INTRODUCTION
Hormones are ductless endocrine glands which
are transported by the blood stream to other parts of the body. All endocrine
structures develop from epithelia or nervous tissue or from both. Hormones do many
things. They regulate metabolism. Metabolism is all of the chemical and energy
reactions that happen in a living thing. Hormones cause the growth and death of
cells and of whole organisms. Hormones also start and control sexual
development. For example, the hormones estrogen and progesterone make girls puberty.
Hormones help keep homeostasis in an organism. Homeostasis means to keep a
constant state inside the body like temperature, amount of water and salts, and
amount of sugar. Hormones released by one gland can also tell other glands to make different hormones.
Hormones have both positive and negative feedback.
Hormone regulation is mostly done by negative feedback
In negative feedback, a
hormone causes an effect. The
cells that make the hormone detect this effect.
Upon detection of the hormone, its production ceases.
A good example of negative feedback is with the
hormone, insulin. Insulin is produced by the pancreas. Insulin is released by
the pancreas in response to consumption of glucose. The amount of glucose in
the blood rises and the pancreas detects this increase. It then secretes
insulin into the blood. Insulin increases glucose uptake in target cells. Some
glucose is used by the cells but some is also converted to and stored in the
form of glycogen. Glucose uptake by cells decreases blood glucose levels - this
decrease is detected by the pancreas and in response, it stops secreting
insulin in to the bloodstream. As insulin levels in the blood decrease, as does
glucose uptake by cells. This negative
feedback therefore helps to maintain normal blood glucose levels and
prevents extreme changes.
Types of Hormones
There are three main
types of hormones: steroid hormones - these are non-polar and do not need a
receptor. The other is peptide hormones. The last is Tyrosine derivative
hormones, produced by the thyroid.
Counter regulatory hormones
Sometimes two or more
hormones control the same thing. For example, blood glucose is very important
to an organism. So it is not controlled by just one hormone. Other hormones
also make the glucose level go up or down. If the glucose level gets too low, the body releases hormones
that do the opposite of
insulin. They do not tell the cells in the body to take up glucose from the
blood. They tell the cells to put glucose back into the blood. These kind of
hormones that work opposite of other hormones are called counter-regulatory hormones.
Counter-regulatory hormones for insulin are glucagon and epinephrine.
Positive feedback
Most important things in
an organism are kept in homeostasis by negative
feedback and counter-regulatory
hormones. However a few things are controlled in different ways. One
rare way is positive feedback.
In negative feedback, the hormone's effect makes a gland stop making hormones.
In positive feedback the opposite happens. The effect of the hormone tells the
gland to make even more
hormones.
An example of positive feedback
is the hormone that causes childbirth
(when babies are born.) The hormone that causes this is oxytocin. This hormone is made by the pituitary gland. When the
baby starts coming out, it stretches the muscle in the cervix (the bottom of
the uterus.) Nerves in the cervix send a message to the pituitary. This message
makes the pituitary release more oxytocin. The oxytocin then causes the muscles
of the uterus to contract, or squeeze.
This causes more stretching in
the cervix. This stretching then tells the pituitary to make even more oxytocin. So levels of
oxytocin keep rising until the squeezing or contractions of the uterus force the baby out.
THE HYPOTHALAMUS
They are paired
thickening in the thalamencephalon and thus nervous in origin.They control most
of the endocrine structures,acting through the pituitary and autonomic
system.Groups of cells produced neurosecretion;droplets produced in the cell traverse
the axons into the pars narvosa,to be stored and later released into the
blood’Other neurosecretions that is releasing factors pass in the blood or the
hypothalamo-hypophyseal portal system into the pars anterior where the
stimulate production of other hormones.
Two hormones plus a number of releasing factors
are produced.Oxytocin stimulates contraction of the uterus during birth,and
promote secretion of milk during suckling.Vasopressin cause a rise in blood
pressure by contaction of arteriols,and is also antidiuretic,favourind
resorption of water and sodiun ions from the renal tubules.Specific releasing
factors affect the secretion of all hormones produced in the pars anterior and
pars intermidia.
