Physiological Mechanisms
Hormonal Control
Dr Smita Bhatia
BP-5, II floor,
Shalimar Bagh (West)
Delhi 110088
Contact: 27483738
Email: smitabhatia@edscientia.com
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Learning objectives
Chemical nature of hormones
Transport of hormones
Mechanism of hormone action
Hormone interactions
Control of hormone secretion
Clearance of hormones
Major endocrine glands, their secretions and disorders
Hypothalamic-hypophyseal axis
Thyroid gland
Parathyroid glands
Adrenal glands
Pancreatic islets
Gonads and placenta
Thymus
Pineal gland
Other endocrine tissues
In addition to other homeostatic mechanisms of the body, one of the two major regulatory
systems of the body is the endocrine system (the other being the nervous system). This
system comprises the endocrine glands that release their secretions, called hormones, into
the blood stream which transports it to the various target organs on which these hormones act
to activate, inhibit or, modify certain functions. Hormones are released into the blood stream
rather than directly reaching the target organs because these glands have no ducts (ductless
glands) to convey their secretions (also because there could be many target organs for a
single hormone so it would not be possible to take these secretions to each and every organ
by means of ducts).
Why do hormones act on certain specific organs and not on others?
This is due to the presence of receptors in/on the target cell. These receptors are protein, or
glycoprotein molecules, which can bind to the hormone. The location of receptors differs
within a cell for different types of hormones. These receptors may be present:

On the cell: For the protein peptide and catecholamine hormones.

In the cytoplasm of the cell: Steroid hormones (since these hormones can readily enter a
cell).

In the cell nucleus: Thyroid hormones (as these hormones can readily enter the cell
because of their lipid soluble nature) where they directly affect the genes.
A specific change occurs after the hormone binds to the receptor (see mechanism of hormone
action). The number of receptors on the cell surface is regulated by the concentration of the
circulating hormone. If the concentration is very high the number of receptors decreases so
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that the cell becomes less sensitive to the hormone. This is known as down-regulation. If the
concentration of the hormone becomes low, the number of receptors increases to increase the
sensitivity of the cell to the hormone. This is known as up-regulation.
Differences between the two major regulatory systems of the body—the endocrine and
the nervous system
Nervous system
Neurotransmitters are released which
act locally
Act on muscle cells, gland cells and
other neurons
Effect of neurotransmitters occurs
within a short span of time (msec)
Effect lasts for a short time (msec)








Endocrine system
Hormones are released which can be
carried anywhere in the body
Act on a variety of cells
Effect of hormones may take seconds
to hours to days to occur
Effect may last for a long time (seconds
to days)
Functions of hormones

Help to regulate the chemical composition and volume of the various components of
the body, e.g. plasma, interstitial fluid.

Help regulate the metabolism and energy balance.

Help regulate the contraction of smooth and cardiac muscle fibres.

Help regulate glandular secretion and some immune system activities.

Control growth and development.

Regulate the functioning of the reproductive system.

Help establish circadian rhythms.

Help regulate the interaction between the environment and the body.
Chemical nature of hormones
Hormones are of different types:
Protein or peptide hormones. These are made up of amino acids. They are water soluble.
Peptides are made up of 3 to 49 amino acids. e g. oxytocin and insulin. Protein hormones are
made up of 50 to 200 amino acids e.g., thyroid stimulating hormone (TSH), follicle
stimulating hormone (FSH). These are produced as biologically inactive precursor molecules
(pre-prohormones) by the rough endoplasmic reticulum of the gland cell. These preprohormones are then cleaved into prohormones which are also biologically inactive.
Prohormones are then packaged into vesicles as hormones by the Golgi body. These vesicles
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are stored in the cytoplasm near the plasma membrane from where they are secreted by
exocytosis on an appropriate stimulus.
Steroid hormones. These hormones are derived from cholesterol e.g., testosterone, cortisol,
progesterone. They are lipid soluble. These are not stored in the cytoplasm but are
synthesized from cholesterol when needed and are secreted directly by passing through the
plasma membrane as they are lipid soluble.
Biogenic amines. They are derived from amino acids. They are of different types:

Thyroid hormones and catecholamines. Thyroxine (T4) and triiodothyronine (T3) are
secreted by the thyroid gland. Catecholamines include epinephrine and non-epinephrine
secreted by the adrenal medulla and dopamine secreted by the hypothalamus and other
brain cells. They are all derivatives of the amino acid tyrosine. Thyroxine is synthesized
in the thyroid follicles where they are stored with thyroglobulin (a glycoprotein). When
needed, thyroxin is released from the thyroglobulin into the blood where it combines with
the thyroxin-binding globulin. Catecholamines are stored in the vesicles in the cytoplasm
which are released by exocytosis when needed. Catecholamines are water soluble while
thyroid hormones are lipid soluble because they are iodinated.

Histamine secreted by the mast cells is derived from amino acid histidine.

Serotonin (or 5-hydroxytrptamine, 5-HT) and melatonin. Both are derived from the
amino acid tryptophan. Serotonin is secreted by certain brain cells and melatonin is
secreted by the pineal gland.

Eicosanoids. These are different types of hormones derived from the fatty acid
arachidonic acid containing 20 carbon atoms. Eicosanoids include prostaglandins (like
PGF2), prostacyclins and leukotrienes. These are water-soluble.

