The document provides an overview of the endocrine system, detailing its components, functions, and the various hormones produced by different glands such as the pituitary, thyroid, and adrenal glands. It explains the mechanisms of hormone action, including their types, regulatory pathways, and interactions with target cells. Additionally, it covers the roles of hormones in metabolism, growth, reproduction, and responses to stress.

1. Endocrine System
Miss. Gayatri K. Bahatakar
Assistant Professor, P. R. Patil Institute of Pharmacy,
Talegaon (SP)

2. • The endocrine system is made up of ductless glands called endocrine
glands that secrete chemical messengers called hormones into the
bloodstream or in the extracellular fluid.
• A hormone is a chemical substance made and secreted by one cell
that travels through the circulatory system or the extracellular fluid to
affect the activities of cells in another part of the body or another
nearby cell.
Functions of the Endocrine System
• Maintains the internal environment in the body (the optimum
biochemical environment)
• Influences metabolic activities
• Integrates and regulates growth and development.
• Controls, maintains and instigates sexual reproduction, including
gametogenesis, coitus, fertilization, fetal growth and development
and nourishment of the newborn

3. Endocrine Glands
• Pituitary
• Thyroid
• Parathyroid
• Adrenal
• Pineal Gland
• Thymus Gland
• Hypothalamus (has both neural functions and releases hormones)
• Pancreas (produces both hormones and exocrine products)
• Gonads (produce both hormones and exocrine products)
• Other tissues and organs also produce hormones – adipose cells, cells
of the small intestine, stomach, kidneys, and heart

5. Nervous System Endocrine System
Neurons release neurotransmitters Endocrine cells release hormones
A neurotransmitter acts on specific
cell right next to it.
Hormones travel to another nearby
cell or act on cell in another part of
the body.
Neurotransmitters have their effects
within milliseconds.
Hormones take minutes or days to
have their effects.
The effects of neurotransmitters are
short-lived.
The effects of hormones can last
hours, days, or years.
Performs short term crisis
management
Regulates long term ongoing
metabolic function
Neurotransmitter acts on specific cell
right next to it.
Hormone can travel to another
nearby cell or it can act on another
part of the body.
Endocrine vs. Nervous System

6. Endocrine vs. Nervous System

7. Hormones
• Are chemical substances secreted by cells into the extracellular fluids
• Regulate the metabolic function of other cells
• Have lag times ranging from seconds to hours
• Tend to have prolonged effects
• Circulate to all tissues but only activate cells referred to as target cells
• Target cells must have specific receptors to which the hormone binds.
• These receptors may be intracellular or located on the plasma membrane.
Types of Hormones
• Amino Acid Derivatives
• These are hormones that are modified from the amino acid called tyrosine. They include the thyroid
hormones and the hormones of the adrenal medulla (epinephrine and norepinephrine). These
hormones are water soluble.
• Peptide Hormones
• These hormones consist of chains of amino acids that vary in size from 3 amino acids (TRH) to 191
amino acids (GH). These hormones are water soluble.
• Lipid Derivatives
• These include hormones that are steroids (e.g. estrogen and testosterone) and fatty acids derivatives
(e.g. prostaglandins). These hormones are lipid soluble.

8. Mechanism of Hormone Action
• Hormones produce one or more of the following cellular changes in
target cells
• Alter plasma membrane permeability
• Stimulate protein synthesis
• Activate or deactivate enzyme systems
• Induce secretory activity
• Stimulate mitosis

