Hormones are organic substances that are secreted by plants and animals and that
function in the regulation of physiological activities and in maintaining
homeostasis. They 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.
. Among animals, the hormones of the vertebrates--particularly those of humans
and other mammals--are the best known. Most vertebrate hormones originate in
specialized tissues, called endocrine tissues, and are carried to their targets
through the bloodstream. Endocrine glands. A major endocrine gland in
vertebrates is the pituitary, which consists of two distinct sections: the
anterior pituitary (or adenohypophysis) and the posterior pituitary (or
neurohypophysis). The anterior pituitary is sometimes called the "master
gland," because it secretes several hormones that affect the other
endocrine glands. For example, the anterior pituitary hormones thyrotropin and
adrenocorticotropic hormone (ACTH) regulate endocrine activity in the thyroid
and the outer region (cortex) of the adrenal glands, respectively. The anterior
pituitary also secretes hormones that affect the sex glands. One of these is
follicle-stimulating hormone (FSH), which stimulates egg production in the
ovaries and sperm production in the testes. Another is luteinizing hormone (LH).
In females, LH works in conjunction with FSH to regulate the female reproductive
cycle and the secretion of female sex hormones. In males, LH controls the
production of the male sex hormones. Other hormones produced in the anterior
pituitary include growth hormone, which is responsible for normal body growth,
and prolactin, which promotes milk production in female mammals. Its designation
as the master gland notwithstanding, the anterior pituitary itself is regulated
by substances called releasing hormones that are secreted by the hypothalamus,
the part of the brain located directly above the pituitary. These hypothalamic
hormones stimulate--or, in some cases, inhibit--the secretions of the anterior
pituitary. The posterior pituitary stores and releases two hormones: oxytocin,
which causes the uterus to contract during birth, and vasopressin, which acts on
the kidneys to restrict the output of urine. These two hormones are actually
produced by the hypothalamus, which is linked directly to the posterior
pituitary. Other endocrine glands in vertebrates include the thyroid,
parathyroids, adrenals, pancreas, and gonads (sex glands). The thyroid produces
hormones that control metabolic rate and oxygen consumption. Hormones from the
parathyroids are concerned with calcium concentration in the blood, and the
pancreas releases insulin and glucagon, hormones that, respectively, lower and
raise the blood-sugar level. Hormones from the adrenal cortex regulate glucose
and sodium metabolism. Those secreted by the central portion (medulla) of the
adrenals affect the heart and the circulatory and respiratory systems; these
hormones are important in helping an individual cope with stress. The heart
itself releases a hormone-- atrial natriuretic peptide--that helps regulate
blood pressure, blood volume, and the salt and water balance within the blood.
(see also Index: thyroid hormone, parathormone) The female sex hormones--the
estrogens and progesterone--are produced by the ovaries. Together with FSH and
LH, these hormones control the cyclical changes in the female reproductive
system--the menstrual cycle in human females and the estrous cycle in other
female mammals. The estrogens also are responsible for female sexual
characteristics. Progesterone is concerned with the maintenance of pregnancy.
Male sex hormones--known as androgens--include testosterone, which is secreted
by the testes. Testosterone is responsible for the maintenance of male sexual
characteristics. Hormone chemistry. Structurally, vertebrate hormones fall into
two main classes. Those of the adrenal cortex and the sex organs are steroids, a
major class of lipid compounds. Virtually all other known vertebrate hormones
consist of amino acids. Most nonsteroidal hormones are composed of chains of
amino acids--either short chains (polypeptides) or long chains (proteins). The
hormones of the adrenal medulla, however, are composed of amino acid derivatives
called amines, those of the thyroid of a single amino acid combined with atoms
of iodine. It is believed that hormones achieve their effects on target tissues
and organs through either of two mechanisms. The steroid hormones and the
hormones of the thyroid can, as a result of their chemical structures, pass
through cell membranes. These hormones apparently enter a target cell and
combine with an intracellular receptor protein. The hormone-receptor complex
then enters the cell's nucleus, where it apparently affects the activity of
specific genes. Genes carry the cell's hereditary blueprint for protein
synthesis, and so the interaction of the hormone-receptor complex with the genes
influences the cell's production of proteins. Because many proteins function as
enzymes within the cell, this influence on protein synthesis can have
far-reaching effects on the cell's activities. The polypeptide, protein, and
amine hormones are believed to operate by a different mechanism. These hormones
do not enter the target cell; instead, they combine with a receptor protein on
the cell's outer membrane. This hormone-receptor complex apparently triggers an
enzyme in the membrane, causing the synthesis of a so-called
"second-messenger" compound within the cell. This second messenger--in
many cases, cyclic adenosine monophosphate (cyclic AMP)--apparently activates
enzyme systems that bring about the desired action by the cell. It is
interesting to note that both proposed mechanisms of hormone action involve the
binding of the hormone to a specific receptor molecule. This feature accounts
for the specificity of hormones; a hormone can have an effect only on cells that
possess the appropriate receptor. Hormones probably exist in all invertebrates.
In insects, neurosecretory cells in the brain produce thoracotropic hormone.
This hormone stimulates glands in the thorax to secrete the hormone ecdysone,
which causes the periodic molting, or shedding, of the hard exoskeleton. Another
insect hormone, called juvenile hormone, maintains the larval state. A decrease
in juvenile hormone triggers the development of the adult characteristics.
Plants also have a hormonal system, which includes auxins, gibberellins, and
cytokinins, all of which promote growth. Plant hormones also include several
growth inhibitors, which regulate such activities as the fall of deciduous
leaves in autumn and the development of dormancy in buds and seeds.