COMMUNICATING JUNCTIONS

COMMUNICATING JUNCTIONS

Cell junctions which permit the intercellular exchange of substances are called communicating junctions, i.e. these junctions permit the movement of ions and molecules from one cell to another cell. Gap junction and chemical synapse are the communicating junctions.

GAP JUNCTION

Gap junction is the intercellular junction that allows passage of ions and smaller molecules between the

cells. It is also called nexus. It is present in heart, basal part of epithelial cells of intestinal mucosa, etc.

Structure of Gap Junction

Membranes of the two adjacent cells lie very close to each other and the intercellular space is reduced from the usual size of 2.5 to 3 nm. Cytoplasm of the two cells is connected by the channels formed by the membranes of both cells. So, the molecules move from one cell to another cell directly through these channels, without having contact with extracellular fluid (ECF). Each channel consists of two halves. Each half belongs to one of the two adjacent cells. Each half of the

ADHERENS JUNCTION

Adherens junction is the cell to cell junction, which connects the actin filaments of one cell to those of

another cell. In some places like epithelial linings, this junction forms a continuous adhesion (zonula adherens) just below the tight junctions. In adherens junction, the membranes of the adjacent cells are held together by some transmembrane proteins called cadherins. Adherens junction provides strong mechanical attachments of the adjacent cells. Adherens junction is present in the intercalated disks between the branches of cardiac muscles . During the contractions and relaxation of heart, the cardiac muscle fibers are held together tightly by means of this junction. The adherens junction present in epidermis helps the skin to withstand the mechanical stress.

FOCAL ADHESION

Focal adhesion is the cell to matrix junctions, which connects the actin filaments of the cell to the

extracellular matrix. In epithelia of various organs, this junction connects the cells with their basal lamina. The transmembrane proteins, which hold the cell membrane and the matrix are called integrins.

DESMOSOME

Desmosome is a cell to cell junction, where the intermediate filaments connect two adjacent cells. Desmosome is also called macula adherens. The membranes of two adjacent cells, which oppose each other, are thickened and become spotlike patches. Intermediate filaments are attach ed with the thickened patches. Some of these filaments are parallel to the membrane and others are arranged in radiating fashion. Desmosomes function like tight junctions. The transmembrane proteins involved in desmosome are mainly cadherins.

HEMIDESMOSOME

Hemidesmosome is a cell to matrix junction, which connects the intermediate filaments of the cell to the

extracellular matrix. This type of cell junction is like half desmosome and the thickening of membrane of only one cell occurs. So, this is known as hemidesmosome or half desmosome. Mostly, the hemidesmosome connects the cells with their basal lamina. The proteins involved in this are integrins.

APPLIED PHYSIOLOGY

1. Dysfunction of adherens junction and focal junction in colon due to mutation of proteins results in colon

cancer. It also leads to tumor metastasis (spread of cancer cells from a primary tumor to other parts of

the body)

2. Dysfunction of desmosome causes bullous pemphigoid (autoimmune disease with tense blistering

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