Get yourself connected
Cells need to interact with their neighbours. By Emma Dixon
In addition to being important in separating the outside and inside of the cell, the membrane allows the cell to interact and communicate with surrounding cells and the extracellular matrix. These interactions are known as ‘junctions’ and can be separated into three types, determined by their structure and function: tight, anchoring and communicating junctions.
Tight junctions (sometimes called occluding junctions)
Tight junctions create a seal between neighbouring cells at the plasma membranes, preventing small molecules ‘leaking’ from one cell to the next. This creates a ‘sheet’ of cells, which can act as a wall within an organ or tissue, keeping molecules on one side by preventing them from moving between the cells in the wall.
In the human digestive system, these cell sheets are used as a wall between the digestive tract and blood vessels, meaning nutrients have to pass through these cells to reach the blood. They have also been shown to be important in maintaining the blood–brain barrier.
As their name suggests, anchoring junctions act to anchor a cell to the cytoskeleton of a neighbouring cell or to the extracellular matrix, through the use of either actin or intermediate filament proteins in the cytoskeleton.
Between two neighbouring cells, anchoring junctions use cadherin proteins at the cell membranes to link the two cells’ cytoskeletons. Junctions that link actin cytoskeletons together are called adherens, and junctions that link intermediate filaments together are called desmosomes.
Anchoring junctions between a cell and the extracellular matrix use integrin proteins at the cell membrane. Junctions that link actin in the cell’s cytoskeleton to the extracellular matrix are called focal adhesions, and junctions that link intermediate filaments to the matrix are called hemidesmosome.
Communicating junctions are different to tight and anchoring junctions, as they not only link cells together but also let them communicate. The main communicating junctions between animal cells are gap junctions. These specialised junctions connect the cytoplasm of neighbouring cells directly to allow molecules, ions and electrical impulses to pass between the cells.
In plant cells, plasmodesmata act in a similar way to gap junctions. They allow cell–cell communication but are themselves lined with plasma membrane and directly connect the endoplasmic reticulum of the two cells.Lead image:
Anne Weston, LRI, CRUK/Wellcome Images