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CHAPTER
17
Part 1
The Endocrine
System
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Endocrine Organs
The Endocrine System: An Overview
•  A system of ductless glands
•  Secrete messenger molecules called
hormones
•  Interacts closely with the nervous system
•  Endocrinology
•  Study of hormones and endocrine glands
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Location of the Major Endocrine Glands
Pineal gland
•  Scattered throughout the body
•  Pure endocrine organs are the …
Hypothalamus
Pituitary gland
Thyroid gland
•  Pituitary, pineal, thyroid, parathyroid, and adrenal
glands
Parathyroid glands
(on dorsal aspect
of thyroid gland)
Thymus
•  Organs containing endocrine cells include:
•  Pancreas, thymus, gonads, and the hypothalamus
•  Plus other organs secrete hormones (eg., kidney,
stomach, intestine)
Adrenal glands
Pancreas
•  Hypothalamus is a neuroendocrine organ
•  Produces hormones and has nervous functions
Ovary (female)
•  Endocrine cells are of epithelial origin
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Hormones
Testis (male)
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Figure 17.1
Control of Hormones Secretion
•  Classes of hormones
•  Amino acid–based hormones
•  Steroids—derived from cholesterol
•  Basic hormone action
•  Circulate throughout the body in blood vessels
•  Influences only specific tissues— those with
target cells that have receptor molecules for
that hormone
•  A hormone can have different effects on
different target cells (depends on the
receptor)
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•  Secretion triggered by three major types of
stimuli:
•  Humoral—simplest of endocrine control
mechanisms
•  Secretion in direct response to changing
ion or nutrient levels in the blood
•  Example: Parathyroid monitors calcium
•  Responds to decline by secreting
hormone to reverse decline
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1
Control of Hormones Secretion
Types of Endocrine Gland Stimuli
•  Secretion triggered by three major types of stimuli
(continued)
1 Capillary blood contains
low concentration of Ca2+,
which stimulates…
•  Neural
•  Sympathetic nerve fibers stimulate cells in the
adrenal medulla
•  Induces release of epinephrine and norepinephrine
•  Hormonal
•  Stimuli received from other glands
•  Certain hormones signal secretion of other
hormones
•  Example: Hypothalamus secretes hormones 
stimulates pituitary  stimulates other glands
(a)
Humoral stimulus
(b)
Neural stimulus
1 Preganglionic sympathetic fibers
stimulate adrenal medulla cells…
Hormonal stimulus
Hypothalamus
CNS (spinal cord)
Capillary (low Ca2+
in blood)
2 …stimulate
the anterior
pituitary gland
to secrete
hormones
that…
Thyroid gland
(posterior view)
Parathyroid
glands
Pituitary
gland
Preganglionic
sympathetic
fibers
Thyroid
gland
Adrenal
cortex
Gonad
(Testis)
Medulla of
adrenal gland
Parathyroid
glands
PTH
Capillary
2 …secretion of parathyroid hormone
2 …to secrete catecholamines
(PTH) by parathyroid glands. PTH acts to (epinephrine and norepinephrine)
increase blood Ca2+.
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(c)
1 The hypothalamus secretes
hormones that…
3 …stimulate other endocrine glands
to secrete hormones
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Control of Hormone Secretion
The Pituitary Gland
•  Always controlled by feedback loops
•  Secretes nine major hormones
•  Attached to the hypothalamus by the infundibulum
•  Two basic divisions of the pituitary gland
•  Adenohypophysis (anterior lobe)
•  Has three major divisions
•  Pars distalis, pars intermedia, and pars
tuberalis
•  Neurohypophysis (posterior lobe) —has two major
divisions
•  Pars nervosa and infundibular
•  Blood concentration declines below a
minimum --> More hormone is secreted
•  Blood concentration exceeds maximum
--> Hormone production is halted
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Figure 17.2
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The Pituitary Gland
The Anterior Lobe
Corpus
callosum
Thalamus
Pineal
Hypothalamus
Mammillary
body
Brain stem
(b)
Acidophil Basophil
(a)
Optic chiasma
Median eminence
of hypothalamus
Anterior lobe
Pars tuberalis
Pars intermedia
Pars distalis
(c)
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Pituitary (hypophysis)
Chromophobe cell
Mammillary
body
Tuber cinereum
Posterior lobe
Infundibulum
Pars nervosa
Capillary with
red blood cells
(d)
•  The pars distalis—largest division of the
anterior lobe
•  Contains five different endocrine cell groups
•  Makes and secretes seven different hormones
•  Tropic hormones regulate hormone
secretion by other glands
•  Include: TSH, ACTH, FSH, LH
•  GH, PRL, and MSH
•  Act directly on non-endocrine target tissues
Spherical cluster
of cells
Figure 17.3
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2
The Anterior Lobe
The Anterior Lobe
•  Growth hormone, GH (somatotropic
hormone)
•  Thyroid-stimulating hormone, TSH
•  Produced by thyrotropic cells
•  Signals thyroid gland to secrete thyroid
hormone
•  Produced by somatotropic cells
•  Stimulates body growth by stimulating
increased protein production and growth of
epiphyseal plates
•  Stimulates growth directly and indirectly by
the liver’s secretion of insulin-like growth
factor-1.