Nitric oxide. Though it is a gas, it is produced as a hormone as well as a neurotransmitter.
It is lipid soluble.
Transport of hormones
The secretion, transport and mechanism of action of these hormones depends on their polar
or non-polar nature i.e., whether they are water soluble or lipid soluble.
The water-soluble hormones do not need any carrier molecules in the plasma, where they can
circulate freely in the aqueous medium. But lipid-soluble hormones cannot be transported as
free molecules in the aqueous plasma and are transported by carrier proteins. In addition to
transporting these hormones these carrier proteins also,

Prevent filtration of small lipid hormones through the glomerulus in the kidneys thus
increasing their half-life.

Provide a readily available stock of these hormones circulating in the blood.
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Mechanism of hormone action
Sequence of events of the action of a lipid molecule
Lipid soluble hormones
Lipid-soluble hormones bind to
the receptors present inside the
target cells because these
hormones can cross the plasma
membrane.
Lipid hormone is released from the blood into the
interstitial space
It crosses the plasma membrane of the cell and binds to
specific receptors inside the cell
The hormone-receptor complex turns certain specific genes
on or off.
Synthesis of certain specific mRNA (and hence specific
proteins) is stimulated or inhibited.
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Water soluble hormones
Since these hormone molecules cannot enter the cell they bind to receptors on the surface of
the target cell and trigger the formation of another molecule within the cell. Here, the
hormone molecule is known as the first messenger molecule and the molecule formed within
the cell due to its binding is known as the second messenger.
Sequence of events of the action of a water molecule (Figure 1)
Water-soluble molecule binds to the receptor (it is a transmembrane protein) on the surface of
the molecule
Hormone-receptor complex activates a membrane bound (bound to the inner side of the plasma
membrane) protein—the G-protein (which binds to a GTP molecule and releases a GDP molecule)
G- protein activates enzyme adenylate cyclase
Adenylate cyclase catalyses the conversion of ATP into cyclic AMP (cAMP). (This cAMP is the
second messenger)
cAMP activates a protein kinase
Protein kinase phosphorylates other cellular proteins
On phosphorylation some cellular proteins get activated while some other get inhibited
Some physiological processes are stimulated or inhibited (depending upon whether the protein
regulating this process has been activated or inhibited)
After some time an enzyme phosphodiesterase breaks down the second messenger to stop this
sequence of events till another hormone molecule binds to the receptors to trigger this again.
Fig 1: Mechanism of G-protein mediated action of water soluble hormones
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Different protein kinases exist in different cells or within the same cell, while one type of
protein kinase may stimulate an activity by phosphorylating a protein another protein kinase
may inhibit another activity by phosphorylating another protein.
In addition to cAMP, other second messengers include cGMP (cyclic guanosyl
monophosphate), inositol phosphate (IP3) and diacyl glycerol (DAG). Nitric oxide which
causes vasodilation by stimulating the relaxation of smooth muscle fibres in blood vessels
acts by stimulating the formation of cGMP (the secondary messenger) which stimulates the
transport of Ca2+ into storage areas of the smooth muscle fibre from the cytosol. When
cytosol Ca2+ ion concentration decreases it results in the relaxation of muscle fibres.
Some hormones cause the opening or closing of specific ion-channels in the cell membrane
to initiate the entry or exit of certain ions to produce a specific effect (this effect may be
produced through the G-protein).
Hormone interactions
The action of a hormone is dependent upon



Its concentration in the plasma
The number of receptors of the hormone
Interaction with other hormones
In addition to the concentration of the hormone and the number of receptors present a
hormone’s interaction with other hormones also affects its effectiveness. The different
types of interactions that a hormone can have with other hormones are:

Permissive effect. When prior exposure to one hormone facilitates the action of another
hormone, e.g. exposure of the uterine cells to estrogen and FSH during follicular phase
facilitates the action of progesterone during the luteal phase of the menstrual cycle.
Exposure to estrogen and FSH also causes the development of receptors for progesterone
in the uterine cells.

Synergistic effect. When the effect of two hormones is greater than their independent
effects. Thus, these hormones work together to produce an effect, e.g. both FSH and
estrogens are required for the development of an ovarian follicle.

Antagonistic effect. When the effect of one hormone is opposite to the effect of another
hormone, e.g. parathyroid hormone from parathyroid gland increases blood calcium
levels while calcitonin from C cells of thyroid reduces blood calcium levels. Normally,
antagonistic hormones are not released at the same time because that would be a waste of
energy.
Control of hormone secretion
Secretion of a particular hormone can be regulated by three mechanisms:

By neural control, e.g., release of epinephrine and nor-epinephrine from the adrenal
medulla is controlled by the sympathetic nervous system.
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
By another hormone, e.g. release of thyroxin from the thyroid gland is stimulated by the
thyroid stimulating hormone (TSH) from the anterior pituitary.

