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All living organisms react to signals from their environment such as light, temperature, sound and chemical substances in order to survive and adapt to their surroundings. In multicellular organisms communication between cells is necessary to regulate growth and differentiation [3]. Neighbouring cells can communicate with each other by direct cell-cell contact through plasma-membrane-bound signalling molecules or through the formation of gap junctions [176]. Cells are able to communicate with other cells some distance away through secretion of chemical signals. The secreted signals can be soluble molecules such as most hormones and neurotransmitters; they bind to receptors on the cell surface of target cells. Hydrophobic signal molecules such as the steroid and thyroid hormones are able to pass the lipid bilayer of the plasma membrane and bind to specific proteins inside the cell [76].