Download III Semester Botany MODULE 7 ENDOCRINOLOGY

III Semester Botany
MODULE 7 ENDOCRINOLOGY
Syllabi: Endocrine glands and their hormones, mode of action (in brief), Hypothalamus, Pituitary,
Thyroid, Parathyroid, Thymus, Islets of Langerhands, Adrenal, Testis and ovary, Hormonal
disorders.
Endocrine Glands
Endocrinology is the science concerned with the endocrine glands and their
secretions (hormones), as well as diagnosis and treatment of disorders of the
endocrine system .The body contains two kinds of glands: exocrine and endocrine.
Exocrine glands secrete their products through ducts into body cavities and onto body
surface, and include salivary, sweat, and digestive glands.
By contrast, endocrine
glands always secrete hormones into the extracellular spaces around the secretory
cells, not into ducts.
The secretion then typically diffuses into capillaries and is
delivered to target cells by blood.
The endocrine glands include the pituitary, thyroid, parathyroids, adrenal,
pineal, and thymus glands. In addition, several organs in your body have areas made
up of endocrine tissues that produce specific hormones, for example the endocrine
pancreas, gonads (ovaries and testes), and hypothalamus. Also, the walls of the small
intestine, stomach, kidneys, and heart contain small pockets of endocrine cells.
Chemistry of Hormones
Most hormones belong to one of the following 4 chemical classes:
1. Peptide hormones: largest, most complex, and most common hormones.
Examples include insulin and prolactin
2. Steroid hormones: lipid soluble molecules synthesized from cholesterol.
Examples include gonadal steroids (e.g testosterone and estrogen) and adrenocortical
steroids (e.g. cortisol and aldosterone).
3. Amines: small molecules derived from individual amino acids.
Include catecholamines (e.g. epinephrine produced by the adrenal medulla), and
thyroid hormones.
4.Eicosanoids:
small
molecules
synthesized
from
fatty
acid
substrates
arachidonic acid) located within cell membranes. Include prostaglandins.
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(e.g.
Pituitary Gland
The pea-size pituitary gland is enclosed by sella turcica (Turk's saddle) of the
sphenoid bone and is connected to the hypothalamus by a funnel-shaped
infundibulum.
In humans, the pituitary gland has two major lobes: the anterior lobe or
adenohypophysis, composed of glandular tissue and the site of production and release
of 6 major hormones, as well as the posterior lobe or neurohypophysis, which is
actually part of the brain and is composed of neurons and pituicytes (glia-like
supporting cells) and is the site of release of two neurohormones produced by the
hypothalamus. The posterior pituitary only stores hormones, it is not involved in the
production of hormones.
The hypothalamus and the Posterior Pituitary
The hypothalamus is an important part of the brain with many functions.
Neurons of the hypothalamus synthesize and secrete two hormones, antidiuretic
hormone (ADH) and oxytocin. The cell bodies of these neurons are found in two nuclei
of the hypothalamus and have axons that descend along a thin stalk of tissue called
the infundibulum and end in the posterior pituitary gland. The hormones are released
at the axon terminals, diffuse into the capillaries and are distributed throughout the
body in the bloodstream.
ADH and oxytocin are peptides and are considered
neurohormones because of their secretion by neurons.
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Antidiuretic Hormone (ADH)
"Diuresis" means urine production. Antidiuretic is a substance that inhibits
formation of urine. The main target tissues for ADH are kidney tubules, which in the
presence of ADH reabsorb more water from urine and return it to the bloodstream,
thus maintaining blood volume. Inability to produce ADH causes diabetes insipidus
(watery urine), in which a person produces copious amounts of urine with a resultant
loss of ions from the blood. The condition can be corrected by the administration of
ADH.
Alcohol suppresses ADH production and release. When ADH is absent, the
kidneys don’t reabsorb as much water. The person drinking alcohol urinates more
often and may become dehydrated as a result. The symptoms of the drinker’s
“hangover”—headache, nausea, dizziness—are largely due to dehydration.
