Download cell junction - Shabeer Dawar

CELL JUNCTION
CELL JUNCTIONS
• In certain tissues the membranes of adjacent
cells may join and form a junction called a cell
junction.
• These are specialized areas of the cell
membrane that provide contact between the
cells.
TYPES OF CELL JUNCTIONS
Two factors are important
1: shape and extent of the contact
• This may be in the form of a spot or puctate area of
limited extent called macula.
• Or this can be like a belt or girdle around the cell
called zonula.
2: The relative closeness and nature of cell contact:
Here the following terms are used
1. Occludens
2. Adherens
3. Gap
CELL JUNCTIONS
1:OCCLUDENS :In this type of junctions the intercellular
space is virtually obliterated and outer surface of the
two cell membranes appear in contact or even fused.
2: ADHERENS: In this type the intercellular space is
apparent being about 20-25nm wide and dense
granular material is present in the intercellular space.
3: GAP: The third types is gap junction which show a
narrow intercellular space of about 3nm.
TYPES OF CELL JUNCTIONS
• On the basis of two factors discussed above
Cell junctions are classified as
1. Maccula adherens(desmosomes or spot
desmosomes)
2. Zonula adherens (belt desmosomes)
3. Zonula occludens(tight junctions)
4. Gap junctions(nexus)
TYPES OF CELL JUNCTIONS
1.MCULA ADHERENS:
• Usually found between epithelial cells
• Discoid structures called desmosomes are present at various levels with their
long axes perpendicular to the basement membrane of epithelium.
• In the region of a desmosome the intercellular gap is about 25nm wide and
contain an adhesive glycoprotein called desmocollin.
• Intracytoplasmic densities called attachment plaques are present beneath
the plasma membrane of both the cells forming the junction.
• Groups of intermediate filaments are inserted into the attachment plaques or
they make hairpin loops and turn back into the cytoplasm
• Hemidesmosomes are sometimes seen which are structures, half of a
desmosome
TYPES OF CELL JUNCTIONS
2. ZONULA ADHERENS(BELT DESMOSOME)
• As the name indicates zonula adherens is present like a girdle
around each cell making the junction.
• The intercellular gap is of normal width(20nm)
• The gap is not bridged by filaments
• This type of gap junction is found among epithelial cells,
fibroblasts, smooth muscle cells etc.
TYPES OF CELL JUNCTIONS
3.ZONULA OCCLUDENS (tight junctions):
• It is like zonula adherens in the form of girdle around
both the cells forming the junction.
• But here the membranes of the cells are very close
and are apparently fused leaving no intercellular
gap.
TYPES OF CELL JUNCTIONS
4.GAP JUNCTIONS (NEXUS):
• The membranes of the adjoining cells are closely apposed but there is a gap of
2-3nm which permit colloidal substances.
• Like desmosomes the nexuses usually form limited attachment plaques.
• Gap junctions are widely distribute in the body and found in cardiac muscles,
smooth muscles, liver,kidny,thyroid,skin etc.
• In smooth and cardiac muscles they help in spreading impulses from cell to cell
• Gap junctions provide communication channels between adjacent cells.
• Their intercellular gap is traversed by hollow tube like structure.
JUNCTIONAL COMPLEX
• In may areas like small intestinal mucosa the
adjacent epithelial cells are joined by a series of
different cell junctions which are collectively called
junctional complex.
• Junctional complex comprises of zonula occludens,
zonula adherens and macula adherens
TYPES OF JUNCTIONS