells, enabling transport and
communication between them.
Plasmodesma:
• Unlike animal cells, every plant cell is surrounded by a
polysaccharide cell wall
•
Neighbouring plant cells are therefore separated by a pair of
cell walls and the intervening lamella, forming an extracellular
domain known as the apoplast.
• Although cell walls are permeable to small soluble proteins
and other solutes, plasmodesmata enable direct, regulated,
symplastic intercellular transport of substances between cells.
•
• There are two forms of plasmodesmata: primary
plasmodesmata, which are formed during cell division, and
secondary plasmodesmata, which can form between mature
cells.
• Structure
• Plasmodesmatal plasma membrane:
• A typical plant cell may have between 103 and 105 plasmodesmata
connecting it with adjacent cells equating to between 1 and 10 per
µm2.
• Plasmodesmata are approximately 50-60 nm in diameter at the
midpoint and are constructed of three main layers, the plasma
membrane, the cytoplasmic sleeve, and the desmotubule.
•
They can transverse cell walls that are up to 90 nm thick.
• The plasma membrane portion of the plasmodesma is a continuous
extension of the cell membrane or plasmalemma
•
It is similar in structure to the cellular phospholipid bilayers.
• Plasmodesmata
• Although each plant cell is encased
in a boxlike cell wall, it turns out that
communication between cells is just as
easy, if not easier, than between animal cells.
Fine strands of cytoplasm, called plasmodesmata, extend through pores in
the cell wall connecting the cytoplasm of each cell with that of its neighbors.
• Plasmodesmata provide an easy route for the movement of ions, small
molecules like sugars and amino acids, and even macromolecules like RNA
and proteins, between cells.
•
The larger molecules pass through with the aid of actin filaments.
Types of epithelia
Classification of cell junctions
Anchoring junctions in an epithelium
Microfilaments
Intermediate filaments
Cell-cell
Cell-matrix
Cell junctions
Tight Junctions: Model
Transcellular & Paracellular Transport
Tight Junctions Form Seals
Tissue culture Model for Junctions
(Madin-Darby Canine Kidney)
Cell Adhesion Moleclues (CAMs)
Desmosomes: Model