•  Adrenocorticotropic hormone, ACTH
•  Stimulates the adrenal cortex to secrete
hormones that help cope with stress
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The Anterior Lobe
The Anterior Lobe
•  Melanocyte-stimulating hormone, MSH
•  Prolactin—produced by prolactin cells
•  In humans, MSH functions in appetite
supression
•  Targets milk-producing glands in the breast—
stimulates milk production
•  Gonadotropins— are produced by
gonadotropic cells and affect the gonads
•  Endocrine cells of the pars distalis
•  Clustered in spheres and branching cords
•  Follicle-stimulating hormone, FSH, and
luteinizing hormone, LH
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The Anterior Lobe
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The Anterior Lobe
Table 17.1
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Table 17.1
3
Hypothalamic Control of Hormone Secretion
from the Anterior Lobe
Hypothalamic Control of Hormone Secretion
from the Anterior Lobe
•  The hypothalamus
•  Releasing hormones
•  Controls secretion of anterior lobe hormones
•  Exerts control by secreting:
•  Releasing hormones—prompt anterior
lobe to release hormones
•  Inhibiting hormones—turn off secretion of
anterior lobe hormones
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The Posterior Lobe
The Anterior Lobe
Hypothalamic neuron
cell bodies
Superior hypophyseal
artery
Hypophyseal
portal system
Primary capillary
plexus
Hypophyseal
portal veins
Secondary
capillary
plexus
Anterior lobe
of pituitary
TSH, FSH, LH,
ACTH, GH, PRL
•  Are secreted like neurotransmitters
•  Enter a primary capillary plexus
•  Travel in hypophyseal portal veins to a
secondary capillary plexus
(hypothalamohypophyseal portal system)
•  From the secondary capillary plexus,
hormones secreted by the anterior lobe
enter general circulation and travel to target
organs
Hypothalamus
1 When appropriately
stimulated, hypothalamic
neurons secrete releasing
and inhibiting hormones into
the primary capillary plexus.
2 Hypothalamic hormones
travel through the portal
veins to the anterior pituitary
where they stimulate or
inhibit release of hormones
from the anterior lobe.
•  Is structurally part of the brain
•  Its axons make up the hypothalamic–
hypophyseal tract
•  Arises from neuronal cell bodies in the
hypothalamus
•  Supraoptic nucleus
•  Paraventricular nucleus
3 Anterior pituitary
hormones are secreted into
the secondary capillary
plexus.
(a) Relationship between the anterior pituitary and the hypothalamus
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Figure 17.4a
The Posterior Lobe
•  Does not make hormones
•  Stores and releases hormones made in the
hypothalamus
•  Releases two peptide hormones
•  Antidiuretic hormone (ADH) [aka, vasopressin]
•  Oxytocin (OT)
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Relationship Between the Posterior Pituitary
and Hypothalamus
1 Hypothalamic neurons
synthesize oxytocin and
ADH.
Paraventricular nucleus
Hypothalamus
Supraoptic nucleus
Optic chiasma
Infundibulum (connecting stalk)
Hypothalamichypophyseal
tract
Inferior
hypophyseal
artery
Axon
terminals
Posterior
lobe of
pituitary
Oxytocin
ADH
2 Oxytocin and ADH are
transported along the
hypothalamichypophyseal tract to the
posterior lobe.