Through negative or positive feedback:
Negative feedback
When the secretion of a hormone is inhibited by
an effect produced in the target cell, e.g. FSH that
stimulates the secretion of estrogen from the
ovary is suppressed when estrogen levels reach a
particular concentration. The effect produced
(secretion of estrogens) by a hormone (FSH from
the anterior pituitary) inhibits the secretion of the
hormone (FSH) that caused it (Figure 2).
Fig 2: Negative feedback control
FSH from anterior pituitary
Follicles in the ovary are stimulated
–

Estrogen secretion
Increased levels of estrogen
Shows the negative feedback
Positive feedback
Secretion of certain hormones is stimulated by the effect that it produces, e.g. oxytocin from
the posterior pituitary enhances uterine contractions during parturition (birth of a baby). This
causes the baby to descend to the cervix, further stretching the cervix which further
stimulates the release of oxytocin. The positive feedback cycle is broken by a sudden change
in the events of the cycle, e.g. in case of oxytocin, the cycle breaks when the baby is born.
In addition to the positive and negative feedback regulation of the hormone secretion there
are periodic variations in their secretion also. These variations are dependent on seasonal
changes, the circadian rhythm (an inherent rhythm), aging, stages of development and sleep,
e.g., the levels of growth hormone increase during early stages of sleep and then reduce.
Clearance of hormones

Binding with tissue. Once a hormone binds to a receptor, it is internalized and the
hormone is degraded and the receptors are recycled.

Metabolic destruction by the tissue, e.g. the water soluble hormones (proteins and
catecholamines) are degraded by enzymes in the blood and tissues and excreted by the
kidneys.

Excretion by liver into bile, e.g. the steroid hormones which are conjugated in the liver
and secreted ("excereted") into the bile.