9. • In general, hormones can act on a target cell in 1 of 2 ways:
Activate Second Messengers (Involves regulatory G proteins)
(This is how amino acid-derived, peptide, and fatty acid-derived hormones work)
• 1. The hormone binds to a G protein-linked receptor on the cell membrane; t he hormone
acts as a first messenger.
• 2. The binding of the hormone to the G protein-linked receptor activates a second
messenger such as cAMP.
• 3. The second messenger than activates or inactivates enzymes in the cell
Activate Genes
(This is how steroid and thyroid hormones work)
• 1. Steroid hormones and thyroid hormones pass directly through the cell membrane of
target cells.
• 2. They bind to receptors in the cytoplasm or in the nucleus. (Thyroid hormones also bind
to receptors in the mitochondria.)
• 3. If they bind to receptors in the cytoplasm, the hormone-receptor complex then enters
the nucleus.
• 4. In the nucleus the hormones directly either turn genes “on” or turn genes “off.” That is
they either cause the gene to start making a protein by transcription and translation or not.
• Thyroid hormones that go to the mitochondria increase the rate of ATP production in the
cell.

10. Humoral Stimuli
• Some hormones are secreted in direct response to changing blood levels of ions and
nutrients
• Example: Concentration of calcium ions in the blood
• Declining blood Ca2+ concentration stimulates the parathyroid glands to secrete PTH
(parathyroid hormone)
• PTH causes Ca2+ concentrations to rise and the stimulus is removed
Neural Stimuli -
• ANS efferent nerve fibres stimulate hormone release
• Ex: Preganglionic sympathetic nervous system (SNS) fibres stimulate the adrenal
medulla to secrete catecholamines.
Hormonal Stimuli
• Some hormones are released in response to hormones produced by other endocrine
organs
• Ex: Many hypothalamic hormones stimulate the anterior pituitary to release
hormones. The pituitary hormones then stimulate other target organs to secrete still
other hormones

11. Hypothalamus
• Is a part of the brain located in the diencephalon, inferior to the thalamus.
• Is made up of neurons and neuroglial cells.
• Produces several different hormones:
• 1. Releasing Hormones
• These stimulate the anterior pituitary gland to release a specific hormone (e.g. GRH-GH)
• 2. Inhibiting Hormones
• These stimulate the anterior pituitary gland to not release a specific hormone (e.g. GRIH-GH)
• 3. Antidiuretic Hormone (ADH) (also called vasopressin)
• Antidiuretic hormone conserves body water by reducing the loss of water in urine.
• This hormone signals the collecting ducts of the kidneys to reabsorb more water and constrict
blood vessels, which leads to higher blood pressure and thus counters the blood pressure drop
caused by dehydration.
• 4. Oxytocin
• Stimulates the smooth muscle of the uterus to contract, inducing labor.

12. • Stimulates the myoepithelial cells of the breasts to contract which
releases milk from breasts when nursing.
• Stimulates maternal behavior.
• In males it stimulates muscle contractions in the prostate gland to
release semen during sexual activity
• The releasing and inhibiting hormones made by the hypothalamus
reach the anterior lobe of the pituitary gland DIRECTLY by a special
set of blood vessels called the hypophyseal portal system.
• The hypothalamus makes antidiuretic hormone (ADH) and oxytocin in
the cell bodies of neurons and then the hormones are transported
down the axons which extend into the posterior pituitary gland. The
posterior pituitary gland The posterior pituitary gland stores and later
releases the hormones as needed.

14. Pituitary (also called Hypophysis)
• Is a two-lobed organ that secretes nine major hormones
• Neurohypophysis – posterior lobe (neural tissue) receives, stores, and
releases hormones (oxytocin and antidiuretic hormone) made in the
hypothalamus and transported to the posterior pituitary via axons.
• Adenohypophysis – anterior lobe, made up of glandular tissue.
Synthesizes and secretes a number of hormones.
• The hypothalamus sends releasing hormones to the anterior pituitary that
stimulates the synthesis and release of hormones from the anterior
pituitary gland
• The hypothalamus also sends inhibiting hormones that shut off the
synthesis and release of hormones from the anterior pituitary gland
• The pituitary gland releases nine important peptide (protein) hormones
• All nine peptide hormones bind to membrane receptors and use cyclic AMP
as a second messenger

