Hormones are the body’s chemical messengers and are part of the endocrine system. Endocrine glands make hormones, which travel through the bloodstream to tissues and organs, and control most of our body’s major systems.
hormone, organic substance secreted by plants and animals that functions in the regulation of physiological activities and in maintaining homeostasis. Hormones carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them. The classical view of hormones is that they are transmitted to their targets in the bloodstream after discharge from the glands that secrete them. This mode of discharge (directly into the bloodstream) is called endocrine secretion. The meaning of the term hormone has been extended beyond the original definition of a blood-borne secretion, however, to include similar regulatory substances that are distributed by diffusion across cell membranes instead of by a blood system.
General features
Relationships between endocrine and neural regulation
Hormonal regulation is closely related to that exerted by the nervous system, and the two processes have generally been distinguished by the rate at which each causes effects, the duration of these effects, and their extent; i.e., the effects of endocrine regulation may be slow to develop but prolonged in influence and widely distributed through the body, whereas nervous regulation is typically concerned with quick responses that are of brief duration and localized in their effects. Advances in knowledge, however, have modified these distinctions.
Nerve cells are secretory, for responses to the nerve impulses that they propagate depend upon the production of chemical transmitter substances, or neurotransmitters, such as acetylcholine and norepinephrine (noradrenaline), which are liberated at nerve endings in minute amounts and have only a momentary action. It has been established, however, that certain specialized nerve cells, called neurosecretory cells, can translate neural signals into chemical stimuli by producing secretions called neurohormones. These secretions, which are often polypeptides (compounds similar to proteins but composed of fewer amino acids), pass along nerve-cell extensions, or axons, and are typically released into the bloodstream at special regions called neurohemal organs, where the axon endings are in close contact with blood capillaries. Once released in this way, neurohormones function in principle similar to hormones that are transmitted in the bloodstream and are synthesized in the endocrine glands.
The distinctions between neural and endocrine regulation, no longer as clear-cut as they once seemed to be, are further weakened by the fact that neurosecretory nerve endings are sometimes so close to their target cells that vascular transmission is not necessary. There is good evidence that hormonal regulation occurs by diffusion in plants and (although here the evidence is largely indirect) in lower animals (e.g., coelenterates), which lack a vascular system.
Hormones have a long evolutionary history, knowledge of which is important if their properties and functions are to be understood. Many important features of the vertebrate endocrine system, for example, are present in the lampreys and hagfishes, modern representatives of the primitively jawless vertebrates (Agnatha), and these features were presumably present in fossil ancestors that lived more than 500 million years ago. The evolution of the endocrine system in the more advanced vertebrates with jaws (Gnathostomata) has involved both the appearance of new hormones and the further evolution of some of those already present in agnathans; in addition, extensive specialization of target organs has occurred to permit new patterns of response.
The factors involved in the first appearance of the various hormones is largely a matter for conjecture, although hormones clearly are only one mechanism for chemical regulation, diverse forms of which are found in living things at all stages of development. Other mechanisms for chemical regulation include chemical substances (so-called organizer substances) that regulate early embryonic development and the pheromones that are released by social insects as sex attractants and regulators of the social organization. Perhaps, in some instances, chemical regulators including hormones appeared first as metabolic by-products. A few such substances are known in physiological regulation: carbon dioxide, for example, is involved in the regulation of the respiratory activity of which it is a product, in insects as well as in vertebrates. Substances such as carbon dioxide are called parahormones to distinguish them from true hormones, which are specialized secretions.
The hormones of vertebrates
Hormones of the pituitary gland
The pituitary gland, or hypophysis, which dominates the vertebrate endocrine system, is formed of two distinct components. One is the neurohypophysis, which forms as a downgrowth of the floor of the brain and gives rise to the median eminence and the neural lobe; these structures are neurohemal organs. The other is the adenohypophysis, which develops as an upgrowth from the buccal cavity (mouth region) and usually includes two glandular portions, the pars distalis and the pars intermedia, which secrete a number of hormones. The hormones secreted by the adenohypophysis are protein or polypeptide in nature and vary in complexity; as a result, their chemical constitution has not always been as fully characterized as has that of structurally simpler molecules of some other endocrine secretions. Functional analysis of these hormones also is difficult, for the targets of certain hormones of the adenohypophysis, called tropic, or trophic, hormones, are other endocrine glands. The action of such tropic hormones can be understood only in the light of the mode of function of the endocrine glands they regulate.
