As living organisms we are constantly receiving and interpreting signals from our environment. These signals can come in the form of light, heat, odors, touch or sound. The cells of our bodies are also constantly receiving signals from other cells. These signals are important to keep cells alive and functioning as well as to stimulate important events such as cell division and differentiation.
Signals are most often chemicals that can be found in the extracellular fluid around cells. These chemicals can come from distant locations in the body (endocrine signaling by hormones), from nearby cells (paracrine signaling) or can even be secreted by the same cell (autocrine signaling).
Signaling molecules may trigger any number of cellular responses, including changing the metabolism of the cell receiving the signal or result in a change in gene expression (transcription) within the nucleus of the cell or both.