PowerLecture:
Chapter 15
The Endocrine System
Learning Objectives



Know the general mechanisms by which
molecules integrate and control the various
metabolic activities in organisms.
Understand how the neuro-endocrine center
controls secretion rates of other endocrine
glands and responses in nerves and
muscles.
Know the major endocrine glands and their
secretions.
Learning Objectives (cont’d)


Know how sugar levels are regulated by
hormones.
Differentiate the modes of action of steroid
and nonsteroid hormones.
Impacts/Issues
Hormones in the
Balance
Hormones in the Balance
Arsenic may be an endocrine disrupter,
especially of glucocorticoids.



Glucocorticoids in turn regulate genes that
protect against cancer.
This may be the link between the consumption
of arsenic in water supplies and increased rates
of bladder, lung, and skin cancers.
Hormones in the Balance
Other endocrine disrupters are also coming
under scrutiny.




The herbicide atrazine has been widely used on
crops and turf grasses.
PCBs, used for many years as fluid insulation in
electrical transformers, have been banned but
still persist in the environment, where they are
linked to reproductive disorders in humans and
animals.
Research is continuing on endocrine
disrupters; the jury is still out.
Useful References for Impacts/Issues
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 EPA:
Endocrine Disruptors Research
Initiative
 InfoTrac: New Report Points Up Growing
Evidence of Endocrine Disrupters. European
Report, May 4, 2006.
How Would You Vote?
To conduct an instant in-class survey using a classroom response
system, access “JoinIn Clicker Content” from the PowerLecture main
menu.
 Some
pesticides may disrupt hormone
function in humans and other animals.
Should they remain in use while researchers
study their safety?


a. No, they could be dangerous; until we know for
sure, it is better to be safe than sorry.
b. Yes, banning them because of potential harm
isn't fair; there should be solid evidence first.
Useful References for
How Would You Vote?
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
Endocrine Disruption Study on
Atrazine Disputed. Pesticide & Toxic
Chemical News, Jan. 13, 2003.
 Water Conserve 2.0: Chemical Affecting
Frogs’ Sexuality; Males Are Acquiring Female
Attributes after Exposure to a Common
Weedkiller, Study Says
Section 1
The Endocrine System:
Hormones
The Endocrine System: Hormones
Hormones are signaling molecules that are
carried in the bloodstream.





Signaling molecules are hormones and
secretions that can bind to target cells and
elicit in them a response.
Hormones are secreted by endocrine glands,
endocrine cells, and some neurons.
Local signaling molecules are released by
some cells; these work only on nearby tissues.
Pheromones are signaling molecules that have
targets outside the body and which are used to
integrate behaviors.
The Endocrine System: Hormones
Hormone sources: The endocrine system.



The sources of hormones (hormone producing
glands, cells, and organs) may be collectively
called the endocrine system.
Endocrine sources and the nervous system
function in highly interconnected ways.
The Endocrine System: Hormones
Hormones often interact.




In an opposing interaction the effect of one
hormone opposes the effect of another.
In a synergistic interaction the combined
action of two or more hormones is necessary to
produce the required effect on target cells.
In a permissive interaction one hormone
exerts its effect only when a target cell has
been “primed” to respond by another hormone.
hypothalamus
pineal gland
pituitary gland
thyroid gland
parathyroid glands
thymus gland
adrenal glands
pancreatic islets
ovaries
testes
Fig. 15.1a, p. 271
Useful References for Section 1
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
Review of the Endocrine System.
Deirdre G. Bauer. MedSurg Nursing, Oct.
2005.
Section 2
Types of Hormones and
Their Signals
Types of Hormones and Their Signals
Hormones come in several chemical forms.





Steroid hormones
are lipids made from
cholesterol.
Amine hormones are
modified amino acids.
Peptide hormones are peptides of only a few
amino acids.
Protein hormones are longer chains of amino
acids.
Types of Hormones and Their Signals


All hormones bind target cells; this signal is
converted into a form that works in the cell to
change activity.
A target cell’s response to a hormone is
dependent on two factors:
•
•
Different hormones activate different cellular
response mechanisms.
Not all cells have receptors for all hormones; the
cells that respond are selected by means of the type
of receptor they possess.
Types of Hormones and Their Signals
Steroid hormones interact with cell DNA.