Excreted by the kidneys.
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Hormones that are bound to plasma proteins have a longer half life.
Half-life of a hormone. The time taken for the levels of a hormone to be reduced to half
of its original concentration is known as its "half-life". Hormones like angiotensin II have
a half-life of less than a minute while others such as the thyroid hormone (bound to
proteins) have a half-life of 1 to 6 days.
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Major endocrine glands, their secretions and disorders
The major endocrine glands include hypothalamus and pituitary (hypothalamo–hypophyseal
system), thyroid, parathyroid, adrenal, pancreas, thymus, pineal gland and the gonads (ovary
and testis) (Figure 3).
Pineal
Hypothalamo–hypophyseal system
Thyroid
Thymus
Adrenal
Pancreas
Ovary
Testis
Fig 3: Position of the major endocrine glands in the body
Hypothalamo–hypophyseal axis
For a long time the pituitary gland (hypophysis) was regarded to be the master gland of the
body as it secretes hormones that control the secretion of other glands in the body. Then it
was discovered that the pituitary itself is regulated by another gland, the hypothalamus,
which secretes a set of regulatory hormones that act on the pituitary. Thus this hypothalamo–
hypophyseal axis regulates the activity of various glands in the body (Figure 4).
H
Hypothalamus
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It is a part of the brain below the thalamus. It is an important connecting link between the
nervous system and the endocrine system because it receives neural inputs from different
regions of the brain and influences the secretions of the various hormones in the body
through the pituitary gland. It integrates all the sensory inputs received by the brain from the
body and acts as a regulatory centre for maintaining body temperature, osmotic balance,
heart rate, respiratory rate, etc.
Releasing or inhibiting hormones
Thyrotropin releasing hormone (TRH)
Control and regulation of hormone
secretion
Stimulates thyrotropin (TSH)
Growth hormone releasing hormone
Stimulates growth hormone release
(GHRH)
Growth hormone inhibiting hormone
Inhibits growth hormone release
(GHIH)
Prolactin releasing hormone (PRH)
Stimulates prolactin release
Prolactin inhibiting hormone (PIH) or
Inhibits prolactin release
dopamine
Adrenocorticotropic hormone releasing
Stimulates adrenocorticotropic hormone
hormone (CRH)
release
Melanocyte stimulating hormone releasing
Stimulates melanocyte stimulating hormone
hormone (MSHRH)
release
Melanocyte stimulating hormone inhibiting
Inhibits melanocyte stimulating hormones
hormone (MSHIH)
release
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Fig 4: Action of releasing and inhibitory hormones from the hypothalamus and
hormones of the anterior pituitary gland
Hypothalamo–hypophyseal portal system
The hypothalamus secretes stimulatory (releasing) and inhibitory hormones or factors which
stimulate or inhibit the release of hormones from the anterior lobe of the pituitary. These
factors are synthesized by the neurosecretory cells of the hypothalamus and released on
appropriate stimulation. These factors are not released in the general circulation but a special
local network of blood vessels between the hypothalamus and hypophysis—the
hypothalamo–hypophyseal portal system (Figure 5).
The hypothalamus receives blood supply from the superior hypophyseal arteries that form the
primary capillary plexus in the hypothalamus which join to form the hypophyseal portal
veins that branch again in the anterior lobe of the pituitary to form a secondary plexus of the
hypophyseal system. The releasing or inhibitory factors released by the hypothalamus are
directly brought to the hypophysis through this portal system so that they do not get diluted
in the general circulation.
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Fig 5: Hypothalamo–hypophyseal portal system
Hypothalamus
Median eminence
Infundibulum
Primary capillary plexus
Pars tuberalis
Adenohypophysis
Neurohypophysis
Pars distalis
Pars nervosa
Hypophyseal portal veins
Secondary capillary plexus
Hypophysis (Pituitary)
The hypophysis or pituitary gland is connected to the hypothalamus though a stalk, the
infundibulum.
The pituitary consists of two lobes, the anterior lobe or adenohypophysis and the posterior
lobe or neurohypophysis. The adenohypophysis has two parts, the lower pars distalis and the
upper pars tuberalis which forms a covering around the infundibulum. The neurohypophysis
has the lobe like pars nervosa and the infundibulum. During embryonic development a third
intermediate lobe called the pars intermedia is present which is lost in the adults but some of
its cells get integrated into the pars distalis.
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Adenohypophysis
It consists of five types of cells that secrete seven types of hormones (Figure 6). These are:
Somatotroph
1. Somatotrophs that secrete the growth hormone (GH) or
somatotropin
2. Thyrotrophs that secrete the thyroid stimulating hormone
(thyrotropin)
Thyrotroph
3. Gonadotrophs secrete the follicle stimulating hormone (FSH) and
the luteinizing hormone (LH). These two hormones together are
known as gonadotropins because they stimulate the gonads to
produce specific hormones.
4. Lactotrophs secrete the hormone prolactin.
Gonadotroph
5. Corticotrophs secrete adrenocorticotrophic hormone (ACTH)
which stimulates the adrenal cortex to produce adrenocorticoids
(cortisol, corticosterone, and aldosterone). Some corticotrophs
are the remnants of the pars intermedia and they secrete the
melanocyte stimulating hormone (MSH).
Lactotroph
Corticotroph
Fig 6: Cell types of the adenohypophysis
Neurohypophysis
This region of pituitary does not synthesize any hormones. It stores and then secretes two
hormones which are synthesized in the neurons of the hypothalamus. There are two sets of
neurons in the hypothalamus, the supraoptic nucleus and paraventricular nucleus which
synthesize hormones and convey them to the posterior pituitary through the nerve fibres of
these neurons. These nerve fibres form the axon terminals in the posterior pituitary where
these hormones are stored and released on appropriate stimulation. These two hormones are:
1. Oxytocin produced by the paraventricular nucleus.
2. Antidiuretic hormone (ADH) or vasopressin produced by the supraoptic nucleus.
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Part of
