fact that as a cell wall expands, the abundance of the primary plasmodesmata decreases. In order to further expand plasmodesmal density during cell wall growth secondary plasmodesmata are produced. The process of secondary plasmodesmata formation is still to be fully understood, however various degrading enzymes and ER proteins are said to stimulate the process. Structure Plasmodesmatal plasma membrane A typical plant cell may have between 1,000 and 100,000 plasmodesmata connecting it with adjacent cells equating to between 1 and 10 per µm2. Plasmodesmata are approximately 50–60 nm in diameter at the midpoint and are constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the desmotubule. They can transverse cell walls that are up to 90 nm thick. The plasma membrane portion of the plasmodesma is a continuous extension of the cell membrane or plasmalemma and has a similar phospholipid bilayer structure. The cytoplasmic sleeve is a fluid-filled space enclosed by the plasmalemma and is a continuous extension of the cytosol. Trafficking of molecules and ions through plasmodesmata occurs through this space. Smaller molecules (e.g. sugars and amino acids) and ions can easily pass through plasmodesmata by diffusion without the need for additional chemical energy. Larger molecules, including proteins (for example green fluorescent protein) and RNA, can also pass through the cytoplasmic sleeve diffusively. Plasmodesmatal transport of some larger molecules is facilitated by mechanisms that are currently unknown. One mechanism of regulation of the permeability of plasmodesmata is the accumulation of the polysaccharide callose around the neck region to form a collar, thereby reducing the diameter of the pore available for transport of substances. Through dilation, active gating or structural remodeling the permeability of the plasmodesmata is increased. This increase in plasmodesmata pore permeability allows for larger molecules, or macromolecules, such as signaling molecules, transcription factors and RNA-protein complexes to be transported to various cellular compartments. Desmotubule The desmotubule is a tube of appressed (flattened) endoplasmic reticulum that runs between two adjacent cells. Some molecules are known to be transported through this channel, but it is not thought to be the main route for plasmodesmatal transport. Around the desmotubule and the plasma membrane areas of an electron dense material have been seen, often joined together by spoke-like structures that seem to split the plasmodesma into smaller channels. These structures may be composed of myosin and actin, which are part of the cell's cytoskeleton. If this is the case