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A system that produces hormones, substances that regulate the body's functioning.A system of this kind may be very simple, involving one or more centers in the nervous system, or it may be complex, involving many glands in the endocrine system.

A comparative endocrinologist studies the evolution of endocrine systems, as well as how these systems affect animals' adaptations to their environment and their ability to reproduce.These studies of nonmammal animals have led to advancements in the study of mammalian endocrinology, including that of humans.Several actions of a pituitary hormone, prolactin, on the regulation of body water and salt content have been discovered in fishes, which led to the demonstration of the same mechanisms in mammals.Earlier studies discovered that secretions produced by the ovary mediate the maturation of eggs (oocytes), but it wasn't until decades later that vertebrates were also shown to have this capability.Researchers studied tadpoles and thyroid hormones for the first time during early 1900s.Also, many mammalian hormones have been isolated and purified through the use of vertebrates as bioassay systems; that is, primitive animals have served as relatively simple, sensitive indicators for the amount of hormone activity in extracts prepared from mammalian endocrine glands.Furthermore, there are a few vertebrate and invertebrate species that have provided a useful "model system" which helped shed light on the nature of hormone receptors and the mechanisms of hormone action.As one example, the ovaducts of chickens have been studied intensively for understanding hormone actions on target tissues via receptors for progesterone and estrogens (hormones released by the gonads).


It is also essential to understand how nonmammals' endocrine systems work when trying to govern natural populations or captive animals.Controlling salmon reproduction artificially has had significant effects on the salmon industry in general.In order to reduce pest insect species, many successful efforts have been based on pheromones.Knowing the endocrinology of a rare species may allow its reproduction in captivity and prevent its extinction.Research could eventually lead to the reintroduction of endangered species into natural habitats.

Evolution of endocrine systems

.In the evolution of the animal kingdom, true endocrine glands emerged later as separate hormone-producing structures.Cells of these endocrine glands might have originated from nerve cells that migrated from the nervous system to various parts of the body during evolution.As far as we know, these independent endocrine glands have only been observed in arthropods (where neurohormones are still the dominant type of endocrine messengers) and in vertebrates (where they are the most developed).

Several previously ascribed to vertebrates are now known to be secreted by invertebrates (for example, insulin in the pancreas).Invertebrates including humans exhibit many of the same hormones found in invertebrates.Several of these molecules are synthesized and employed by unicellular animals and plants as chemical regulators, much like hormones in higher animals.Accordingly, the evolutionary history of endocrinological regulators goes back into ancient times, and the major changes that occurred centered on the different uses these molecules were put to during evolution.

Vertebrate endocrine systems

.Agnatha, or jawless fishes, is the most primitive order of fishes.Agnatha are jawed fishes of the classes Chondrichthyes and Osteichthyes, which evolved from the Agnatha millions of years ago.The Chondrichthyes refer to the cartilaginous fishes, such as sharks and rays, while the Osteichthyes are the bony fishes.There are four teleost subgroups of bony fishes: goldfish, trout, bass, and teleosts, the most advanced. Teleost fishes are the first to develop lungs and the first to invade land.The class Amphibia, comprised of frogs and toads, has evolved from the teleosts.Amphibians gave rise to the reptiles, which became more adapted to land and diverged along multiple evolutionary lines.In the evolutionary tree of reptiles, turtles, dinosaurs, crocodiles, snakes, and lizards all descend from the primitive reptiles.The different groups of reptiles that later evolved into birds and mammals are known as the Aves and Mammalia respectively.Biologically, amphibians, reptiles, birds, and mammals are called tetrapods (four-footed vertebrates).

Developed over millions of years, the human endocrine system is a product of evolution.As a result, it does not come as a surprise that the endocrine glands and hormones of the human endocrine system are analogous to those of more primitive vertebrates.By studying these animals, it is possible to observe the emergence of the hypothalamic-pituitary-target organ axis, as well as many other endocrine glands, during the evolution of fishes, which preceded the evolution of terrestrial vertebrates.