Plant cells are surrounded by cell walls which are barriers for cell-cell communication.  This barrier is overcome by specialized junctions called plasmodesmata. They are similar to gap junctions, connecting the cytosol of adjacent cells. Small molecules (<1000 Da), such as ions, amino acids, and sugars, can diffuse freely through plasmodesmata.  These small molecules include signaling molecule and transcription factors. The size of the channel is also regulated to allow molecules up to 10,000 Da in size.  The permeability of these channels is dependent on many factors, including Ca2+ concentration. An increase in cytosolic Ca2+ concentration will reversibly limit passage through the plasmodesmata. Unlike gap junctions, the cell membranes of adjacent cells merge to form a continuous channel called an annulus. Additionally, within the channel, there is an extension of the endoplasmic reticulum, called a desmotubule, which spans between the cells. The cell-cell interactions facilitated by plasmodesmata play an important role in development of plant cells and tissues and defense against viral infection.