pituitary
Adeno-
Principal cell type
Hormones
Principal actions
Somatotroph
Growth
Growth of body cells especially of
hormone (GH)
bones of limbs, stimulates protein
Dwarfism—Reduced secretion of GH from the anterior
synthesis and inhibits protein
pituitary results in stunted growth so the person
breakdown, stimulates hydrolysis
remains a dwarf. In African Pygmies and Lévi-Lorain
of fats, retards use of blood
dwarfs, however, the secretion of GH from the
glucose for ATP production
hypothalamo-hypophyseal tract is normal but
(diabetogenic effect).
Somatomedin C (a mediator of growth hormone
hypophys
is
Target
organs
General
Disorders
Hyposecretion
action) levels are low. Levels of GH reduce with age.
Hypersecretion
Gigantism—This occurs due to overactivity of the
somatotrophs or some tumors in this region of the
pituitary causes increased secretion of GH. If this
happens before adolescence (before the closure of
epiphyseal plates) the person is abnormally tall.
If this happens after adolescence the bones become
thicker and the soft tissue continues to grow. In his
condition, called acromegaly, the hands and feet
become greatly enlarged, the lower jaw protrudes
out, the forehead slants forwards, and the tongue
liver and kidneys also become enlarged.
Thyrotroph
Thyrotropin or
Controls secretion of thyroid
Thyroid
thyroid
hormones
gland
Adrenocorticot
Controls secretion of adrenal
Adrenal
ropic hormone
cortex hormones.
cortex
stimulating
hormone
(TSH)
Corticotroph
(ACTH)
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Lactotroph
Prolactin (PRL)
Along with other hormones
Mammary
stimulates milk production,
glands
participates in control of
reproduction, osmoregulation,
growth and metabolism
Gonadotroph
Follicle
In males, stimulates
stimulating
spermatogenesis. In females
hormone
stimulates growth of ovarian
(FSH)
follicles.
Luteinizing
In females, also causes secretion
hormone (LH)
of estrogen & proferone and
Or
together with FSH, it triggers
Interstitial cell
ovulation, stimulates conversion of
stimulating
ovarian follicles into corpus
hormone
luteum.
(ICSH)
In males stimulates stimulation of
Gonads
Gonads
testosterone from interstitial cells
of Leydig.
Neuro-
No hormones are
Stimulates contraction of uterine
Uterine
phypophy
synthesized here. Its
Oxytocin (OT)
muscles during birth; initiates
muscles and
sis
hormones are
ejection of milk.
mammary
synthesized in
glands
hypothalamus
Antidiuretic
Stimulates reabsorption of water
Kidney,
Hyposecretion causes diabetes insipidus. Inability
hormone
and reduction in urine output;
blood
of the posterior pituitary to secrete enough ADH can
(ADH) or
stimulates constriction of blood
vessels,
be due to head injury, some infections or it may be
vasopressin
vessels to increase blood pressure,
sweat
congenital. It can result in loss of water from the
reduces sweat secretion from
glands
body due to the formation of very dilute urine as
sweat glands.
enough water is not reabsorbed by the kidney
tubules resulting in severe dehydration.
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Thyroid gland
The thyroid gland is an H-shaped gland that lies over the trachea below the larynx with the
right and left lateral lobes on either side of it. The lobes are connected by a mass of tissue,
called the isthmus. The gland consists of microscopic spherical sacs called thyroid follicles.
These contain a colloid, composed of the glycoprotein thyroglobulin bound to thyroid
hormones triiodothyronine (T3) and tetraiodothyronine or thyroxine (T4), which fills most of
the thyroid gland.
Basement membrane
Colloidal secretion
Blood capillary
Larynx
Parafollicular cells
Thyroid gland
Follicular cell
Trachea
Colloidal secretion
Cuboidal epithelium
Blue arrow shows parafollicular or C-cells that
secrete calcitonin which helps lower calcium
levels. These C-cells are actually named for
being "clear" (as it is lightly stained). Notice
that they are in the interstitium and do not
normally touch the follicles.
The simple cuboidal epithelium lining the
follicles produce the hormones T3 and T4
which are stored in the follicles with a
glycoprotein, thyroglobulin. Notice that the
thyroid is the only gland to store its
hormones extracellularly.
Source: Courtesy: http://www.kumc.edu/instruction/medicine/anatomy/histoweb/endo/endo.htm ©1996
The University of Kansas
17
Thyroid
Hormone
Principle actions
Disorders
Follicular
Triiodothyronine
Increases basal metabolic
Hypersecretion is called hyperthyroidism and hyposecretion is called hypothyroidism.
cells
(T3)
rate, stimulates synthesis
Hyperthyroidism (toxic goiter, thyrotoxicosis or Grave’s disease). Is caused by an
of proteins, increases use
autoimmune disorder where antibodies bind to receptors to TSH mimicking its action in
of glucose and fatty acids
stimulating the thyroid gland. These antibodies are called thyroid-stimulating
for ATP production,
immunoglobulins.
Thyroxine or
increases heart strength,
It may also be caused by a tumour in the thyroid tissue. Symptoms of hyperthyroidism
tetraiodothyronine
accelerates body growth
include a high state of excitability, increased sweating, intolerance to heat, weight loss, hand
(T4)
and contribute to the
tremors, psychic disorders and protrusion of the eyeballs in most patients.
development of nervous
Hypothyroidism. There is a reduced secretion of thyroid hormones because of another type
system in the embryo.
of autoimmune disorder where antibodies destroy the secretory cells. It may also be caused
cell type
by a deficiency of iodine as it is needed for the synthesis of thyroid hormones. The gland
enlarges in order to increase the secretion of hormones. This state of enlarged thyroid gland
is called goiter. Hypothyroidism in adults causes myxedema where there is accumulation of
a gel-like fluid in the interstitial spaces. 0ther symptoms include swelling of the face,
bagginess under the eyes, sluggishness, reduced cardiac output, etc.
Hypothyroidism in fetal life, infancy or childhood causes a condition called cretinism. It could
be congenital or caused by iodine deficiency. Symptoms include mental retardation and
improper body growth.
Parafollicular
Calcitonin (CT)
Lowers blood levels of
cells (C-
ionic Ca2+ and phosphates
cells)
by inhibiting bone
resorption by osteoclasts
and stimulates uptake of
calcium and phosphates
into the bone matrix. This
effect is more
predominant in children
than in adults.
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Parathyroid glands
