_Cell cell cross talk.ppt
- 1. CELL JUNCTIONS Specialized cell junctions occur at points of cell-cell and cell-matrix contact in all tissues and they are particularly plentiful in epithelia. Cell junctions may be classified into three functional groups. http://biology.kenyon.edu/edwards/project/greg/pd.htm
- 2. Occluding junctions: I. Tight junctions (found only in vertebrates) II. Septate junctions (invertebrates mainly) Anchoring junctions: Actin filaments attachment sites: I. Cell-cell junctions (adherens junctions) II. Cell-matrix junctions (focal adhesions) Intermediate filaments attachment sites: I. Cell-cell junctions (desmosomes) II. Cell-matrix junctions (heterodesmosomes) Communicating junctions: I. Gap junctions II. Chemical synapses III. Plasmodesmata (Plants only) Functional classification of cell junctions
- 4. Plasmodesmata perform many of the functions as Gap junctions The tissues of the plants are organized on different principles from those of an animal. This is because the plant cells are imprisoned between rigid cell walls composed of an extra cellular matrix rich in cellulose and other polysaccharides. The cell walls of adjacent cells are firmly cemented to those of their neighbours which eliminates the need for the anchoring junctions to hold the cell in place. But a need for direct cell-to –cell interaction remains. Thus, plants have only one intercellular junctions called plasmodesmata (singular: plasmodesma). Like gap junctions they directly connect the cytoplasm of the adjacent cells. In plants however, the cell wall between a typical pair of adjacent cells is atleast 0.1 μm thick and so a structure very different from gap junctions is required to mediate communication across it.
- 5. Plasmodesmata solve this problem. With a few specialized exceptions every cell in a higher plant is connected to its neighbor by these structures which form fine cytoplasmic channels through the intervening cell walls.
- 7. It has been suggested that plasmodesmata transports gibberellic acid in algae. Electric coupling between cells has been associated with the density of plasmodesmata between the cells, and electrical signals play a large role in the initiation of a wound response in plants. Plasmodesmata have been shown to be sensitive to various levels of calcium, pH, and light, demonstrating that the mechanisms of plasmodesmata transport are more complex than previously thought. Plasmodesmata tend to close when there is a significant pressure difference between adjacent cells, again speaking to a role of plasmodesmata in a wound response upon a loss of turgor pressure.
- 8. The permanent opening or closing of plasmodesmata serve to signify symplastic domains in plants, often compartmentalizing various plant functions (a sort of division of labor), such as in guard cells or in the sieve element-companion cell complex in stems. Plasmodesmata serve as directors of plant growth and development and may help to determine a program of cell differentiation, such as sealing off root and stem epidermal cells from the rest of the plant. This notion has been supported by a study in which RNA encoding the transcription factor KNOTTED1 (KN1) was localized to all cell layers of the developing maize meristem except the outermost, but the KN1 protein was found in all cell layers (reviewed by Zambryski, 1995). Thus, KN1 is most likely being transported to the outer layer through plasmodesmata, providing a channel through which this and other important signalling and regulatory molecules can pass into other cells.