Steroid hormones, such as estrogen and
testosterone, are lipid-soluble and therefore
cross plasma membranes readily.
•
•

Once inside the cell, they penetrate the nuclear
membrane and bind to receptors in the nucleus,
either turning on or turning off genes.
Switching genes on or off changes the proteins that
are made by the cell, thus effecting a response.
Some steroid hormones bind receptors in the
cell membrane and change membrane
properties to affect change to the target cell’s
function.
1 A steroid hormone
molecule moves from the
blood into the fluid that
bathes a target cell.
2 Being a
lipid-soluable
molecule, the
steroid hormone
diffuses across
the target
cell’s plasma
membrane.
5 In the
cytoplasm,
the resulting
protein carries
out the cell’s
response to
the hormone
signal.
change in
cell activity
3 The hormone diffuses
through the cytoplasm,
then on through the
nuclear envelope.
Inside the nucleus, it
will bind with a receptor
molecule.
receptor
hormonereceptor
complex
4 Now the
hormonereceptor
complex
triggers
gene activity
in the DNA
Fig. 15.2a, p. 273
Types of Hormones and Their Signals
Nonsteroid hormones act indirectly, by way
of second messengers.



Nonsteroid hormones include the amine,
peptide, and protein hormones.
Nonsteroid hormones cannot cross the plasma
membrane of target cells, so they must first
bind to a receptor on the plasma membrane.
•
•
Binding of the hormone to the receptor activates the
receptor; it in turn stimulates the production of a
second messenger, a small molecule that can relay
signals in the cell.
Cyclic AMP (cyclic adenosine monophosphate) is
one example of a second messenger.
1 A glucagon molecule
diffuses from blood into the
fluid that bathes the plasma
membrane of a liver cell.
glucagon receptor
at target cell’s
membrane
cyclic AMP + Pi
ATP
2 Glucagon binds with the
receptor, and the binding
activates adenylate cyclase.
This enzyme catalyzes the
formation of cyclic AMP
inside the target cell.
4 Protein kinase A
converts phosphorylase
kinase to active form.
This enzyme activates a
different enzyme, which
breaks down glycogen to
its glucose monomers.
3 The cyclic AMP
now activates
protein kinase A.
5 Protein kinase A
also inhibits an
enzyme required for
glycogen synthesis.
Fig. 15.2b, p. 273
Useful References for Section 2
The latest references for topics covered in this section can
be found at the book companion website. Log in to the
book’s e-resources page at www.thomsonedu.com to access
InfoTrac articles.
 InfoTrac:
Can PYY Cure Obesity? U.S. News
& World Report, Sept. 15, 2003.
Section 3
The Hypothalamus and
Pituitary Gland: Major
Controllers
The Hypothalamus and Pituitary Gland
The hypothalamus and pituitary gland
work jointly as the neural-endocrine control
center.



The hypothalamus is a portion of the brain that
monitors internal organs and conditions.
The pituitary is connected to the hypothalamus
by a stalk.
•
•
The posterior lobe consists of nervous tissue and
releases two hormones made in the hypothalamus.
The anterior lobe makes and secretes hormones
that control the activity of other endocrine glands.
hypothalamus
optic
chasma
Location of
the pituitary
pituitary stalk
gland:
anterior
lobe
posterior
lobe
membrane
around brain
Fig. 15.1b, p. 271
The Hypothalamus and Pituitary Gland
The posterior pituitary lobe produces ADH
and oxytocin.




Neurons in the hypothalamus produce
antidiuretic hormone (ADH) and oxytocin,
which are released from axon endings in the
capillary bed of the posterior lobe.
ADH (or vasopressin) acts on the walls of
kidney tubules to control the body’s water and
solute levels by stimulating reabsorption.
Oxytocin triggers uterine muscle contractions to
expel the fetus and acts on mammary glands to
release milk.
a Secretory neurons
in the hypothalamus
synthesize ADH or
oxytocin.
b The ADH Oxytocin
moves downward
inside the axons of
the secretory neurons
and accumulates in
the axon endings.
d The
hormone
molecules
move into the
general
circulation.
c Action potentials
trigger the release of
these hormones,
which enter blood
capillaries in the
posterior lobe of
the pituitary.
kidney
nephrons
ADH oxytocin
mammary
glands
muscles in
uterus
wall
Fig. 15.3, p. 274
The Hypothalamus and Pituitary Gland
The anterior pituitary lobe produces six
other hormones.