3 Oxytocin and ADH are
stored in axon terminals
in the posterior pituitary.
4 Oxytocin and ADH are
released into the blood
when hypothalamic
neurons fire.
(b) Relationship between the posterior pituitary and the hypothalamus
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Figure 17.4b
4
The Posterior Lobe
•  ADH (vasopressin)
•  Made in supraoptic nucleus
•  Targets kidneys to reabsorb water
•  Oxytocin
•  Produced in the paraventricular nucleus
•  Induces smooth muscle contraction of
reproductive organs, ejects milk during breast
feeding, and signals contraction of the uterus
during childbirth
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Table 17.1
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The Thyroid Gland
The Thyroid Gland
Hyoid bone
Thyroid cartilage
•  Located in the anterior neck
•  Largest pure endocrine gland
•  Composed of follicles and areolar connective
tissue
•  Produces two hormones
Common carotid
artery
Inferior thyroid
artery
•  Thyroid hormone (TH)
•  Calcitonin
Right
subclavian
artery
Trachea
Epiglottis
External carotid
artery
Superior thyroid
artery
Isthmus of
thyroid gland
Left subclavian
artery
Left lateral
lobe of thyroid
gland
Aorta
(a) Gross anatomy of the thyroid gland, anterior view
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The Thyroid Gland
Figure 17.5a
The Parathyroid Glands
Colloid-filled Follicular cells
follicles
(secrete thyroid hormone)
•  Lie on the posterior surface of the thyroid
gland
•  Contain two types of endocrine cells
•  Chief cells
•  Produce parathyroid hormone (PTH)
•  Increases blood concentration of Ca2+
•  Oxyphil cells
•  Function unknown
Parafollicular cell
(secretes calcitonin)
(b) Photomicrograph of thyroid gland follicles (160×)
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Figure 17.5b
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5
The Parathyroid Glands
The Adrenal (Suprarenal) Glands
Pharynx
(posterior
aspect)
Thyroid
gland
Esophagus
Parathyroid
cells
(secrete
parathyroid
hormone)
Oxyphil
cells
Parathyroid
glands
Trachea
•  Pyramid-shaped glands located on the
superior surface of each kidney
•  Supplied by about 60 suprarenal arteries
•  Nerve supply is almost exclusively
sympathetic fibers
Capillary
(a) Location of parathyroid
glands, posterior view
(b) Photomicrograph of
parathyroid gland
tissue (360×)
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Figure 17.6a, b
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The Adrenal (Suprarenal) Glands
The Adrenal Medulla
•  Two endocrine glands in one
•  Chromaffin cells
•  Adrenal medulla—a cluster of neurons
•  Derived from neural crest
•  Part of the sympathetic nervous system
•  Adrenal cortex—forms the bulk of the gland
•  Derived from somatic mesoderm
•  All adrenal hormones help one cope with
danger, terror, or stress
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•  Are modified ganglionic sympathetic neurons
•  Secrete amine hormones epinephrine and
norepinephrine
•  Enhance “fight-or-flight” response
•  Hormones are stored in secretory vesicles
•  Are arranged in spherical clusters and some
branching cords
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The Adrenal Cortex
The Adrenal Cortex
•  Secretes lipid-based steroid hormones
•  Cortex is composed of three layers (zones)
•  Hormones are corticosteroids
•  Adrenal corticosteroids are of two main
classes
•  Zona glomerulosa—cells arranged in
spherical clusters
•  Zona fasciculata—cells arranged in parallel
cords; contains lipid droplets
•  Zona reticularis—cells arranged in a
branching network
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•  Mineralocorticoids
•  Glucocorticoids
•  Also secretes androgens
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Mineralocorticoids
Glucocorticoids
•  Aldosterone—secreted by the zona
glomerulosa
•  Cortisol is the main type
•  Secreted by zona fasciculata and zona
reticularis
•  Helps the body deal with stressful situations
•  Secreted in response to decline in blood
volume or blood pressure
•  Is the terminal hormone of the reninangiotensin mechanism
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The Adrenal Gland–Gross and Microscopic
Stress and the Adrenal Gland
Short-term stress
More prolonged stress
Stress
Hormones
secreted
Capsule
Zona
glomerulosa
Nerve impulses
Hypothalamus