16. Hormones of the Anterior Pituitary Gland (Adenohypophysis)
Growth Hormone (GH or somatotropin)
• GH produced by somatotropic cells of the anterior lobe
• Stimulates most cells, but target bone and skeletal muscle
• Stimulates the liver and other tissues to secrete insulin-like growth factor I
(IGF-I or somatomedin)
• IGF-I stimulates proliferation of chondrocytes (cartilage cells), resulting in
bone growth.
• GH stimulates cell growth, replication, and protein synthesis through release
of IGF-I.
• Direct action promotes lipolysis to encourage the use of fats for fuel and
inhibits glucose uptake
• Antagonistic hypothalamic hormones regulate GH
• Growth hormone–releasing hormone (GHRH) stimulates GH release
• Growth hormone–inhibiting hormone (GHIH or somatostatin ) inhibits GH
release

18. 2. Thyroid Stimulating Hormone (TSH or Thryotropin)
• Travels to the thyroid gland (target cells) where it stimulates the release of thyroid hormones in response
to low temperatures, stress, and pregnancy
• Thyrotropin releasing hormone (TRH) from the hypothalamus promotes the release of TSH
• Rising blood levels of thyroid hormones act on the pituitary and hypothalamus to block the release of
TSH
3. Adrenocorticotropic Hormone (ACTH or Corticotropin)
• Travels to the adrenal gland (target cells) where it stimulates the release of corticosteroids (such as
cortisol) in the adrenal cortex.
• Corticotropin-releasing hormone (CRH) from the hypothalamus promotes the release of ACTH in a daily
rhythm
• Internal and external factors such as fever, hypoglycemia, and stressors can trigger the release of CRH
4. Follicle Stimulating Hormone (FSH)
• Travels to the gonads (target cells) and stimulates sperm or egg cell production and maturation and
estrogen secretion
• Gonadotropin-releasing hormone (GnRH) from the hypothalamus promotes the release of FSH during
and after puberty

19. 5. Leutinizing Hormone (LH)
• Travels to the ovaries in females (target cells) and stimulates ovulation, maturation
of follicles (together with FSH) and stimulates the corpus luteum to secrete
progesterone.
• In males LH travels to the testes (target cells) to stimulate secretion of
testosterone.
• LH is also referred to as interstitial cell-stimulating hormone (ICSH)
6. Prolactin (PL)
• Travels to the mammary glands (target cells) and stimulates the development of
mammary glands to produce milk.
• In males scientists think prolactin influences the sensitivity of cells in the testes
(interstitial cells) to the effects of luteinizing hormone (LH)
• Prolactin-releasing hormone (PRH) from the hypothalamus stimulates the release
of prolactin
• Prolactin-inhibiting hormone (PIH) from the hypothalamus inhibits the release of
prolactin
• Blood levels rise toward the end of pregnancy, suckling stimulates PRH release and
encourages continued milk production

20. Hormones of the Posterior Pituitary Gland (Neurohypophysis)
• The neurohypophysis contains axons from neurons in the hypothalamus
1. Antidiuretic Hormone (ADH or vasopressin)
• Made by neurons of the supraoptic nucleus in the hypothalamus
• Signals the collecting ducts of the kidneys to reabsorb more water and
constrict blood vessels, which leads to higher blood pressure and thus
counters the blood pressure drop caused by dehydration or other reasons
2. Oxytocin
• Made by neurons of the paraventricular nucleus of the hypothalmus
• Stimulates the smooth muscle of the uterus to contract, inducing labor
• Stimulates the myoepithelial cells of the breasts to contract which releases
milk from breasts when nursing.
• Stimulates maternal behavior.
• In males it stimulates muscle contractions in the prostate gland to release
semen during sexual activity.