These are small masses of tissue, partially embedded in the posterior surface of the lateral
lobes of the thyroid gland.
Oxyphil cells
Chief cells
Larynx
Thyroid gland
Parathyroid glands
Trachea
Parathyroid cells (Chief cell) in string-like
arrangement on the right and large, clear
oxyphil cells (whose function is unknown)
to the left.
Hormone and
Principal actions
Disorders
Parathyroid
Increases blood Ca2+ and PO42+ levels.
Hypoparathyroidism occurs when the
hormone (PTH)
Increases bone resorption by osteoclasts;
parathyroid hormone is not secreted in
from Chief cells
and promotes formation of calcitriol,
adequate amounts. This results in decrease in
source
which increases rate of dietary Ca
2+
and
Ca2+ ion concentration of blood; very low levels
Mg2+ absorption, decreases the excretion
of calcium result in tetany that could be fatal.
of calcium from the kidneys.
Hyperparathyroidism is increased secretion of
parathyroid hormone and results in an
increased plasma Ca2+ ion concentration due to
increased bone resorption. This results in
weakened bones, depressed peripheral and
central nervous system, muscle weakness,
constipation, lack of appetite and depressed
relaxation of the heart muscle during diastole.
Secondary hyperparathyroidism may be caused
by vitamin D deficiency where there is a
compensatory hyperactivity of the parathyroid
gland.
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Adrenal glands
A pair of adrenal (supra-renal) glands are located, one on each side of the spinal cord, above
each kidney. Each gland consists of an outer cortex and inner medulla. The cortex has three
distinct layers—zona glomerulosa, zona fasciculata and zona reticularis, each secreting
different types of steroids. Medulla as groups of large cells which secrete epinephrine and
norepinephrine on sympathetic stimulation (that is why adrenal medulla is considered to be
an extension of the sympathetic nervous system).
Adrenal medulla
Adrenal cortex
Adrenal glands
Kidney
Capsule
Zona glomerulosa
Zona fasciculata
Zona reticularis
Medulla
Source: Courtesy: http://www.kumc.edu/instruction/medicine/anatomy/histoweb/endo/endo.htm ©1996
The University of Kansas
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Gland part
Cell type
Hormone
Principal action
Target
Disorders
organ
Cortex
Zona
Mineralocorticoid
Controls electrolyte and water balance,
Kidney
glomerulosa
s (mainly
increases blood levels of Na and H2O,
Addison’s disease. This may be caused by atrophy of the
cells
aldosterone)
decreases blood levels of K+ by
cortical cells due to an autoimmune disorder. It may also
+
Hyposecretion disorder is called hypoadrenalism or
stimulating kidney tubules to reabsorb
be caused by tuberculous infection or cancer. It causes
more Na , Cl and water and less K .
reduced blood volume, hyponatremia, hyperkalemia,
Promotes Na+ resorption and K+ and
reduced cardiac output, sluggishness, increased
HCO3 excretion in sweat glands. It also
susceptibility to any kind of stress and increased
stimulates Na resorption in the large
pigmentation of the mucous membranes and skin.
+
–
+
-
+
intestine.
Hypersecretion is called hyperadrenalism or Cushing’s
syndrome. It may be caused by an abnormal function of
the hypothalamus that causes hypersecretion of the
corticotrophin releasing hormone which is turn causes an
increased secretion of ACTH and cortisol. It may also be
due to an abnormally high secretion of ACTH from the
pituitary or hypersecretion of cortisol due to an adrenal
cortex adenoma. Symptoms include abnormal deposition of
fat in the thoracic and upper abdominal regions, edema,
acne, hirsuitism (due to increased levels of androgens).
Hypersecretion of only aldosterone from the zona
glomerulosa of the adrenal cortex caused by a tumour in
this region is known as Conn’s syndrome. It is
characterized by hypokalemia, hypernatremia, increased
blood volume. Muscle paralysis may occur due to
hyperkalemia (which interferes with normal transmission of
the action potential), It causes an reduced renin secretion
from the kidneys due to increased blood volume.
Zona
Glucocortiocoids
Raises blood glucose level, promotes
Liver,
21
fasciculata
Cortisol (main),
gluconeogenesis in the liver and reduces
adipocytes
cells
corticosterone
glucose utilization by cells, reduces
and other
protein stores in all other cells of the
body cells
body, except liver cells and plasma,
promotes mobilization of fatty acids from
adipose tissue and enhances oxidation of
fatty acids in the cells, provides general
resistance to long term stress by blocking
inflammatory and allergic responses.
Zona
Androgens
Assists in early growth of axillary and
reticularis
(main), e.g.
pubic hairs in both sexes; in females, it
Gonads
Hypersecretion of androgens from the zona reticularis
because of a tumour in this part of the adrenal cortex
cells
dihydroepiandros
contributes to libido and is a source of
causes the adrenogenital syndrome. It causes
terone (DHEA)
estrogen after menopause
masculinization of the body. If it occurs in a female there is
and
a development of male characteristics such as a beard, a
androstienedione
deeper voice, deposition of proteins in the muscles,
baldness, etc. If it occurs in a prepubertal male it causes
precocious development of secondary sexual characters.
Medulla
Chromaffin
Epinephrine
Stimulates elevation of blood glucose by
Skeletal
cells
(adrenaline)
conversion of liver glycogen to glucose;
muscles,
Norepinephrine
raises blood pressure; accelerates the
cardiac
(nor-adrenaline)
rate and force of heart beat; causes
muscles,
constriction of skin and visceral
smooth
capillaries; causes dilation of vessels of
muscles,
heart and skeletal muscles; increases
blood
lipid breakdown, oxygen consumption,
vessels, fat
erection of hair, dilation of pupils; initiate
cells
stress response
22
Pancreatic islets
The pancreas is both an endocrine and exocrine gland. It is a flattened organ, about 12.5–15
cm long, located in the curve of the duodenum. Roughly 99% of pancreatic cells are exocrine
present in clusters called acini. Interspersed among them are a group of endocrine cells
forming lobules known as pancreatic islets or Islets of Langerhans. These islets contain four
types of cells— alpha (, beta (, delta (, and F cells.
Pancreas
Types of cells in the Islet of Langerhans
Capillary
F cell
Alpha cell
Beta cell
Delta cell
23
Cell type
Hormone
Principal action
Target organ
Disorders
Alpha cells
Glucagon