THE
PITUITARY: The pituitary body lies beneath the thalamencephalom. It is formed by
fusion of a hollow down-growth from the brain,the infundibulum,with a dorsal
invagination from stomodaeum,the hypopysis and it thus nervous and epithelia in
origin.The infundibulun becomes the pars navosa;the endocrine parts hypophysis
are the pars anterior and pars imtermedia.The former secrets six
hormones.Somatotropin STH or growth hormones stimulates rate of protein
synthesis, excess produces gigantism, and deficiency, dwarfness; the hormones
also promotes production of glucagon in pancreas.Thyrotropic hormones stimulates develpoment of the thyroid and
promotes production of thyrosin.adrenocorticotropic hormones induce production
of corticoids from the adrenal.Follicle stimulating hormone promotes develpoment
of Grafian follicles in females,and of sperm in males.Luteinizing hormone LTH
stimulate ovulation and formation of corpora lutea. Prolactin affect the
development of the mammary glands and the maintenance of milk scretion. The
pars produces two hormone both melanocyte-stimulating; they affect formation
and distribution of melanin granules in the chromatophores
THYRIOD
GLAND:
Is a mass of soft, red vesicular tissue on each side of the laryns. It develops
from epithelium in the floor of the pharyns and I homologous with the endostyle
of lower chordates. The hormone tyroxin regulate the basic rate of metabolism
i.e BRM; deficiency lower the BRM and lead to hypotyriodism. In the young, this
causes retardation of development, a condition known as cretinism or mentally
retarded dwarfness; in adult deficiency lead to sluggish metabolism, atrophy of
gland and swelling of neck; the condition of mysoedema. Excess of thyrocin
causes hyperthyroidism; there is a high BRM, restless activity, rapid heart
beat and general wasting of the body.The gland becomes enlargeg and the eye
balls protrude;a condition known as exophthalmic goitre. Deficiency of iodine
may cause simple goitre.The effects of thyroxin in promoting metamorphosisin
amphibian and some fishes are notable.
THE SUPRARENAL GLANDS are small, yellow, ovoid structures, one anterior to
each kidney. The cortex is derived from neural crests and the medulla from
coelomic epithelium. The medulla secretes adrenalin and nor –adrenalin after
sympathetic stimulation. This produce
dramatic changes in the body in response to fear, stress and shock. The three
zones of the cortex secrets different hormones ;the outer zone produce
mineralocorcorticoids called aldosterone and dexycorticosterone which play a
vital part in osmoregulator; the middle zone secrete the glucocortcoids, cortisone
and cortisol, which affect carbohydrate metabolism; the inner zone produce
androgen and oestrogen which promotes development of the secondary sexual
characteristics.
THE PARATHYROID; are two pairs of small glandular masses, lateral to
the thyroid, and derived from epithelium the third and forth visceral and
clefts. The secrtion, parathormone affect calcium metabolism, playing a large
part in exchange of calcium and potassium sulphate ion between bone and blood. It
also affect excretion and resorption of these ions.
THE THYMUS GLAND: derived from epithelium dorsal to the visceral
clefts, is a large, soft, pink mass above the heart; it degenerate considerably
with age. In the young, it is the main source of lymphocytes and is important
in immunological reactions. The hormone, thyroxin, stimulate lymphocyte
production in the lymphatic glands.
THE PINEAL BODY: Is a hollow dorsal outgrowth of the thalamencephalon
in lamprey larvae and some reptiles, it has a complete eye-like structure, and
I the formal, it has an undoubted effect on skin colour. In amphibians, the
hormone melatonin cause blanching of the skin; the same effect may occur in
mammals. Melatonin certainly stimulates maturation of the gonads, probably by
stimulating secretion of gonadotropic releasing factors from the hypothalamus.