22. Thyroid Glands
• The thyroid gland contains numerous thyroid follicles that release 2 hormones: thyroxine
(T4) and triiodothyronine (T3)
• Thyroid hormones are held in storage but eventually attach to thyroid binding globulins
(TBG); some are attached to transthyretin or albumin
• Thyroid hormones regulate metabolism; they diffuse into target cells and bind to
mitochondria, thereby increasing ATP production; they also bind to receptors activating
genes that control energy utilization
• Thyroid hormones increase protein synthesis, and promote glycolysis, gluconeogenesis,
and glucose uptake
• Thyroid hormones are necessary for normal growth as they stimulate release of GH from
the anterior pituitary
• Thyroid hormones are very important for brain development
• C Cells in between the thyroid follicles produce calcitonin.
• Calcitonin decreases the concentration of calcium in the blood where most of it is stored
in the bones; it stimulates osteoblast activity and inhibits osteoclast activity, resulting in
new bone matrix formation.
• which help regulate calcium concentration in body fluids

24. Parathyroid Glands
• The parathyroid glands are four or so masses of tissue
embedded posteriorly in each lateral mass of the
thyroid gland
• Parathyroid hormone (PTH) is the most important
endocrine regulator of calcium and phosphorus
concentration in extracellular fluid
• PTH has the opposite effect of calcitonin.
• PTH stimulates osteoclasts which increases blood
calcium levels.
• PTH causes reabsorption of Ca+2 from kidneys so it is
not excreted in the urine
• PTH stimulates synthesis of calcitriol (hormone made in
the kidney which the active form of Vitamin D which
increases Ca+2 absorption from small intestine

26. Adrenal Glands
• The adrenal glands are located superior to each kidney.
• Each adrenal gland has a pyramid shape.
• Each adrenal gland has an inner medulla and outer cortex:
• Adrenal Cortex
• Adrenal Medulla

28. Adrenal Cortex
• Makes and secretes over 30 different steroid hormones (collectively called corticosteroids)
• The adrenal cortex has 3 regions (zones) that each make a major type of hormones:
1. Mineralocorticoids (e.g. aldosterone)
• Stimulates the kidneys to reabsorb sodium if blood pressure drops
• It also secretes (eliminates) potassium
2. Glucocorticoids (e.g. cortisol)
• These hormones help you to cope with stress
• Cortisol increases the level of sugar in the blood by stimulating the production of glucose from
fats and proteins (gluconeogenesis)
• It also reduces swelling
• In large doses, cortisol inhibits the immune system.
• It stimulates gluconeogenesis, mobilization of free fatty acids, glucose sparing. Also acts as an
anti-inflammatory agent
3. Gonadocorticoids (e.g.testosterone, estrogens, and progesterone )
• The adrenal gland also makes small amts of the sex hormones (mostly androgens
(testosterone) and lesser amounts of estrogens and progesterone)
• Scientists not certain what role these hormones play; but know that when over secreted they
can cause problems

29. Adrenal Medulla
• secretes the hormones epinephrine and norepinephrine when stimulated by
sympathetic neurons of the autonomic nervous system (ANS)
• Both epinephrine and norepinephrine contribute to the bodies' "fight or flight"
response, just like the sympathetic nervous system.
• They have the same effects as direct stimulation by the sympathetic NS (increase
heart rate, breathing rate, blood flow to skeletal muscles, and concentration of
glucose in the blood), but their effects are longer lasting
• Norepinpehrine is similar to epinephrine, but it is less effective in the conversion
of glycogen to glucose.
• ~75 - 80% epinephrine
• ~25-30% norepinephrine

31. Pancreas
• Located along the lower curvature of the small intestine (duodenum)
• The pancreas contains both exocrine and endocrine cells
• The exocrine portion secretes digestive enzymes into the duodenum via the
pancreatic duct
• The endocrine portion has clusters of endocrine cells within the pancreas called
pancreatic islets or Islets of Langerhans
• Alpha cells secrete glucagon
• Beta cells secrete insulin
• Glucagon increases the levels of glucose in the blood by stimulating the liver to
breakdown glycogen into glucose during fasting or starvation
• Insulin lowers blood glucose by increasing the rate of glucose uptake and
utilization
• Glucagon raises blood glucose by increasing the rates of glycogen breakdown and
glucose manufacture by the liver

