Hormone A hormone is a class of signaling molecules produced by glands in multicellular organisms that are transported by the circulatory system to target distant organs to regulate physiology and behaviour. Hormones have diverse chemical structures, mainly of 3 classes: The glands that secrete hormones comprise the endocrine system. The term hormone is sometimes extended to include chemicals produced by cells that affect the same cell autocrine or intracrine signalling or nearby cells paracrine signalling.
The nervous system and the endocrine system are closely related to one another in their function, for both serve to coordinate activity. The endocrine glands of mammals generally have more complex regulatory functions than do those of lower vertebrates.
This is particularly true… Evolution of endocrine systems The most primitive endocrine systems seem to be those of the neurosecretory type, in which the nervous system either secretes neurohormones hormones that act on, or are secreted by, nervous tissue directly into the circulation or stores them in neurohemal organs neurons whose endings directly contact blood vessels, allowing neurohormones to be secreted into the circulation Invertebrate endocrine system, from which they are released in large amounts as needed.
True endocrine glands probably evolved later in the evolutionary history of the animal kingdom as separate, hormone-secreting structures.
Some of the cells of these endocrine glands are derived from nerve cells that migrated during the process of evolution from the nervous system to various locations in the Invertebrate endocrine system. These independent endocrine glands have been described only in arthropods where neurohormones are still the dominant type of endocrine messenger and in vertebrates where they are best developed.
It has become obvious that many of the hormones previously ascribed only to vertebrates are secreted by invertebrates as well for example, the pancreatic hormone insulin.
Likewise, many invertebrate hormones have been discovered in the tissues of vertebrates, including those of humans. Some of these molecules are even synthesized and employed as chemical regulators, similar to hormones in higher animals, by unicellular animals and plants.
Thus, the history of endocrinologic regulators has ancient beginnings, and the major changes that took place during evolution would seem to centre around the uses to which these molecules were put. Vertebrate endocrine systems Vertebrates phylum Vertebrata are separable into at least seven discrete classes that represent evolutionary groupings of related animals with common features.
The class Agnatha, or the jawless fishes, is the most primitive group. Class Chondrichthyes and class Osteichthyes are jawed fishes that had their origins, millions of years ago, with the Agnatha.
The Chondrichthyes are the cartilaginous fishes, such as sharks and rays, while the Osteichthyes are the bony fishes. Familiar bony fishes such as goldfish, trout, and bass are members of the most advanced subgroup of bony fishes, the teleosts, which developed lungs and first invaded land.
From the teleosts evolved the class Amphibiawhich includes frogs and toads.
The amphibians gave rise to the class Reptilia, which became more adapted to land and diverged along several evolutionary lines. Among the groups descending from the primitive reptiles were turtles, dinosaurs, crocodilians alligators, crocodilessnakes, and lizards. Birds class Aves and mammals class Mammalia later evolved from separate groups of reptiles.
Amphibians, reptiles, birds, and mammals, collectively, are referred to as the tetrapod four-footed vertebrates. The human endocrine system is the product of millions of years of evolution. By examining these animals it is possible to document the emergence of the hypothalamic-pituitary-target organ axis, as well as many other endocrine glands, during the evolution of fishes that preceded the origin of terrestrial vertebrates.
The hypothalamic-pituitary-target organ axis The hypothalamic-pituitary-target organ axes of all vertebrates are similar. The hypothalamic neurosecretory system is poorly developed in the most primitive of the living Agnatha vertebrates, the hagfishes, but all of the basic rudiments are present in the closely related lampreys.
In most of the more advanced jawed fishes there are several well-developed neurosecretory centres nuclei in the hypothalamus that produce neurohormones.
These centres become more clearly defined and increase in the number of distinct nuclei as amphibians and reptiles are examined, and they are as extensive in birds as they are in mammals.
Some of the same neurohormones that are found in humans have been identified in nonmammals, and these neurohormones produce similar effects on cells of the pituitary as described above for mammals.
Two or more neurohormonal peptides with chemical and biologic properties similar to those of mammalian oxytocin and vasopressin are secreted by the vertebrate hypothalamus except in Agnatha fishes, which produce only one. The oxytocin-like peptide is usually isotocin most fishes or mesotocin amphibians, reptiles, and birds.
The second peptide is arginine vasotocin, which is found in all nonmammalian vertebrates as well as in fetal mammals.
Chemically, vasotocin is a hybrid of oxytocin and vasopressin, and it appears to have the biologic properties of both oxytocin which stimulates contraction of muscles of the reproductive tract, thus playing a role in egg-laying or birth and vasopressin with either diuretic or antidiuretic properties.
The functions of the oxytocin-like substances in nonmammals are unknown. The pituitary glands of all vertebrates produce essentially the same tropic hormones: The production and release of these tropic hormones are controlled by neurohormones from the hypothalamus.
The cells of teleost fishes, however, are innervated directly. Thus, these fishes may rely on neurohormones as well as neurotransmitters for stimulating or inhibiting the release of tropic hormones.
Among the target organs that constitute the hypothalamic-pituitary-target organ axis are the thyroid, the adrenal glands, and the gonads. Their individual roles are discussed below. The thyroid axis Thyrotropin secreted by the pituitary stimulates the thyroid gland to release thyroid hormones, which help to regulate development, growth, metabolismand reproduction.Endocrine system, any of the systems found in animals for the production of hormones, substances that regulate the functioning of the organism.
Such a system may range, at its simplest, from the neurosecretory, involving one or more centres in the nervous system, to the complex array of glands found in the human endocrine system. Vertebrate Physiology Bio This web page contains notes to accompany lectures in Vertebrate Physiology, Biology , taught by Dr.
Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, , USA.. Endocrine system. An endocrine system consists of those glandular cells, tissues, and organs whose products (hormones) supplement the rapid, short-term coordinating functions of the nervous system.
Some evidence, although not always direct, has been reported for hormones in a wide variety of invertebrates. An endocrine system consists of those glandular cells, tissues, and organs whose products (hormones) supplement the rapid, short-term coordinating functions of the nervous system.
Vertebrate Physiology Bio This web page contains notes to accompany lectures in Vertebrate Physiology, Biology , taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, , USA.. Endocrine system. 1 Hormones and the Endocrine System Angelica Pereyra Biology Overview endocrine system invertebrate vertebrate endocrine . Neuroendocrine controls exist already in lower invertebrates, and during evolution, endocrine glands have appeared in molluscs, although endocrine cells may have appeared earlier. The present review discusses at first the different strategies used in the past and nowadays to isolate hormones and .
Some evidence, although not always direct, has been reported for hormones in a wide variety of invertebrates. 1 Hormones and the Endocrine System Angelica Pereyra Biology Overview endocrine system invertebrate vertebrate endocrine glands hormones mode of action of.
Endocrine system, any of the systems found in animals for the production of hormones, substances that regulate the functioning of the organism. Such a system may range, at its simplest, from the neurosecretory, involving one or more centres in the nervous system, to the complex array of glands found in the human endocrine system.