Oxytocin
Oxytocin, the other hormone made in the hypothalamus, causes uterine
contraction during childbirth and milk letdown when a baby is nursing. The more the
uterus contracts during labor, the more nerve impulses reach the hypothalamus,
causing oxytocin to be released. Similarly, the more a baby suckles, the more oxytocin
is released. In both instances, the release of oxytocin from the posterior pituitary is
controlled by positive feedback—that is, the stimulus continues to bring about an
effect that ever increases in intensity.
Recently, it has been suggested that oxytocin is important in sexual arousal
and orgasm in both males and females. In nonsexual relationships, oxytocin may be
responsible for promoting nurturing and affectionate behaviour (a "cuddle hormone").
Hypothalamus and Anterior Pituitary
The hypothalamus exerts control of the anterior pituitary by secreting tropic
hormones which are hormones that regulate the secretion of other hormones by either
stimulating or inhibiting their secretion. The anterior pituitary is non-neural tissue
which unlike the posterior pituitary has no direct nervous connection with the
hypothalamus. The tropic hormones secreted by the neurosecretory cells of the
hypothalamus reach the target cells of the anterior pituitary by the hypothalamicpituitary portal system.
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Tropic hormones from the hypothalamus act upon cells that secrete tropic
hormones in the anterior pituitary. Except for one, all hormones secreted by the
hypothalamus and anterior pituitary are peptides.
Hypothalamus
Tropic
Effect
Anterior Pituitary
Effect/ Target Organ
Prolactin
Mammary gland
development/milk secretion
Prolactin releasing
hormone (PRH)
+
Prolactin inhibiting
hormone (dopamine)
-
Thyrotropin releasing
hormone (TRH)
+
Thyroid stimulating
hormone (TSH)
Secretion of thyroid
hormone by thyroid gland
Corticotropin Releasing
Hormone (CRH)
+
Adrenocorticotropic
hormone (ACTH)
Adrenal Cortex
Growth and energy
metabolism/secretion of
insulin-like growth factors
by liver
Growth hormone
releasing hormone
(GHRH)
+
Growth hormone
Growth hormone
inhibiting hormone
(GHIH)
-
Growth hormone
+
Follicle stimulating
hormone
(FSH)/Luteinizing
hormone (LH)
Gonadotropin releasing
hormone (GNRH)
FSH: development of sex
cells/LH: secretion of sex
hormones and stimulation
of ovulation
Hormones of the anterior pituitary include:
1. Growth hormone (GH) or Somatotropin (STH)
2. Thyroid-stimulating hormone (TSH) or Thyrotropin
3. Adrenocorticotropic hormone (ACTH) or Corticotropin
4. Follicle-stimulating hormone (FSH)
5. Luteinizing hormone (LH)
6. Prolactin (PRL)
(These last two are collectively known as Gonadotropins)
Growth hormone
GH is responsible for general somatic growth by promoting increase in cell size
and the rate of mitosis. Although its major targets are the bones and skeletal muscles,
the effect on bones is only indirect: instead GH stimulates the liver to produce several
small proteins called somatomedins which directly stimulate growth of both cartilage
and bone, including direct effect on the epiphyseal plates of long bones.
In addition GH:
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1. Increase uptake of amino acids from the blood and increase protein synthesis
2. Increase uptake of sulfur needed for the synthesis of chondroitin sulfate into
cartilage matrix
3. Increase breakdown of fat leading to ↑ blood levels of fatty acids, and
4. Decrease the rate of glucose uptake and metabolism
Hypersecretion of GH in children results in gigantism, a condition in which
growth is extremely rapid. The person becomes abnormally tall (even 2.4 m or more)
but has relatively normal body proportions. On the other hand, hypersecretion of GH
in adults results in acromegaly (enlarged extremities). This condition is characterized
by overgrowth of bony areas still responsive to GH, namely bones of the feet, face, and
hands. Thickening of soft tissues leads to malformed facial features and an enlarged
tongue.
Hyposecretion of GH in children results in slowed long bone growth, a condition
called pituitary dwarfism. Such individuals attain a maximum height of only 1.2 m
and may have normal body proportions. However, lack of GH is typically accompanied
by deficiencies of other anterior pituitary hormones, including TSH and gonadotropins
(FSH and LH) resulting in malproportioned individuals who fail to mature sexually.
