ABSTRACT
When things change, cells respond. Every cell, from the humble bacterium to the most sophisticated eukaryotic cell, monitors its intracellular and extracellular environment, processes the information it gathers, and responds accordingly. Long before multicellular creatures roamed the Earth, unicellular organisms had developed mechanisms for responding to physical and chemical changes in their environment. These almost certainly included mechanisms for responding to the presence of other cells. The fundamental features of cell signaling have been conserved throughout the evolution of the eukaryotes. Cells in multicellular animals communicate by means of hundreds of kinds of extracellular signal molecules. These include proteins, small peptides, amino acids, nucleotides, steroids, retinoids, fatty acid derivatives, and even dissolved gases such as nitric oxide and carbon monoxide. A typical cell in a multicellular organism is exposed to hundreds of different signal molecules in its environment. Most cells respond to many different signals in the environment, and some of these signals may influence the response to other signals.