Aldosterone
CRH (corticotropinreleasing hormone)
Zona
fasciculata
Medulla
Cortex
Adrenal gland
Spinal cord
Cortisol
and
androgens
Cortex
Zona
reticularis
Medulla
Kidney
To target in blood
Adrenal medulla
(secretes amino acid–
based hormones)
Adrenal
medulla
(a)Drawing of the histology of the
adrenal cortex and a portion of
the adrenal medulla
Corticotroph cells
of anterior pituitary
Preganglionic
sympathetic
fibers
Epinephrine
and
norepinephrine
(b) Photomicrograph (140X)
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Figure 17.7a, b
Catecholamines
(epinephrine and
norepinephrine)
Short-term stress response
1.Increased heart rate
2.Increased blood pressure
3.Liver converts glycogen to glucose and releases
glucose to blood
4.Dilation of bronchioles
5.Changes in blood flow patterns leading to decreased
digestive system activity and reduced urine output
6.Increased metabolic rate
Adrenal cortex
(secretes steroid
hormones)
ACTH
Mineralocorticoids
Glucocorticoids
Long-term stress response
1.Retention of sodium
and water by kidneys
2.Increased blood volume
and blood pressure
1.Proteins and fats converted
to glucose or broken down
for energy
2.Increased blood glucose
3.Suppression of immune
system
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The Pineal Gland
The Pancreas
•  Located on the roof of the diencephalon
•  Shaped like a pine cone
•  “Pineal sand” is radiopaque
•  Located in the posterior abdominal wall
•  Contains endocrine and exocrine cells
•  Used as a landmark to identify other brain
structures in X rays
•  Pinealocytes secrete melatonin
•  A hormone that regulates circadian rhythms
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Figure 17.8
•  Exocrine cells
•  Acinar cells —secrete digestive enzymes
•  Endocrine cells
•  Pancreatic islet cells —islets of Langerhans
•  About one million islets—scattered
throughout the pancreas
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The Pancreas
The Pancreas
•  Main endocrine cell types
•  Pancreatic islets contain two rare cell types
•  Alpha cells (α cells)—secrete glucagon
•  Signals liver to release glucose from glycogen
•  Raises blood sugar
•  Beta cells (β cells)—secrete insulin
•  Signals most body cells to take up glucose
from the blood
•  Promotes storage of glucose as glycogen in
liver
•  Lowers blood sugar
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•  Delta (∂) cells
•  Secrete somatostatin
•  Inhibits secretion of insulin and glucagon
•  F (PP) cells
•  Secrete pancreatic polypeptide
•  May inhibit exocrine activity of the
pancreas
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The Thymus
The Gonads
•  Located in the lower neck and anterior thorax
•  Important immune organ
•  Site at which T-lymphocytes arise from
precursor cells
•  Main sources of sex hormones
•  Testes and ovaries
•  Male
•  Interstitial cells secrete androgens
•  Primarily testosterone
•  Promotes the formation of sperm
•  Maintains secondary sex characteristics
PLAY
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Male Hormones
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The Gonads
Other Endocrine Structures
•  Female
•  Endocrine cells occur within
•  Ovaries
•  Androgens secreted by the theca folliculi
•  Converted to estrogen by follicular granulosa
cells
•  Estrogen
•  Maintains secondary sex characteristics
•  Progesterone
•  Prepares the uterus for pregnancy
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•  The heart
•  Atria contain atrial natriuretic peptide
(ANP)
•  The GI tract
•  Enteroendocrine cells
•  The placenta
•  Sustains the fetus and secretes several
steroid protein hormones
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Other Endocrine Structures
•  The kidneys
•  Cells of the juxtaglomerular apparatus
(JGA) secrete renin
•  Endothelial cells and interstitial connective
tissue—secrete erythropoietin
•  The skin
•  Modified cholesterol molecules convert to a
precursor of vitamin D
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Pituitary Disorders
•  Gigantism
•  Hypersecretion of GH in children
•  Pituitary dwarfism
•  Hyposecretion of GH
•  Diabetes insipidus
•  Pars nervosa does not make enough ADH
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Disorders of the Pancreas: Diabetes Mellitus
Diabetes Mellitus
•  Caused by
•  Type 2 diabetes
