A basic feature of multicellular animals is the formation of diverse kinds of tissue, such as skin, blood, or muscle, where cells are organized in specific ways. Cells must also be able to communicate with each other and have markers of individual identity. 

The nature of the physical connections between the cells of a tissue in large measure determines what the tissue is like. Indeed, a tissue’s proper functioning often depedns critically on how the individual cells are arranged within it. 

Most cadherins function as transmembrane adhesion proteins that indirectly link the of the cells they join. This occurs in Cell Junctions like the adherens junctions where the cytoplasmic tails of cadherins interacts indirectly with actin filaments by means of a group of intracellular anchor proteins called “catenins” and with desmosomes where the cytoskeleton is intermediate filaments.

Occluding junctions: include the tight junctions whose role is particularly prominent in the small intestine where they prevent transport proteins at the apical surface of the epithelia cell (surface facing the lumen) from mixing with those in the basolateral surfaces of the cell. In addition tight junctions insure that spaces between epithelia cells are sealed so that transported molecules cannot diffuse back into the gut. Major transmembrane proteins in a tight junction include the claudins and occludins. 

Adherens (Anchoring) junctions: 

Adheres junction connect the cytoskeleton of a cell to the cytoskeleton of other cells using the transmembrane protein cadherin or to the extracellular matrix through the use of the TM protein integrin. In both cases, there is an intracellular coupling to either actin or intermediate filaments depending on the type of intracellular anchor proteins involved. 

Cadherin sueprfamily: is a large family of Ca2+ dependent adhesive moelcuels found in viturally all all metazoan animals. 

Desmosomes: are a cadherin based unciton unique to vertegrates. They contain the cadherins desmocollin and desmoglein, which interact with intermediate filaments of ctoskeletons instead of acin. Desmosomes join adjacent cells. These connections support tissues again mechanical stress. 

Hemidesmosomes and focal adhesions: connect cells to the basal lamins or other ECM. In this calse the proteins that interat with the ECM are integrins. 

Communicating junctions: 

Communicating junctions include the gap junctions which form channels through transmembrane proteins called connexins. 6 connexins are required to form a channel (called a “connexon”) and when the connexons in the PM of 2 cells in contact are aligned, they form a continuous aqueous channel that connects the two cell interiors. These junctions allow communicaiton between cells by diffusion thorugh small openings. Communicating junctions permit small molecuels or ions to pass from one cell to the other. In animals, these direct communicaiton channels are called gap junctions and in plant, plasmodesmata. 

The various anchoring junctions are contained in the chart below: