Building Bodies - Cell junctions
Above: adherens junctions rivet cells together, binding or adhering them strongly to one-another. [Click image for full size.]

Adherens junctions

Adherens junctions also bind epithelial cells together, where they may form a band around the circumference of each cell,
zonula adherens (the adhesion zones). They may also bind cells together at discrete spots (focal adhesions). They
may also bind cells to the
extracellular matrix (ECM) at spots called adhesion plaques.

It is thought that adherens junctions are required for contact inhibition and mutations in these genes seem to be responsible
for certain cancers. Contact inhibition is the inhibition of growth and cell division when a cell contacts neighbours on all sides
(or all edges in an epithelium sheet of cells). This prevents tissues growing uncontrollably (when cells do they form tumours -
cell masses called neoplasms). If a tissue such as the skin is damaged, then contact inhibition is removed from those cells
adjacent to the wound and so these cells spread, grow and divide until the wound is sealed.

More animal cell junctions

Binding cells together is a key process in building multicellular bodies from cells. In animals there are four main types of cell

Adherens junctions
    2. Desmosomes
    3. Gap junctions
    4. Tight junctions

These junctions involve proteins that act as rivets or bolts to bind neighbouring cells together. These junctions may occur
wherever neighbouring cells contact one another and these contact areas are often highly folded to form inter-digitating
processes that interlock and help bind the cells more strongly.
The plaques in adherens junctions are composed chiefly of catenin, which connects the junction to the actin-filament network
of the cytoskeleton (cell skeleton). This allows adherens junctions to respond dynamically as the actin cytoskeleton is
capable of rapid contraction and enables tension to be maintained throughout a tissue.
Desmosomes respond more
passively as they contain plaques of desmoplakin that connect to the intermediate filaments of the cytoskeleton. Intermediate
filaments give cells and tissues passive strength and toughness. A good example is keratin, the main protein that gives skin,
scales and horns their toughness. Bundles of keratin fibres span the desmosomes on different sides of the cell, so that when
tension is applied to the tissue, as when it is stretched, the cable become taught and resist further stretching.

Hemidesmosomes are similar, but attach a cell to the extracellular matrix (ECM) rather than to another cell. The ECM
typically consists of glycoproteins secreted by the cells and forms a hydrated gel-like mesh, forming a supporting external
skeleton for the cells. The ECM may be a three-dimensional mesh, but in epithelial tissue it forms a two-dimensional sheet
that underlies the cells and is called the
basement membrane. (Remember epithelium, plural - epithelia, is a general name
for any covering tissue, sheets of cells that cover surfaces, be they external surfaces like the skin or internal ones like the
lining of the lumen of blood vessels, the latter linings of internal structures are also called endothelium. Epithelium is not the
'opposite' of endothelium, as that would be ectothelium, an external covering.)

Gap Junctions

These are junctions in animal cells that create channels that cross the cell membranes, connecting the cytoplasms of
neighbouring cells together. These channels are tiny, allowing only small molecules and ions to cross from one cell to
another. These junctions are formed from proteins in the cell membranes that form hollow tubes through which small
molecules and ions (with a molecular mass below 1000)
electrochemical signals, such as Ca2+ (a second messenger) or
Na+ can be delivered from one cell to its neighbours. If you touch a single cell in an epithelial sheet, then not only that cell will
respond, but also its neighbours, as an electrical signal passes from the stimulated cell to the neighbouring cells via the gap
Gap junctions electrically couple cells together, allowing electrical signals to pass from one cell to its neighbours. Gap
junctions are extremely important for coordinating cells in a tissue and tissues requiring precise coordination have lots of
gap junctions, for example, cardiac muscle, which must beat in synchrony, or the smooth muscle of the uterus wall ready for
child-birth. They also occur in certain locations in the nervous system as
electrical synapses which are faster than
chemical synapses, but are bidirectional rather than unidirectional, that is they allow signals to travel in either direction. Gap
junctions allow cells to communicate rapidly with their nearest neighbours.

In bone, the bone cells or osteocytes are embedded in a well-developed ECM, making up the bulk of the bone, which
consists of the strong rope-like protein collagen embedded in a mineral (stony) matrix of calcium hydrogen phosphate
crystals. The bone cells appear quite isolated from one-another, but they give out slender protoplasmic projections that form
a 3D network throughout the bone. Where a projection from one osteocyte meets that of another, gap junctions occur,
electrically coupling the osteocytes together. This electrical network of osteocytes acts as a stress sensor, detecting where
stresses are greatest and least in the bone and remodelling the bone accordingly - adding bone where it is needed.
Epithelial cells are typically electrically coupled to their neighbours via gap junctions, allowing them to collectively sense and
respond to stresses from fluid flow, for example.

Gap junctions can be opened or closed by the cell. If a cell becomes damaged it starts to leak toxic calcium, which although
an important messenger in cells is toxic in high concentration. Other noxious materials are released inside a dying cell and to
prevent neighbouring cells from being damaged, the gap junctions adjacent to the damaged cell can be closed. Opening
and closing of gap junctions presumably allow electrical synapses to regulate how sensitive they are, that is what the
threshold is for a signal to pass from one cell to the next.

Keeping things water-tight!

Many organs contain fluids and what leaks into or out of the fluid from the tissues must be controlled. The epithelium lining
the lumen of such organs can regulate how leaky or water-tight it is by forming
tight-junctions. When tightly sealed, the
tight-junctions prevent leakage, but when 'unzipped' they allow fluid to flow across the epithelium.
Junctions Between Plant Cells