•  Insufficient secretion of insulin
•  Resistance of body cells to the effects of
insulin
•  Type 1 diabetes
•  Develops suddenly, usually before age 15
•  T cell–mediated autoimmune response
destroys beta cells
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• 
• 
• 
• 
Adult onset
Usually occurs after age 40
Cells have lowered sensitivity to insulin
Controlled by dietary changes and regular
exercise
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Disorders of the Thyroid Gland
Disorders of the Thyroid Gland
•  Grave’s disease
•  Myxedema
•  Most common type of hyperthyroidism
•  Immune system makes abnormal antibodies
•  Stimulates the oversecretion of TH by
follicle cells
•  Leads to nervousness, weight loss, sweating,
and rapid heart rate
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•  Adult hypothyroidism
•  Antibodies attack and destroy thyroid tissue
•  Low metabolic rate and weight gain are
common symptoms
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Disorders of the Thyroid Gland
Thyroid Disorders
•  Endemic goiter
•  Due to lack of iodine in the diet
•  Cretinism
•  Hypothyroidism in children
•  Short, disproportionate body, thick tongue,
and mental retardation
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Disorders of the Adrenal Cortex
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Figure 17.10
Thyroid Disorders
•  Cushing’s syndrome
•  Caused by hypersecretion of glucocorticoid
hormones—usually a pituitary tumor
•  Addison’s disease
•  Hyposecretory disorder of the adrenal cortex
•  Deficiencies of both mineralocorticoids and
glucocorticoids
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The Endocrine System Throughout Life
•  Thyroid gland
•  Forms from a thickening of endoderm on the
floor of the pharynx
•  Parathyroids and the thymus gland
•  From endoderm lining the pharyngeal
pouches
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Figure 17.11
Embryological Origin of Selected Endocrine
Organs
•  Pineal gland
•  Originates from ependymal cells
•  Pituitary gland—dual origin
•  Adenohypophysis originates from the roof
of the mouth
•  Neurohypophysis grows inferiorly from the
floor of the brain
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Embryological Origin of Selected Endocrine
Organs
Embryological Origin of Selected Endocrine
Organs
Hypophyseal
pouch
Pharyngeal
pouches
Future thymus
•  Adrenal gland—dual origin gland
•  Adrenal medulla—from neural crest cells of
nearby sympathetic trunk ganglia
•  Adrenal cortex—from mesoderm lining the
coelom
Brain
Future parathyroid
glands
Hypothalamus
(a) Week 5. Thyroid, thymus,
and parathyroid glands form
from pharyngeal endoderm.
Hypophyseal pouch extends
superiorly from ectoderm in
the roof of the mouth.
Neurohypophyseal
bud
Future thyroid
gland
Esophagus
Lung bud
Future mouth
Hypophyseal
pouch
(b) Week 6. Inferior extension
of the floor of the
diencephalon forms the
neurohypophyseal bud.
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Neurohypophyseal
bud
Anterior
Posterior
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Embryological Origin of Selected Endocrine
Organs
Hypophyseal
pouch
(c) Week 7. Hypophyseal
pouch pinches off the
surface ectoderm and is
closely associated with the
neurohypophyseal bud.
Neurohypophyseal
bud
Third
ventricle
of brain
(d) Week 8. Hypophyseal
pouch forms the
anterior lobe of pituitary;
neurohypophyseal bud
forms the posterior lobe.
Distinct portions of
each differentiate.
Anterior lobe
Posterior lobe
Pars tuberalis
Infundibulum
Pars distalis
Pars nervosa
Figure 17.12a, b
The Endocrine System Throughout Life
•  Endocrine organs operate effectively until old age
•  Anterior pituitary
•  Increase in connective tissue and lipofuscin
•  Decrease in vascularization and number of hormonesecreting cells
•  Adrenal cortex
•  Normal rates of glucocorticoid secretion continue
•  Adrenal medulla
•  No age-related changes in catecholamines
Pars intermedia
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Figure 17.12c, d
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The Endocrine System Throughout Life
•  Thyroid hormones
•  Decrease slightly with age
•  Parathyroid glands
•  Little change with aging
•  GH, DHEA, and the sex hormones
•  Marked drops in secretion with age
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