CELL JUNCTION CELL JUNCTIONS In certain tissues the

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 -25 nm 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 3 nm.

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 25 nm 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(20 nm) • 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 -3 nm 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