As in other organisms, because hormones are a fundamental way that multicellular organisms "solve" the problem of intercellular communication and the regulation of local gene expression to serve the whole, vertebrates have evolved a substantial array of hormone-producing glands, genes, and systems that affect all aspects of life: reproduction, growth and development, maintenance of a stable internal environment, mental functions, physical activity, food seeking and satiety, many behaviors, and regulation of energy balance (Ojeda and Griffin 2000). Rudiments of these systems are found in chordate "relics" like amphioxus. As noted earlier—and as we have come to expect—at least some of these are evolutionarily related to substances found in invertebrates.
The vertebrate endocrine system classically consists of a number of ductless glands that produce hormones, with different functions, for release directly into the bloodstream (Table 10-3). However, a number of other glands and cells secrete hormones, and in fact some cells produce hormone for use within the cell and do not secrete it at all. These "nonclassical" hormone-secreting organs and their products are listed in Table 10-4 (Kacsoh 2000; Ojeda and Griffin 2000).
Hormones can have several effects on a target tissue, and several hormones may have the same effect. Also, as in plants and insects, hormone concentrations are controlled by feedback mechanisms, that is, they are up- or downregulated by the concentration of hormones or other compounds in the blood. Increasing levels of glucose in the blood, for example, stimulate the release of insulin, a polypeptide hormone, from the pancreas. Increased amounts of insulin in the blood in turn stimulate glucose uptake by the liver and its conversion to glycogen, and subsequent lower blood glucose levels lead to decreased secretion of insulin and slower glucose uptake.
The pancreas illustrates an interesting point about the evolution of organ systems. It is an evolutionary and developmental outcropping of the gut and also secretes digestive enzyme from a different set of cells; this indicates the general
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