Corticotropin (ACTH) stimulates the adrenal
cortex.
Thyrotropin (TSH) stimulates the thyroid
gland.
Follicle-stimulating hormone (FSH) causes
ovarian follicle development and egg
production.
The Hypothalamus and Pituitary Gland




Luteinizing hormone (LH) also acts on the
ovary to release an egg.
Prolactin (PRL) acts on the mammary glands
to stimulate and sustain milk production.
Somatotropin (STH), also known as growth
hormone (GH), acts on body cells in general to
promote growth.
Most of these hormones are releasers that
stimulate target cells to secrete other
hormones; other hormones from the
hypothalamus are inhibitors and block
secretions.
a Cell bodies of
different secretory
neurons in the
hypothalamus secrete
releasing and
inhibiting hormones.
b The hormones are
picked up by a capillary
bed at the base of the
hypothalamus.
c The bloodstream
delivers hormones to a
capillary bed in the
anterior lobe of pituitary.
e Hormones from
anterior lobe cells
enter small blood
vessels that lead to
the general
circulation.
ACTH TSH
d Molecules of the releasing or
inhibiting hormone diffuse out of
capillaries and act on endocrine
cells in the anterior lobe.
FSH
LH
PRL
STH(GH)
most cells
(growth-promoting
effects)
adrenal
glands
thyroid gland
testes in males,
ovaries in females
mammary
glands
Fig. 15.4, p. 275
Useful References for Section 3
The latest references for topics covered in this section can
be found at the book companion website. Log in to the
book’s e-resources page at www.thomsonedu.com to access
InfoTrac articles.
 The
Pituitary Society
 InfoTrac: The Scent of Trust. Prevention, Oct.
2005.
Section 4
Factors That Influence
Hormone Effects
Factors That Influence Hormone Effects
Problems with control mechanisms can
result in skewed hormone signals.


Endocrine glands in general only release small
quantities of hormones and control the
frequency of release to make sure there isn’t
too much or too little hormone.
Factors That Influence Hormone Effects
Abnormal quantities of hormones can lead
to growth problems.



Gigantism results from an
oversecretion of growth
hormone in childhood.
Pituitary dwarfism
results from an
undersecretion of GH.
Figures 15.5a and 15.14
Factors That Influence Hormone Effects


Acromegaly is a
condition resulting from
an oversecretion of GH in
adulthood leading to
abnormal thickening of
tissues.
Diabetes insipidus
occurs when ADH
secretions fall or stop,
leading to dilute urine and
the possibility of serious
dehydration.
Figure 15.5b
Factors That Influence Hormone Effects
Hormone interactions, feedback, and other
factors also influence a hormone’s effects.


At least four factors influence the effects of any
given hormone.
•
•
•
•

Hormones often interact with one another.
Negative feedback mechanisms control secretion of
hormones.
Target cells may react differently to hormones at
different times.
Environmental cues can affect release of hormones.
Hormones throughout the body are affected in
similar ways.
Video: Hormone Replacement Therapy
 This
video clip is available in CNN Today
Videos for Anatomy & Physiology, 2003,
Volume VII. Instructors, contact your local
sales representative to order this volume,
while supplies last.
Useful References for Section 4
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac: Acromegaly. Ana
PokrajacSimeunovic and Peter Trainer. Chemist &
Druggist, Nov. 19, 2005.
Section 5
The Thymus, Thyroid,
and Parathyroid Glands
The Thymus, Thyroid,
and Parathyroid Glands
Thymus gland hormones aid immunity.
Thyroid hormones affect metabolism,
growth, and development.



The thyroid gland secretes thyroid hormone
(TH), which has effects on metabolism, growth,
and development; the thyroid gland also
secretes calcitonin, which helps regulate
calcium levels in the blood.
thyroid
cartilage
(Adam’s
apple)
blood
vessel
thyroid
gland
trachea
(windpipe)
Fig. 15.6a, p. 278
Stimulus
Blood level of thyroid
hormone falls below a
set point.
Response
+
Hypothalamus
TRH
Anterior Pituitary
TSH
Rise in the blood level
of thyroid hormone
inhibits secretion of
TRH and THS.
Thyroid Gland
Thyroid hormone
is secreted.
Fig. 15.6b, p. 278
Stimulus
Blood level of thyroid
hormone falls below a
set point.
Response
+
Hypothalamus
–
TRH
–
Anterior Pituitary
TSH
Rise in the blood level
of thyroid hormone
inhibits secretion of
TRH and TSH.
Thyroid Gland
Thyroid hormone
is secreted.
Stepped Art
Fig. 15.6b, p. 278
The Thymus, Thyroid,
and Parathyroid Glands

