Plant cells have rigid cell walls that maintain cell shape but hinder cellular communication. Microscopic channels in the cell walls called plasmodesmata directly connect the cytoplasm of adjacent cells, allowing molecular exchange for communication. Primary plasmodesmata are created during cell division, while secondary plasmodesmata are formed between the existing cell walls of neighboring cells. Plasmodesmata are membrane-lined, containing a narrow tube-like central structure called the desmotubule. The Desmotubule is formed by an extension of the ER that connects adjacent cells. The cytosol flows freely between the two cells making a continuous network called the symplast. Cells control the passage of molecules by regulating the diameter of the plasmodesmata. When callose, a polysaccharide, accumulates, the channel is constricted, allowing only water and small molecules to pass. When callose breaks down, the channel is dilated, allowing larger molecules to pass through. By degrading plasmodesmata, cells lose symplastic connectivity with their neighbors.