Publisher Summary The capacity of the mammalian organism to function as an integrated unit is made possible by two principal control mechanisms, the nervous system and the endocrine system. Although the endocrine system has been traditionally regarded as a system of glands capable of releasing chemical mediators, which act on targets distant from the site of production, this distinction has become less sharp over recent years. It has become evident that there are intimate links between the nervous, endocrine and immune systems at the level of the hypothalamus and pituitary gland that serve to coordinate these systems. The nervous system also releases chemical substances, which not only act as local mediators but also as circulating hormones. Hormones released by cells in endocrine glands also have local effects in the cells in which they are synthesized. Although, endocrine organs are usually resistant to direct toxic effects of xenobiotics, they are extremely sensitive to stimulation or inhibition by a variety of trophic or antitrophic substances and by end-organ feedback. Changes in endocrine organs may occur following administration of hormones, hormone modulators, trophic factors and their synthetic analogues. Several lines of evidence indicate that the secretory function of endocrine tissue is closely related to proliferation of endocrine cells. A powerful and increasingly sophisticated tool in the characterization of specific endocrine and neuroendocrine cells, their products and relationships with other cell markers is immunocytochemistry.