Adenohypophysis
Growth hormone (somatotropin)
Growth hormone is a protein, the primary structure of which has been fully established for the human and bovine forms of the hormone. It is probably universally distributed in gnathostomes (vertebrates with jaws), in which it is essential for the maintenance of growth, but its presence in agnathans (jawless vertebrates) has not yet been established with certainty. The physical and chemical properties of growth hormone, which differ from species to species, are associated with marked differences in biological activity. Only part of the molecule, however, is actually responsible for its biological activity, for up to 25 percent of it can be lost without causing any decline in potency.
Humans respond to growth hormones obtained from other primates, but the rat responds to those from a wide range of species. Even more striking, growth of teleost (bony) fishes, which stops if the pituitary gland is removed, can be restarted by treatment with mammalian growth hormone; on the other hand, preparations of pituitary glands from these fishes have no effect on the growth of mammals. The growth hormones of lungfishes, which are closely related to the terrestrial vertebrates, and of sturgeons, which are primitive members of the evolutionary line that led to bony fishes, affect mammalian growth, perhaps because these hormones have a more generalized molecular structure.
Growth is such a complex process that definition of the growth hormone’s mode of action is difficult. One of its known effects is an increase in the rate of protein synthesis, which is to be expected, since growth involves the deposition of new protein material. In addition, growth hormone affects the metabolism of certain ions (including sodium, potassium, and calcium), promotes the release of fats from fat stores, and influences carbohydrate metabolism in ways that tend to cause an increase in the level of glucose in the bloodstream. The last action creates a demand for an increased output of insulin (a hormone secreted by the pancreas), which acts to return the blood glucose level to normal. Prolonged treatment of dogs with growth hormone can overstrain the pancreatic tissue in which insulin is synthesized and bring about a diabetic condition, with insulin being formed in inadequate quantities. It is unlikely, however, that this is a factor in establishing diabetes mellitus in humans. Excess secretion of growth hormone does, however, have damaging effects in humans, for it produces overgrowth of the skeleton. If this occurs in youth, before the closure of the epiphyses (ends) of the long bones, it results in gigantism. If it occurs afterward, it causes acromegaly, in which the disturbance is more serious, with enlargement of the bones and soft tissues and consequent distortion of the skull.
Prolactin
Prolactin is a protein hormone that in female mammals initiates and maintains the secretion of milk, the mammary glands having been previously prepared for this function by the action of other hormones. In the female rat, prolactin also maintains the secretion of the hormone progesterone, which is formed by the corpus luteum, an endocrine gland of the ovary. Thus, prolactin is a gonadotropin, its target being an endocrine gland. Moreover, the molecular structures of prolactin and growth hormone are similar, which may explain why they show some overlap in biological properties. In particular, administration of prolactin promotes some growth in many terrestrial vertebrates. Human growth hormone has prolactin-like luteotropic properties.
