Ligand-induced stimulation of gonadotropin receptors

In multicellular organisms, elaborate cell-to-cell communication networks coordinate the growth, differentiation, and metabolism of the multitude of cells in the diverse tissues and organs. The many cell types use a large variety of signalling molecules and mechanisms for intercellular communication. Besides direct cell-to-cell contacts, cellular communication is also conveyed by secreted chemical messengers. Signal molecules that are synthesized and secreted by signalling cells induce cellular responses only in those cells that express specific receptor molecules for them. Distinct types of intercellular communication are mediated through such a mechanism: (1) neural communication, in which neurotransmitters are released at synaptic junctions from nerve cells and act across a narrow synaptic cleft on a postsynaptic cell; (2) paracrine conununication, in which secreted products of cells diffuse locally and affect neighboring cells of different type; (3) autocrine communication, when local chemical messengers act on the secreting cell itself or on neighboring cells of the same type, and (4) endocrine communication, in which specific cells of endocrine organs secrete signal molecules (hormones) that are carried by the blood to their specific target ceils, which may be distributed widely throughout the body.