Accelerates breakdown of glycogen into glucose
Liver adipose
Hypersecretion of insulin is
(cells)
Causes lypolysis
in liver.
tissue
called hyperinsulinism. It

Beta cells
Insulin
Promotes conversion of other nutrients, such as
may be caused by an
amino acids and lactic acid, into glucose in the
adenoma of an Islet of
liver (gluconeogenesis).
Langerhans. It results in

Enhances the release of glucose into blood.
hypoglycemia (reduced

Stimulates glucose transport from blood to
Liver, muscle,
blood glucose levels) which
muscles and adipose cells, and stimulates liver
adipose tissue, and
could be fatal.
to take up glucose.
body cells
(cells)

Inhibits gluconeogenesis in the liver.
Hyposecretion of insulin or

Promotes both oxidation and conversion of
hypoinsulinism causes
glucose into glycogen in liver and muscle cells.
diabetes mellitus (Type I).
Inhibits metabolic breakdown of stored glycogen
In Type II diabetes mellitus
in liver and muscle cells.
the amount of insulin
Promotes synthesis of fats from glucose by
secreted by pancreatic 
adipose tissue and also inhibits metabolic
cells is normal but the
breakdown of fat.
response of the cells is not
Promotes uptake of amino acids by liver and
(insulin resistance).
muscle cells, and stimulates protein synthesis
Diabetes mellitus causes