When diagnosed before puberty, pituitary dwarfism can be treated with GH
replacement therapy. Hypersecretion of GH in adults generally causes no problems.
Thyroid-Stimulating Hormone (TSH)
TSH controls development and secretory activity of the thyroid gland.
TSH release is induced by the TRH from the hypothalamus.
Adrenocorticotropic Hormone (ACTH)
ACTH stimulates the adrenal cortex to release corticosteroid hormones.
ACTH release is induced by hypothalamic corticotropin-releasing hormone (CRH).
FSH and LH (Gonadotropins)
In both sexes, gonadotropins release is stimulated by gonadotropin-releasing hormone
(GnRH) from the hypothalamus.
Gonadotropins regulate the function of the gonads (testes and ovaries):
FSH stimulates production of gametes (sperm or egg)
LH promotes production of sex steroids (testosterone or estrogen).
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Prolactin (PRL)
In humans, it is responsible for mammary growth and development and
lactogenesis (milk production) at the end of pregnancy in the female, as well as
regulation of testicular function in the male.
PRL release is stimulated by prolactin-releasing hormone (PRH - possibly
serotonin) and inhibited by prolactin-inhibiting hormone (PIH or dopamine). In
females, a brief rise in PRL levels just before the menstrual period partially accounts
for breast swelling and tenderness experienced by some women at that time, but since
this PRL stimulation is very brief, the breasts do not produce milk. In contrast, PRL
levels increase dramatically toward the end of pregnancy and milk production by the
breasts becomes possible. After birth, the infant's suckling stimulates PRH release in
the mother allowing for continued milk production.
Pineal gland
The pineal gland which is located in the brain secretes the hormone melatonin.
Melatonin appears to regulate circadian rhythms (day-night cycles).
Thyroid Gland
The thyroid gland is a large gland located in the neck, where it is attached to
the trachea just below the larynx. The parathyroid glands are embedded in the
posterior surface of the thyroid gland. The thyroid gland is composed of a large
number of follicles, each a small spherical structure made of thyroid cells filled with
triiodothyronine (T3), which contains three iodine atoms, and thyroxine (T4), which
contains four iodine atoms. These are the two forms of thyroid hormone; T3 is thought
to have the greatest effect on the body.
Effects of Thyroid Hormones
To produce triiodothyronine and thyroxine, the thyroid gland actively acquires
iodine. The concentration of iodine in the thyroid gland can increase to as much as 25
times that of the blood. If iodine is lacking in the diet, the thyroid gland is unable to
produce the thyroid hormones. In response to constant stimulation by the anterior
pituitary, the thyroid enlarges, resulting in a simple, or endemic, goiter. Some years
ago, it was discovered that the use of iodized salt allows the thyroid to produce the
thyroid hormones, and therefore helps prevent simple goiter.
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Thyroid hormones increase the metabolic rate. They do not have a single target
organ; instead, they stimulate all cells of the body to metabolize at a faster rate. More
glucose is broken down, and more energy is utilized.
If the thyroid fails to develop properly, a condition called congenital
hypothyroidism results. Severe hypothyroidism during the fetal development and in
infants is called cretinism. The child has a short disproportionate body, a thick tongue
and neck, and is mentally retarded. Cretinism may reflect a genetic deficiency of the
fetal thyroid gland or maternal factors such as lack of iodine in the diet.
The occurrence of hypothyroidism in adults produces the condition known as
myxedema, which is characterized by lethargy, weight gain, loss of hair, slower pulse
rate, lowered body temperature, and thickness and puffiness of the skin. The
administration of adequate doses of thyroid hormones restores normal function and
appearance.
.
In the case of hyperthyroidism (over secretion of thyroid hormone), as seen in
Graves’ disease, the thyroid gland is overactive, and a goiter forms. This type of goiter
is called exophthalmic goiter. The eyes protrude because of edema in eye socket
tissues and swelling of the muscles that move the eyes. The patient usually becomes
hyperactive, nervous, and irritable, and suffers from insomnia.
