|MadSci Network: Cell Biology|
Gap junctions are composed of proteins that assemble to form pores in the membranes of cells called connexons. A connexon on one cells linked to a connexon from a neighboring cell to form an open channel between the cells. This channel is wide enough to allow the exchange of small intracellular signalling molecules between cells such as calcium and cyclic AMP, but not larger macromolecules like proteins or nucleic acids. In tissues that contain electrically excitable cells (for example smooth muscle, cardiac and nerve cells) allow actions potentials to spread rapidly without having the delay associated with chemical synapses. In addition it is known that gap junctions play a role in embryonic development. When cells begin to differentiate they start to lose their gap junctions and only maintain connections with cells of the same tissue type maintaining the ability to behave as a cooperative assembly. It is also thought that gap junctions may play a role in long range signalling. This would happen for example when a signalling molecule is in high concentration in one area within a given tissue, but low in a different area. This concentration gradient could provide cells with positional information to control differentiation of a tissue.
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