hyperglycemia, glycosuria
while inhibiting protein breakdown.
Delta cells
Somatostatin (is a
Inhibits secretion of glucagon and insulin; reduces
Pancreas (and 
(glucose in urine) polyuria
(cells)
paracrine agent)
motility of stomach, duodenum and gall bladder;
cells),
(increased urine output),
reduces secretion and absorption in the digestive
gastrointestinal
tissue injury, increased
tract.
tract
metabolism of fat,
Pancreatic
Inhibits somatostatin secretion, gall bladder
Pancreas, gall
ketoacidosis and depletion
polypeptide
contraction and secretion of pancreatic digestive
bladder
of body proteins.
(is a paracrine agent)
enzymes.
F cells
24
Gonads and placenta
The testis in males and ovaries in females secrete sex hormones during puberty. These
hormones are steroids and responsible for controlling various secondary sexual characters
during puberty. The placenta also releases some hormones that are responsible for the
maintenance and certain changes during pregnancy.
Ovary
Testis
Ovarian follicle
The Graafian follicle is identified by the large
antrum (A) and the cumulus oophorous (arrow)
that surrounds the actual oocyte and projects
into the antrum.
Placenta
Corpus luteum
Progesterone from the corpus luteum maintains
the uterus for implantation. Granulosa luteal
cells (GL) and theca luteal cells (TL).
Source: Courtesy: http://www.kumc.edu/instruction/medicine/anatomy/histoweb/female/female.htm ©1996
The University of Kansas
Testis
Sperms in different
stages of development
Seminiferous tubule
Interstitial cells of Leydig
25
Gland type
Hormones
Principal action
Disorders
Estrogen (estradiol
Stimulates the development and maintenance
Hypogonadism: when there is reduced estrogen secretion because
and estrone)
of female sexual characteristics such as high
of poorly formed ovaries or genetically abnormal ovaries female
pitch, female voice and female pattern and
eunuchism occurs. The female secondary sexual characteristics fail
distribution of body hair at puberty.
to develop and there is a prolonged growth of bones. The ovarian
Together with gonadotropic hormones of the
cycles are irregular or there may be complete amenorrhoea.
and part
Ovary
Ovarian follicle
anterior pituitary gland they also regulate
menstrual cycle and development of
Hypersecretion of estrogens: may occur in case of a granulosa cell
secondary sex organs.
tumour which usually occurs after menopause. Symptoms include
hypertrophy of the endometrium and irregular bleeding.
Corpus luteum
Progesterone and
Progesterone prepares and maintains the
estrogen
uterine lining for pregnancy, stimulates
mucosal lining of the fallopian tubes to
secrete a nutrient-rich fluid, prevents the
uterine myometrium from undergoing
contractions. Prepares the breast for milk
secretion. Estrogen stimulates uterine lining
for implantation to maintain pregnancy,
prepares the mammary glands for lactation
and regulates oogenesis.
Relaxin
Facilitates accommodation of the growing
fetus. Relaxes pubic symphysis and helps
dilate uterine cervix near the end of
pregnancy.
Inhibin
Regulates oogenesis by inhibiting FSH and
GnRH secretion.
Testis
Interstitial cells
Testosterone
Stimulates the descent of testis and male
Hypogonadism: where there is a loss of testes or if there is a
26
of Leydig
pattern of development (before birth);
reduced secretion of GnRH from the hypothalamus (adipose genital
stimulates development and maintenance of
syndrome or Fröhlich’s syndrome or hypothalamic eunuchism). In
male sexual characteristics and expression of
the absence of testosterone in an adult some of the secondary
male characteristics such as beard,
sexual characteristics are lost. In a child these characteristics fail to
moustache and low-pitch voice; stimulates
develop.
spermatogenesis, growth spurt, protein
Sertoli cells
Inhibin
synthesis and muscle development, bone
Hypergonadism: refers to an increased secretion of testosterone
growth, stimulate secretion of erythropoietin
due to tumour of Leydig cells. This causes an abnormally increased
from the kidneys; increases basal
muscle growth, reduced height (as the epiphyseal plates close
metabolism.
early) and excessive development of male sexual characteristics.
Regulates spermatogenesis by inhibiting FSH
secretion.
Placenta
Human chorionic
Stimulates progesterone release from the
gonadotropin
corpus luteum and maintains it. It has an
(HCG)
interstitial cell stimulating effect in a male
fetus. Simulates mammary gland growth
or
during pregnancy. Has weak growth hormone
Human placental
like effects. Decreases insulin sensitivity and
lactogen
glucose utilization by the mother’s cells so
that glucose is made available to the fetus. It
also mobilizes fatty acids from mother’s fat
stores.
27
Thymus
The thymus is located behind the sternum. It consists of two lobes
separated from one another by a connective tissue capsule.
Extensions of this capsule penetrate in the form of septa or
trabeculae to divide each lobe into lobules. Each lobule has a
lighter staining central medulla surrounded by a darkly staining
outer cortex. The cortex contains T cells which proliferate and
mature in the thymus; dendritic cells that assist the maturing T
cells and epithelial cells with long processes form a framework for
the maturing T cells. The medulla consists of more mature T cells,
epithelial cells and macrophages. Clusters of flattened degenerate
epithelial cells are arranged in concentric layers called Hassall’s
(thymic) corpuscles. In infants, the thymus is large but it starts
Thymus
degenerating after puberty and is almost absent in old age.
Source: http://www.cytochemistry.net/microanatomy/immune_system/lymphoid_tissues.htm
© copyright 1998 Gwen V. Childs, Ph.D. URL Address: http://cellbio.utmb.edu/microanatomy/
Gwen V. Childs, Ph.D., WebMistress gvchilds@utmb.edu
Thymus
Hormones
Principal action
Thymosin, thymic humoral factor,
Promote the proliferation and
thymic factor, thymopoietin
maturation of T-cells (derived from
lymphocytes)
28
Pineal gland
It is a small endocrine gland attached to the roof of the third ventricle of the brain at the
midline. It is covered by a capsule formed by pia mater and consists of masses of neuroglia
and secretory cells called pinealocytes.
It secretes the hormone melatonin, which is believed to help in maintaining the biological
clock, as it is produced when no light stimulus is present and its production ceases when eye
receives light stimulus.
Cerebral cortex
Pineal gland
Hypothalamus
Pituitary
Pineal gland
Hormones
Principal action
Melatonin
Involved with the setting of the
biological clock in the body.
Controls seasonal fertility in some
animals.
Other endocrine tissues
Some tissues other than those described already, contain endocrine cells which secrete
hormones. Along with these hormones some growth factors are also produced which
stimulate cell growth and division.
29
Production site
Hormone or
Principal action
growth factor
Gastrointestinal tract
G-cells of the stomach
Gastrin
Promotes secretion of gastric juice and increases motility
of the stomach.
Enteroendocrine cells of
Glucose-dependent
Stimulates release of insulin by pancreatic  cells, inhibits
the duodenum
insulinotropic peptide
gastric secretion.
(GIP)
Secretin
Stimulates secretion of pancreatic juice rich in HCO3– ions
and bile; reduces gastric secretion and motility.
Cholecystokinin
Stimulates secretion of pancreatic juice rich in enzymes,
(CCK)
release of bile from the gall bladder and brings about the
feeling of fullness after eating.
Vasoactive intestinal
Inhibits gastric secretion and motility
polypeptide (VIP)
Oxyntic cells of the
Ghrelin
Stimulates food intake
Angiotensinogen gets
Causes vasoconstriction, enhances reabsorption of sodium
converted to
and chloride ions and water, stimulates the release of
angiotensin I which
aldosterone from adrenal cortex which further stimulates
gets converted to
reabsorption of sodium and chloride ions from the kidney
angiotensin II
tubules. All this results in increased blood volume and
stomach and cells of
the intestine
Liver
blood pressure.
Kidneys
Erythropoetin (EPO)
Increases rate of red blood cell formation.
Calcitriol (active
Aids in the absorption of dietary calcium and phosphorus.
vitamin D)
Heart
Atrial natriuretic
Decreases blood pressure and blood volume by
peptide (ANP)
stimulating the excretion of Na+ ions from the kidney
tubules.
Adipose tissue
Leptin
Suppresses appetite, stimulates the release of
corticotropin releasing hormone that decreases food
intake, increases sympathetic activity resulting in an
increased metabolic rate and energy expenditure,
suppresses the release of appetite stimulators from the
hypothalamus.
Submaxillary salivary
Epidermal growth
Stimulates proliferation of epithelial cells, fibroblasts,
gland
factor (EGF)
neurons and astrocytes; suppresses some cancer cells and
30
secretion of gastric juice by the stomach.
Nerve growth factor
Stimulates the growth of ganglia in embryonic life,
(NGF)
maintains sympathetic nervous system, and stimulates
differentiation of neurons.
Blood platelets
Platelet-derived
Found in blood; stimulates proliferation of neuroglial cells,
growth factor (PGF)
smooth muscle fibres, and fibroblasts; may have a role in
wound healing; may contribute to the development or
artherosclerosis.
Pituitary and brain
Fibroblast growth
Stimulates proliferation of many cells derived from
factor (FGF)
embryonic mesoderm (fibroblasts, adrenocortical cells,
smooth muscle fibers, chondrocytes and endothelial cells);
stimulates cell migration and growth and production of
fibronectin (an adhesion protein).
Normal and tumor
Tumor angiogenesis
Stimulates growth of new capillaries, organ regeneration
cells
factor (TAFs)
and wound healing.
Various cells
Transforming growth
Some have activities similar to epidermal growth factor,
factors
others inhibit proliferation of many cell types.
31