Parathyroid Glands
The parathyroid glands are small in size and are found on the posterior aspect of
the thyroid gland. Typically, there are four of them but the actual number may vary.
PTH, a small protein, is the single most important hormone controlling calcium
homeostasis. Its release is triggered by falling blood calcium levels and inhibited by
hypercalcemia (high blood calcium).
PTH release has the following consequences on these target organs:
1. Osteoclasts (bone absorbing cells) are stimulated to digest bone and release ionic
calcium and phosphates to the blood.
2. Kidneys are stimulated to reabsorb calcium and excrete phosphate
3. Absorption of calcium by the intestine increases. Vitamin D is required for
absorption of calcium from ingested food. For vitamin D to exert this effect, it must
first be converted by the kidneys to its active form and it is this conversion that is
directly stimulated by PTH.
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Calcium is essential for so many functions, including transmission of action
potentials, muscle contraction, pacemaker activity in the heart, and blood clotting,
precise control of ionic calcium levels in body fluids is absolutely critical. As a result
both hyper- and hypoparathyroidism can have severe consequences.
Hyperparathyroidism
Is rare, usually the result of a parathyroid gland tumor, and results in severe
loss of calcium from the bones. The bones soften and deform as their mineral salts are
replaced by fibrous connective tissue. The resulting hypercalcemia leads to, among
others, depression of the nervous system leading to abnormal reflexes and weakness
of the skeletal muscles, as well as formation of kidney stones as excess calcium salts
are deposited in kidney tubules.
Hypoparathyroidism
It is a PTH deficiency, which is a common consequence of parathyroid trauma
or removal during thyroid surgery. The resulting hypocalcemia increases excitability of
neurons and may lead to tetany resulting in uncontrollable muscle twitches and
convulsions, which if untreated may progress to spasms of the larynx, respiratory
paralysis and death.
Thymus
The thymus lies close to the heart and is critical for normal immune function
because T-lymphocytes mature here (which is the reason they are called T (T for
thymus) - lymphocytes). The thymus also secretes the hormone thymosin which
regulates T cell function.
Adrenal Glands
The adrenal glands sit atop the kidneys. Each adrenal gland consists of an
inner portion called the adrenal medulla and an outer portion called the adrenal
cortex. The adrenal medulla is under nervous control, and the adrenal cortex is under
the control of ACTH (also called corticotropin), an anterior pituitary hormone. Stress of
all types, including emotional and physical trauma, prompts the hypothalamus to
stimulate the adrenal glands.
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Adrenal Medulla
Epinephrine (adrenaline) and norepinephrine (noradrenaline) produced by the
adrenal medulla rapidly bring about all the body changes that occur when an
individual reacts to an emergency situation. The release of epinephrine and
norepinephrine achieves the same results as sympathetic stimulation—the “fight-orflight” responses: increased heart rate, rapid respiration, dilation of the pupils, etc.
Thus, these hormones assist sympathetic nerves in providing a shortterm response to
stress.
Adrenal Cortex
There are three layers in the adrenal cortex, and each produces a different set
of steroid hormones. The hormones produced by the adrenal cortex provide a longterm response to stress. The two major types of hormones produced by the adrenal
cortex are the mineralocorticoids and the glucocorticoids. The mineralocorticoids
regulate salt and water balance, leading to increases in blood volume and blood
pressure. Glucocorticoids influence metabolism of most body cells, help us resist
stress, and are considered to be absolutely essential to life. The most important
glucocorticoid
in
humans
is
cortisol,
but
small
amounts of
cortisone
and
corticosterone are also produced. The glucocorticoids regulate carbohydrate, protein,
and fat metabolism, leading to an increase in blood glucose level. Cortisone, the
medication often administered for inflammation of joints, is a glucocorticoid.
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The adrenal cortex also secretes small amounts of both male and female sex
hormones -Gonadocorticoids (Sex Hormones) regardless of one’s gender. Both male
and female sex hormones promote skeletal growth in adolescents. The male hormones
from the adrenal gland stimulate the growth of axillary and pubic hair at puberty. In
addition, male hormones help to sustain the sex drive, or libido, in both men and
women.
