STABLE INTERCELLULAR BRIDGES CHANNELS FOR CELL-TO-CELL COMMUNICATION

Numerous systems for cell-to-cell communication have evolved in multi-cellular organisms. Plasmodesmata, gap junctions, tunneling nanotubes or stable intercellular bridges are channels that ensure a direct cytoplasmic connection between interconnected cells. Stable intercellular bridges (SIBs, also termed cytoplasmic bridges) are wide channels (up to 15 mu m in diameter) that allow the transfer of macromolecules, cytoplasm or even organelles such as, mitochondria or centrioles between cells. In fact, the cells that are interconnected by SIBs form a kind of functional syncytium and are usually termed cell cysts, clusters or nests. SIBs are formed by incomplete cytokinesis when the transient intercellular bridge is not split (cell abscission is not finished), after which it is somehow stabilized and forms SIB. Cytoplasmic bridges are widespread among animals, especially in germ cells where they interconnect developing gametes. However, they have also been found in some somatic tissues. Several roles for SIBs have been proposed, e.g., they may be necessary for haploid and genetically different spermatids to remain phenotypically diploid. They may provide the directional transfer of macromolecules and cell organelles toward the future egg cell during oogenesis. SIBs were described over 50 years ago, but we have only recently begun to learn how these structures are formed, what functions they perform and how important they are for intercellular communication.