Iodine-deficient diets interfere with proper
synthesis of thyroid hormones.
•
•
Simple goiter is an enlargement
of one or both lobes of the thyroid
gland in the neck; enlargement
follows low blood levels of thyroid
hormones (hypothyroidism).
Graves disease and other forms of
hyperthyroidism result from too
much thyroid hormone in the blood.
Figure 15.7
The Thymus, Thyroid,
and Parathyroid Glands
PTH from the parathyroids is the main
calcium regulator.


Humans have four parathyroid
glands, which secrete
parathyroid hormone (PTH),
the main regulator of blood
calcium levels.
•
•
More PTH is secreted when blood calcium levels
drop below a certain point; less is secreted when
calcium rises.
Calcitonin contributes to processes that pull calcium
out of the blood.
Decrease in calcium ion
concentration in blood
Parathyroid
glands
PTH
stimulates
Calcium ion
level increases
Osteoclasts
release
calcium ions
from bone
Kidney tubules
increase
calcium ion
reabsorption
Intestine
increases
calcium ion
absorption
Fig. 15.8, p. 279
Decrease in calcium ion
concentration in blood
Parathyroid
glands
PTH
stimulates
Calcium ion
level increases
Osteoclasts
release
calcium ions
from bone
Kidney tubules
Increase calcium
ion reabsorption
Intestine
Increases calcium
ion absorption
Stepped Art
Fig. 15.8, p. 279
The Thymus, Thyroid,
and Parathyroid Glands


Rickets in children arises from a vitamin D
deficient diet; vitamin D is needed to aid
absorption of calcium from food.
Hyperparathyroidism sees so much calcium
being withdrawn from a person’s bones that the
bone tissue is dangerously weakened.
Useful References for Section 5
The latest references for topics covered in this section can
be found at the book companion website. Log in to the
book’s e-resources page at www.thomsonedu.com to access
InfoTrac articles.
 The American
Thyroid Association
 InfoTrac: Primary Hyperparathyroidism (The
Effective Physician). William E. Golden;
Robert H. Hopkins. Internal Medicine News,
Dec. 1, 2005.
Section 6
Adrenal Glands and
Stress Responses
Adrenal Glands and Stress Responses
The adrenal cortex produces
glucocorticoids and mineralocorticoids.


One adrenal gland is located on top of each
kidney; the outer part of each gland is the
adrenal cortex, the site of production for two
major steroid hormones.
Adrenal Glands and Stress Responses

Glucocorticoids raise the level of glucose in
the blood.
•
•
•
The main glucocorticoid, cortisol, is secreted when
the body is stressed and blood sugar levels drop;
cortisol promotes gluconeogenesis, a mechanism
for making glucose from amino acids derived from
protein breakdown.
Cortisol also dampens the uptake of glucose from
the blood, stimulates the breakdown of fats for
energy, and suppresses inflammation.
Hypoglycemia can result when the adrenal cortex
makes too little cortisol; this results in chronically low
glucose levels in the blood.
Stimulus
Hypothalamus
a Blood level of
cortisol falls below
a set point
Response
b CRH
Anterior Pituitary
adrenal
cortex
ACTH
adrenal
medulla
Adrenal Cortex
f Both the
hypothalamus
and pituitary
detect rise in
blood level of
cortisol and
slow its
secretion.
Cortisol is secreted,
with these effects:
c Cell uptake of glucose from blood slows in many tissues,
especially muscles (not the brain).
d Proteins are broken down in many tissues, muscles
especially. The amino acids are converted to glucose, and
used in the assembly or repair of cell structures.
kidney
e Fats in adipose tissue are broken down to fatty acids that
enter blood as an alternative energy source, indirectly
conserving glucose for the brain.
Fig. 15.9, p. 281
Adrenal Glands and Stress Responses


Mineralocorticoids regulate the
concentrations of minerals such as K+ and Na+
in the extracellular fluid; aldosterone is one
example that works in the nephrons of the
kidneys.
The adrenal cortex also secretes sex hormones
in the fetus and at puberty.
Adrenal Glands and Stress Responses
Hormones from the adrenal medulla help
regulate blood circulation.