Hyperadrenalism
When the level of adrenal cortex hormones is high due to hypersecretion, a
person develops Cushing’s syndrome. It can be caused by a cortisol-secreting tumor in
the adrenal glands, ACTH-secreting tumor of the pituitary, or ACTH secreted by
abdominal carcinoma. The symptoms include a persistent hyperglycemia, dramatic
loss of muscle and bone proteins, and water and salt retention, leading to
hypertension and edema. The face is moon-shaped due to edema. Masculinization may
occur in women because of excess adrenal male sex hormones.
Hypoadrenalism
When the level of adrenal cortex hormones is low due to hyposecretion, a
person develops Addison’s disease. It involves significant reduction in plasma glucose
and sodium, very high levels of potassium and loss of weight. Without cortisol, glucose
cannot be replenished when a stressful situation arises. Even a mild infection can lead
to death. The usual treatment is corticosteroid replacement therapy.
Pancreas
The pancreas is a long organ that lies transversely in the ab-domen between the
kidneys and near the duodenum of the small intestine. It is composed of two types of
tissue. Exocrine tissue produces and secretes digestive juices that go by way of ducts
to the small intestine. Endocrine tissue, called the pancreatic islets (islets of
Langerhans), produces and secretes the hormones insulin and glucagon directly into
the blood.
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The two antagonistic hormones, insulin and glucagon, both produced by the
pancreas, help maintain the normal level of glucose in the blood. Insulin is secreted
when the blood glucose level is high, which usually occurs just after eating. Insulin
stimulates the uptake of glucose by most body cells. Insulin is not necessary for the
transport of glucose into brain or red blood cells, but muscle cells and adipose tissue
cells require insulin for glucose transport. In liver and muscle cells, insulin stimulates
enzymes that promote the storage of glucose as glycogen. In muscle cells, the glucose
supplies energy for muscle contraction, and in fat cells, glucose enters the metabolic
pool and thereby supplies glycerol for the formation of fat. In these ways, insulin
lowers the blood glucose level.
Glucagon is secreted from the pancreas, usually between meals, when the blood
glucose level is low. The major target tissues of glucagon are the liver and adipose
tissue. Glucagon stimulates the liver to break down glycogen to glucose and to use fat
and protein in preference to glucose as energy sources. Adipose tissue cells break
down fat to glycerol and fatty acids. The liver takes these up and uses them as substrates for glucose formation. In these ways, glucagon raises the blood glucose level.
Diabetes Mellitus
Diabetes mellitus is a fairly common hormonal disease in which insulinsensitive body cells are unable to take up and/or metabolize glucose. Therefore, the
blood glucose level is elevated—a condition called hyperglycemia. Because body cells
cannot access glucose, starvation occurs at the cell level. The person becomes
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extremely hungry—a condition called polyphagia. As the blood glucose level rises,
glucose will be lost in the urine (glycosuria). Glucose in the urine causes excessive
water loss through urination (polyuria). The loss of water in this way causes the
diabetic to be extremely thirsty (polydipsia). Glucose is not being metabolized, so the
body turns to the breakdown of protein and fat for energy. Fat metabolism leads to the
buildup of ketones in the blood, and excretion of ketones in the urine (ketonuria).
Because ketones are acidic, their buildup in the blood causes acidosis (acid blood),
which can lead to coma and death.
Secondary Endocrine Organs
A number of organs, in addition to their primary functions, secrete hormones or
participate in their activation. These secondary endocrine organs include:
Heart - Secretes atrial natriuretic peptide which regulates sodium reabsorption by the
kidneys.
Kidneys - Secretes erythropoietin (EPO) which stimulates production of red blood cells
in the bone marrow.
Digestive organs - Secrete several hormones that regulate digestion and absorption of
food.
Liver - Secretes insulin-like growth factors (IGF's) which promote tissue growth.
Skin and Kidneys - Activate vitamin D3 which regulates blood calcium levels.
Gonads
The ovaries and testes in addition to producing the sex cells are major sites for
the production of sex hormones.
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