34. Regulation of Glucose Metabolism During Exercise
• Glucagon secretion increases during exercise to promote liver glycogen breakdown
(glycogenolysis)
• Epinephrine and norepinephrine further increase glycogenolysis
• Cortisol levels also increase during exercise for protein catabolism for later
gluconeogenesis
• Growth Hormone mobilizes free fatty acids
• Thyroxine promotes glucose catabolism
• As intensity of exercise increases, so does the rate of catecholamine (epinephrine and
norepinephrine) release for glycogenolysis
• During endurance events, the rate of glucose release very closely matches the muscles'
need
• When glucose levels become depleted, glucagon and cortisol levels rise significantly to
enhance gluconeogenesis
• Glucose must not only be delivered to the cells, it must also be taken up by them. That
job relies on insulin. Exercise may enhance insulin’s binding to receptors on the muscle
fiber.
• Up-regulation (receptors) occurs with insulin after 4 weeks of exercise to increase its
sensitivity (diabetic importance).

35. Regulation of Fat Metabolism During Exercise
• When low plasma glucose levels occur, the catecholamines are released to
accelerate lipolysis
• Triglycerides are reduced to free fatty acids by lipase which is activated by:
• Cortisol
• Epinephrine
• Norepinephrine
• Growth Hormone
• Hormonal Effects on Fluid and Electrolyte Balance
• Reduced plasma volume leads to release of aldosterone which increases Na+ and
H2O reabsorption by the kidneys and renal tubes.
• Antidiuretic Hormone (ADH) is released from the posterior pituitary when
dehydration is sensed by osmoreceptors, and water is then reabsorbed by the
kidneys.

36. Hormones and Stress
• Stress = any condition that threatens homeostasis
• GAS (General Adaptation Syndrome) is our bodies response to stress-
causing factors
• Three phases to GAS:
• Alarm Phase (immediate, fight or flight, directed by the sympathetic
nervous system)
• Resistance Phase (dominated by glucocorticoids)
• Exhaustion Phase (breakdown of homeostatic regulation and failure
of one or more organ systems)

38. Pineal Gland
• Melatonin production in the brain depends on this tiny gland
• Deep within the brain is the tiny pineal gland, an organ that produces the
body’s melatonin, an influential hormone that helps to regulate sleep and
wakefulness and the circadian patterns that have broad effects on health.
Anatomy
• The pineal gland (or pineal body) is a small, pine-cone shaped organ that lies
within the roof of the third ventricle, deep within the brain. Autopsy studies
have shown that the average size of the pineal gland is similar to that of a grain
of rice. The ventricles are fluid-filled spaces, and the third ventricle extends
from the large lateral ventricles to the narrow cerebral aqueduct, passing
between the two halves of the part of the brain called the diencephalon.
• It is located within an area called the epithalamus, just behind the thalamus
and above the cerebellum, resting at the back of the brain, near the brain stem.
There is a small fluid-filled pineal recess that projects into the stalk of the
pineal body, allowing for the hormones it produces to more easily be diffused
throughout the brain.

39. Structure
• The cells that make up the tissue of the pineal gland in humans and other mammals
include hormone-producing pinealocytes and the supportive interstitial cells. Nerve
cells, or neurons, might influence the pinealocytes by secreting specific chemicals
called neurotransmitters. Nerve fibers reach the gland via the pineal stalk and contain
many substances, including:
• GABA
• Orexin
• Serotonin
• Histamine
• Oxytocin
• Vasopressin
• The pinealocyte cells have receptors for all these neurotransmitters, suggesting
influence from these other chemicals that are common within the brain.
• In humans and other mammals, this influence is extended beyond the brain to a
collection of neurons located in the sympathetic superior cervical ganglia and the
parasympathetic sphenopalatine and otic (parasympathetic, sympathetic and motor
roots)ganglion. This connection is a relay from the pineal gland to the suprachiasmatic
nucleus (SCN), located in the hypothalamus.