THE PANCREAS; is derived from the endodermal epithelium. The
islets of lngerhans conyains two types of cells; the large cells which secretes
insulin and the smaller cells which secretes glucagons. The former promotes
synthesis of glycogen in the liver from blood hexose; the latter favours breakdown of liver glycogen to
glucose. The bigger cells are sensitive o to blood sugar level and smaller
cells are stimulated by STH. Deficiency of insulin results in diabetes
mellitus.
The ovaries, like the testes are derived from coelomic
epithelium.The follicular phase of oestrogen cycle is activated by FSH; this induces development of the follicle
stimulating them to produce oestrogen, the most potent being oestradiol. These
activate the reproductive tract in preparation for pregnancy and stimulate
development of mammary glands and secondary sexual characteristics. The luteal
phase is activated by LH which induce pair of ruptured follicles and growth of corpora
lutea;the latter activated by prolactin ,secretes progesterone which inhibits
further ovulation, prepare the uterus for implantation and induces further
development of the mammae. Relaxin, from the corpora lutea, promotes dilataion of
the cervix and relaxation of pubic symphysis during birth
The placenta is epithelial in origin ,part from the
uterine lining, part from the trophoblast. Three hormones are secreted; a
gonadotropins, oestroegn and progesterone; influence growth of the uterus and
mammae after ovarian secretions have waned, and assist in maintenance of corpora lutea until birth
The testes, after stimulation by FSH, produces at
least two androgens, androsterone and testosterone,which play a part in
spermatogenesis, control activity of seminal vesticle and prostrate gland and
helps in the formation of the male sexual characteristics.
In the alimentary canal, the gastric mucosa secretes
gastrin, which induced continued flow of gastric juice; entrocrinin, which
starts enzymes secretion in the duodenum, and gastric secretin, which promotes
secretion of salt solution from the pancreas. The duodenal mucosa produces
secretin, which stimulates further flow of pancreatic salt solution; pancreomysin
induces secretion of pancreatic enzymes; cholecystokinin causes contraction of
the gall bladder and entrogastrone inhibits acid secretion by the stomach.
NERVE-ENDING: Secrete hormones which excite adjacent cells, sympathetic
fibres secrete sympathin, identical in effect with adrenalin; parasympathetic
and somatic motor fibres produces acetycholine, which also facilitates
transmission of impulses across synapses.
Tissues hormones; histamine and acetycholine, are both
powerful depressor, which are probably present in inactive form in all cells. They
are activated when cells are damaged and play important part in the process of
healing.
CONCLUSION
Hormones
are ductless glands that transfer information and instructions between cells in
animals, often described as body’s chemical messengers and these chemicals are
transported to target organs or tissues. Hormones regulates growth and
development ,control the functions of various tissue ,support reproductive
functions and regulate metabolism[the process used to breakdown food to create
energy]
REFERENCES
"Hormones". http://www.nlm.nih.gov/medlineplus/hormones.html.
Crisp TM, Clegg ED, Cooper RL, Wood WP, Anderson DG, Baetcke KP, Hoffmann
JL, Morrow MS, Rodier DJ, Schaeffer JE, Touart LW, Zeeman MG, Patel YM (1998). "Environmental endocrine
disruption: An effects assessment and analysis". Environ.
Health Perspect. 106 (Suppl 1):
11–56. PMC 1533291. PMID 9539004. www.ncbi.nlm.nih.gov/pmc/articles/PMC1533291/.
Beato M, Chavez S and Truss M (1996). "Transcriptional regulation by
steroid hormones". Steroids 61
(4): 240–251.
Hammes SR (2003). "The further redefining
of steroid-mediated signaling". Proc Natl Acad Sci USA 100 (5): 21680–2170. doi:10.1073/pnas.0530224100.
PMC 151311. PMID 12606724.
//www.ncbi.nlm.nih.gov/pmc/articles/PMC151311/.
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