Prolactin itself shows remarkable variety in biological action from one vertebrate group to another. It promotes the production of so-called crop-milk with which pigeons feed their young, and the associated changes in structure and arrangement of the wall of the crop provide a convenient means to assay the hormone. In certain newts (Triturus species) prolactin induces the change of behaviour that drives young animals into the water (water-drive action). In bony fishes, prolactin is concerned with the regulation of the level of sodium in blood plasma. It therefore is essential in some teleost species for the maintenance of life in fresh water. Although other teleosts (e.g., eels) can survive in fresh water after hypophysectomy, this means only that prolactin is but one factor in a complex regulatory mechanism involving several factors. Mammalian prolactin can regulate sodium metabolism when given to eels. Yet, although other convincing evidence suggests that the hormone must be present in the pituitaries of certain teleosts, preparations of their glands tested on pigeons do not have a typical crop-stimulating action. This evidence is best accounted for by supposing that the prolactin molecule has undergone evolutionary changes in its molecular structure and biological properties with respect to particular species and has also established specific adaptive relationships with target organs such as the crop and mammary glands
Adrenocorticotropic hormone
Adrenocorticotropic hormone (ACTH; also called corticotropin) is present in all jawed vertebrates but has not yet been decisively demonstrated in agnathans. It regulates the activity of part of the outer region (cortex) of the adrenal glands. In mammals its action on the adrenal cortex is limited to areas called the zona reticularis and zona fasciculata, in which important steroid hormones (e.g., glucocorticoids, such as cortisol and corticosterone) are formed; ACTH does not affect the synthesis of the mineralocorticoid hormone aldosterone, which takes place chiefly in the outer cortical region (zona glomerulosa). Evidence strongly suggests that the action of ACTH is mediated by a substance known as CAMP (cyclic 3′,5′-adenosine monophosphate), the rate of synthesis of which increases in adrenal tissue in the presence of ACTH; CAMP in turn promotes synthesis of enzymes necessary for the formation of cortisol and corticosterone. The relationship between ACTH and the adrenal cortex is an example of the negative feedback characteristic of endocrine systems; i.e., a decrease in the level of glucocorticoids circulating in the bloodstream evokes an increase in the secretion of ACTH, which, by stimulating the secretory activity of its target gland (the adrenal cortex), tends to restore to normal the level of glucocorticoids in the bloodstream. The release of ACTH can also be influenced by the level of circulating epinephrine (adrenaline), which is not surprising in view of the close functional relationship between the hormones of the adrenal cortex and medulla.
The ACTH of mammals is a polypeptide molecule consisting of 39 amino acids, only the first 20 of which are required for full activity. This region, often referred to as the active centre, is constant in composition in all mammals studied thus far; the remainder of the molecule varies slightly in amino acid composition among different species. Since, however, the mammalian hormone is active in all vertebrates, ACTH structure probably varies little from one class to another. The concept that biological activity is localized in an active centre of a complex molecule is applicable to other polypeptide and protein hormones, including growth hormone, the structure of which, as noted previously, can be partly lost without causing loss of activity. The concept of an active centre, however, raises the question of the function of the rest of the molecule. It may serve as the site of antigenic properties or of structural features important in establishing relations with specialized receptors in target cells.
Thyrotropin (thyroid-stimulating hormone)
Thyrotropin (also called thyroid-stimulating hormone, or TSH) regulates the thyroid gland through a feedback relationship similar to that for ACTH; thyrotropin increases the secretion of the hormones from the thyroid gland and, if its action is prolonged, evokes increases in cell number (hyperplasia) and in size of the gland. One consequence of an overactive thyroid in humans is a bulging of the eyes (exophthalmos). The cause of this is obscure, although it has been thought to result from the action of a distinct exophthalmos-producing substance that, while closely associated with thyrotropin, can be chemically separated from it. Thyrotropin, which is probably absent from agnathans, is a glycoprotein—i.e., a protein combined with carbohydrate. Its molecular weight is estimated to be about 26,000 to 30,000 in mammals. Some variability occurs in the degree of response obtained when a hormonal preparation from one species is tested on other species. This suggests, as with prolactin, that it has undergone molecular evolution.
Follicle-stimulating hormone
Follicle-stimulating hormone (FSH) is a type of gonadotropin; it is concerned with the regulation of the activity of the gonads, or sex organs, which are endocrine glands as well as the sources of eggs and sperm. FSH stimulates development of the graafian follicle, a small vesicle containing an egg, in the ovary of the female mammal. In the male it promotes the development of the tubules of the testes and the differentiation of sperm. FSH, like thyrotropin, is a glycoprotein, with an estimated molecular weight (in humans) of 41,000 to 43,000. The effects of FSH are discussed further in the section Hormones of the reproductive system, below.
Many hormones are secreted by special glands, such as thyroid hormone produced by the thyroid gland.
MAJOR TYPES OF HORMONE
Eicosanoids-
They are hormones made from lipids. They send messages near the cells that make the hormones.
Amino acid derived hormones
These are hormones that one nerve cell sends to another. Most of them are neurotransmitters.
Peptides, polypeptides and protein hormones
Examples of such hormones include growth hormones and insulin
Steroid hormones
These hormones are derived from cholesterol. They include testosterone and estradiol.
CLASSIFICATION OF HORMONES
Hormones are classified into five different categories. The categories include:
- According to chemical nature
- According to mechanism of action
- According to nature of action
- According to the effect and
- According to their stimulation of Endocrine glands.
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