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Endocrinology (from Greek ἔνδον, endo, "within"; κρῑνω, krīnō, "to separate"; and -λογία, -logia, "the study of") is a branch of biology and medicine dealing with the endocrine system, its diseases, and its specific secretions called hormones, as well as the integration of developmental events proliferation, growth, and differentiation (including histogenesis and organogenesis), and also the psychological or behavioral activities of metabolism, growth and development, tissue function, sleep, digestion, respiration, excretion, mood, stress, lactation, movement, reproduction, and sensory perception as caused by hormones. Endocrinology is concerned with study of the biosynthesis, storage, chemistry, biochemical and physiological function of hormones and with the cells of the endocrine glands and tissues that secrete them. Various specializations exist, including behavioral endocrinology[1][2][3] and comparative endocrinology. The endocrine system consists of several glands, all in different parts of the body, that secrete hormones directly into the blood rather than into a duct system. Hormones have many different functions and modes of action; one hormone may have several effects on different target organs, and, conversely, one target organ may be affected by more than one hormone. In the original 1902 definition by Bayliss and Starling (see below), they specified that, to be classified as a hormone, a chemical must be produced by an organ, be released (in small amounts) into the blood, and be transported by the blood to a distant organ to exert its specific function. This definition holds for most "classical" hormones, but there are also paracrine mechanisms (chemical communication between cells within a tissue or organ), autocrine signals (a chemical that acts on the same cell), and intracrine signals (a chemical that acts within the same cell).[4] A neuroendocrine signal is a "classical" hormone that is released into the blood by a neurosecretory neuron (see article on neuroendocrinology). Hormones act by binding to specific receptors in the target organ. As Baulieu notes, a receptor has at least two basic constituents: a recognition site, to which the hormone binds; and an effector site, which precipitates the modification of cellular function.[5] Between these is a "transduction mechanism" in which hormone binding induces allosteric modification that, in turn, produces the appropriate response. |
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