40. • The SCN is of vital importance because this is the primary pacemaker for the
circadian rhythm within the body, affected by the perception of light
detected by the retina and sent along the retinohypothalamic tract.
Function
• The most important function of the pineal gland is the production of
melatonin. Melatonin is synthesized from molecules of the neurotransmitter
serotonin. Once produced, it is secreted from the pineal gland. It has important
effects on circadian rhythm, including impacts on sleep and possible effects on
seasonal reproduction in animals.
• Within the pineal gland, serotonin (which is derived from the amino acid called
tryptophan) undergoes a transformation, when an acetyl group and then a
methyl group are added to yield melatonin. This is accomplished with two
enzymes:serotonin-N-acetyltransferase and hydroxyindole-O-methyltranferase.
The production of melatonin is impaired by light exposure.

41. How does light affect melatonin production within the pineal gland?
• From the Latin meaning “about a day,” the word "circadian" refers to numerous
physiological processes that are coupled to the timing of light and darkness.
Though inclusive of sleep and wakefulness, this circadian timing likely extends to
the release of hormones, the use of energy to optimize metabolism, and the
coordination of the body’s interconnected systems.
• Light that passes through the retina of the eye activates specific receptors called
intrinsically photosensitive retinal ganglion cells (ipRGC). These cells contain the
photopigment called melanopsin. From here, the signal is relayed from the eyes
to the pineal gland.
• First, the message is passed along the retinohypothalamic tract that extends from
the retinal cells to the SCN in the anterior hypothalamus in the brain. The
paraventricular nucleus of the hypothalamus then sends the signal along to the
preganglionic sympathetic neurons in the spinal cord, to the superior cervical
ganglion, and finally on to the pineal gland.
• The pineal gland can then alter its production of melatonin, based on the amount
of light that is perceived by the eyes. This has led the pineal gland to be called the
“third eye” of the body, due to its ability to respond to the perception of light.

42. • When melatonin is produced, it is not released into a vacuum to do as it pleases.
As is true for many processes within the body, there is a balance that is preserved.
This balance is called homeostasis. When the pineal gland secretes melatonin, this
feeds back via action on the MT1 and MT2 melatonin receptors on the SCN. This
interplay affects the control of the circadian system within the body, with broader
implications for potential disease.
• There are a few other curious effects of melatonin that are not fully understood
within humans. It is known that in animal models melatonin can decrease
gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus. This
may have an inhibitory effect on reproductive functions. In mammals, this may
slow the maturation of sperm and eggs and reduce the function of the
reproductive organs.
• It is theorized that it may affect the seasonal reproductive functions of some
animals. When nights are longer in the winter months, and the access to food may
be reduced, the increased darkness may lead to higher melatonin levels and
reduced fertility. This may make it less likely for some animal species to have
young that may not survive the lean times of winter. The significance of this,
especially among humans, is unknown.

43. Associated Conditions
• The pineal gland and its production of melatonin are central to the circadian rhythm disorders
that affect sleep. It may exacerbate insomnia in delayed sleep phase syndrome, for example. It
also may have a role in seasonal affective disorder, sometimes known as winter depression. In
addition, when the pineal gland is affected by tumors, the effects may lead to brain surgery.
Circadian Rhythm Disorders
• These conditions occur when the synchrony between the patterns of wakefulness and sleep do
not align with societal norms or the natural rhythm of light and darkness. Characterized by
irregular bedtimes and awakenings, the affected person will experience insomnia and
improperly timed sleepiness. The circadian sleep disorders include:
• Delayed sleep phase syndrome: Night owls who have difficulty getting to sleep and trouble
waking at an early time
• Advanced sleep phase syndrome: Characterized by early sleep onset and early morning
awakening
• Free-running, or non-24: Most often found in blind people without light perception, the timing
of sleep may shift gradually over weeks or months
• Irregular sleep-wake rhythm: Shorter intervals of sleep over the 24-hour day, rather than a
prolonged sleep period overnight

