Animal cells can signal to one another in various ways. (A) Hormones produced in endocrine glands are secreted into the bloodstream and are often distributed widely throughout the body. (B) Paracrine signals are released by cells into the extracellular fluid in their neighborhood and act locally. (C) Neuronal signals are transmitted along axons to remote target cells. (D) Cells that maintain an intimate membrane-to-membrane interface can engage in contact-dependent signaling. Many of the same types of signal molecules are used in endocrine, paracrine, and neuronal signaling. The crucial differences lie in the speed and selectivity with which the signals are delivered to their targets.  Fair Use and Copyright info