Communicating cells may be close together or far apart. Multicellular organisms release signaling molecules that target other cells. Cells may communicate by direct contact. Both animals and plants have cell junctions that connect to the cytoplasm of adjacent cells. Signaling substances dissolved in the cytosol can pass freely between adjacent cells. Animal cells can communicate by direct contact between membrane-bound cell surface molecules. Such cell-cell recognition is important to such processes as embryonic development and the immune response. In other cases, messenger molecules are secreted by the signaling cell. Some transmitting cells release local regulators that influence cells in the local vicinity. One class of local regulators in animals, growth factors, includes compounds that stimulate nearby target cells to grow and multiply. This is an example of paracrine signaling, which occurs when numerous cells simultaneously
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