44. Pineal Gland Tumors
• Cancer may rarely affect the pineal gland. In fact, less than 1 percent of brain tumors occur in
the pineal gland, but 3%–8% of brain tumors in children are found here.2 Generally, pineal
gland tumors occur more among young adults, those individuals between 20 and 40 years of
age. There are only a handful of tumors that may affect the pineal gland within the brain. In
fact, there are only three types of true pineal cell tumors. These include:
• Pineocytoma: Slow growing, often classified as a grade II tumor
• Pineoblastoma: Generally more aggressive, either classified as a grade III intermediate form
or more malignant grade IV
• Mixed pineal tumor: Contains a combination of cell types, making a clean classification less
possible
• These tumors may grow large enough to obstruct the normal flow of the cerebrospinal fluid
within the ventricles. It is estimated that 10%–20% of the pineal gland tumors may also
spread via this medium, especially the more aggressive pineoblastoma variant. Fortunately,
these cancers rarely metastasize elsewhere in the body.
Symptoms that develop with a pineal gland tumor may include:
• Impaired eye movements causing double vision
• Headache
• Nausea
• Vomiting

45. Thymus
Anatomy
• The thymus is located at the top of the sternum (breastbone), near the collarbone. It
resides between the sternum and the aortic arch. At birth, the thymus measures
approximately 1 to 2 inches wide by one-half inch thick. The organ grows in size throughout
childhood, until it begins to shrink in adolescence.
• The thymus is encapsulated by a wall made of collagen-type tissue. Inside, the organ is
divided into two main lobes with irregular lobules (sub-lobes), each of which contains
several structures and cell types:
• Cortex: Nearest the organ’s wall, the cortex region contains developing T cell lymphocytes
• Medulla: A region near the center of each lobule, the medulla holds fully developed T cells
• Epithelioreticular cells: These cells create walls that divide the organ into a latticework of
sections that hold developing and mature T cells
• Blood vessels: The capsule and lobular walls contain blood vessels to supply oxygen to the
organ’s tissues
• Lymphatic vessels: Similar to blood vessels, lymphatic vessels carry lymphatic fluid through
the body’s lymph system, including the thymus
• Macrophages: These immune system cells destroy T cells that have not developed properly

46. Anatomical Variations
• The shape of the thymus can vary widely in infants, sometimes stretching above the
clavicle. Infants can be born with an enlarged thymus that puts pressure on the
trachea (windpipe), heart, or other structures. It is not always recommended that
the thymus be removed in these cases, as it can have a negative effect on immune
system development.
Function
• The only purpose of the thymus is to produce white blood cells called T
lymphocytes (T cells). They are called T cells because they are primarily produced in
the thymus. The thymus produces some T cells before birth and continues the
process from birth through adolescence.
T cells come in several varieties that perform various roles in the immune response.
The most common types of T cells and their roles are:
• T4 or CD4 cells: Alert other white blood cells to pathogens, so they can be
destroyed
• T8 or CD8 cells: Control the overall immune system response by suppressing the
activities of other white blood cells
• Killer T cells: This specific type of CD8 cell recognizes and destroys foreign cells,
cancer cells, and those infected with a virus.

47. Associated Conditions
• Although the thymus stops producing T cells in adolescence and gradually
shrinks away, it can be affected by cancer. The two main types of cancer that
can arise in the thymus are:
• Thymoma: A tumor of the thymus
• Thymic cancer: A type of thymoma that often spreads (metastasizes)
Thymoma and thymic cancer are rare. The risk of developing cancer of the
thymus increases if a person has one of these other medical conditions:
• Myasthenia gravis: A chronic autoimmune and neuromuscular disease
• Lupus: An autoimmune disease that causes chronic, systemic (body-wide)
inflammation
• Rheumatoid arthritis: An autoimmune disease that causes chronic
inflammation of the joint tissues

48. Disorders