In multicellular animals, physiological regulatory systems require information and cell-to-cell communication. Most intercellular communication is by means of chemical signals that bind to receptors, as described in Chapter 7. Some types of chemical signals discussed in other chapters include growth factors, morphogens, and cytokines. In future chapters you will learn about chemical signals used by the nervous system called neurotransmitters. Each of these examples involves a chemical message secreted by cells and received by other cells (target cells) that have appropriate receptors. These four classes of chemical signals are named according to the types of actions they have: growth factors influence cell division, morphogens influence developmental processes, cytokines control immune system cells, and neurotransmitters enable nerve cells to communicate with each other and with target cells such as muscles. In this chapter we cover another broad class of chemical signals called endocrines, which are secreted by epithelial cells directly into the extracellular fluid (ECF; see Key Concept 39.2). From the ECF, endocrines can diffuse locally and also into the blood, where they circulate throughout the body.