21 Cell-to-cell signalling
Intercellular communication
In multicellular organisms where different functions are carried out by differentiated cells, mechanisms for intercellular communication are required to ensure the efficient integration of cellular activities, or homeostasis. Signalling between cells may occur directly, through gap junctions between cells (Ch. 30) or activation of contact-dependent signalling pathways (Ch. 33), or indirectly, via the secretion of extracellular chemical messengers, which cause a response in a target cell when recognized by specific receptors.
Chemical signalling
Inter6cellular chemical signals may be classified according to their functions into hormones, local mediators and neurotransmitters (Fig. 3.21.1).
• Hormones.These are secreted by specialized cells, usually found in endocrine glands, into the circulation and are carried to target cells where they have their effect. Endocrine signalling coordinates the activities of distally related tissues.