The inner part of the adrenal gland, the adrenal
medulla, secretes epinephrine and
norepinephrine.
Secretion by the adrenal medulla influences
these molecules to behave like hormones to
regulate blood circulation and carbohydrate use
during stress.
Adrenal Glands and Stress Responses
Long-term stress can damage health.




Stress triggers the fight-flight response and the
release of cortisol, epinephrine, and
norepinephrine; constant release of these
molecules can contribute to hypertension and
cardiovascular disease.
Excess cortisol suppresses the immune
system, making individuals susceptible to
disease.
Social connections for support and exercise for
health can reduce the effects of stress.
Video: Peanut Allergies
 This
video clip is available in CNN Today
Videos for Anatomy & Physiology, 2004,
Volume VIII. Instructors, contact your local
sales representative to order this volume,
while supplies last.
Useful References for Section 6
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
Prenatal Exposure to Stress and
Stress Hormones Influences Child
Development. Elysia Poggi Davis; Curt A.
Sandman. Infants & Young Children, July–
Sept. 2006.
Section 7
The Pancreas:
Regulating Blood Sugar
The Pancreas: Regulating Blood Sugar

The pancreas has both exocrine and
endocrine functions; the endocrine cells are
located in clusters called pancreatic islets.
stomach
pancreas
small
intestine
Fig. 15.10, p. 282
The Pancreas: Regulating Blood Sugar

Each pancreatic islet secretes three
hormones:
•
•
•
Alpha cells secrete glucagon, which causes
glycogen stored in the liver to be converted to
glucose, which then enters the bloodstream.
Beta cells secrete insulin, which stimulates the
uptake of glucose by liver, muscle, and adipose cells
to reduce levels in the blood, especially after a meal.
Delta cells secrete somatostatin, which can inhibit
the secretion of glucagon and insulin.
Figure 15.9
a Stimulus
Increase in blood glucose
f Stimulus
Decrease in blood
glucose
PANCREAS
b alpha cells
x
_
glucagon
c beta cells
+
insulin
d Body cells, especially in muscle
and adipose tissue, take up and
use more glucose.
Cells in skeletal muscle and liver
store glucose in the form of
glycogen.
e Response
Decrease in blood glucose
g alpha cells h beta cells
_
+
x
glucagon
insulin
i Cells in liver break
down glycogen faster.
The released glucose
molecules enter blood.
j Response
Increase in blood
glucose
Fig. 15.10a, p. 282
Useful References for Section 7
The latest references for topics covered in this section can
be found at the book companion website. Log in to the
book’s e-resources page at www.thomsonedu.com to access
InfoTrac articles.
 Journal
of the Pancreas
 InfoTrac: Perfecting a ‘Pancreas’: Scientists
Fine-Tune a Device to Be Used by Diabetics.
Jamie Talan. Newsday, May 15, 2006.
Section 8
Disorders of Glucose
Homeostasis
Disorders of Glucose Homeostasis
Diabetes mellitus is a
disease resulting from the
secretion of too little insulin.



Without insulin, cells can’t
remove glucose from the
blood; the kidneys remove the excess in urine,
creating imbalances in water-solute
concentrations.
Metabolic acidosis, a lower than optimal blood
pH, can result because of this imbalance.
Figure 15.11
Disorders of Glucose Homeostasis
In type 1 diabetes (also known as
“juvenile-onset diabetes”) the insulin is no
longer produced because the beta cells
have been destroyed by an autoimmune
response.



Only about 1 in 10 diabetics have this form of
diabetes.
Treatment is by insulin injection.
Disorders of Glucose Homeostasis
Type 2 diabetes is a global health crisis.


In type 2 diabetes the insulin levels are near
normal but the target cells cannot respond to
the hormone.
•
•

Beta cells eventually break down and produce less
and less insulin.
Excess glucose in the blood damages capillaries.
Cardiovascular disease, stroke, heart attack,
and other serious complications arise.
Disorders of Glucose Homeostasis
Metabolic syndrome is a warning sign.


Prediabetes describes individuals with slightly
elevated blood sugar levels that have an
increased risk for developing type 2 diabetes;
about 20 million Americans fall into this
category and do not know it.
Disorders of Glucose Homeostasis


A composite of features collectively called
metabolic syndrome also describe risk for
diabetes; these features include: “apple
shaped” waistline, elevated blood pressure, low
levels of HDL, and elevated glucose and
triglycerides.
Type 2 diabetes can be controlled with a
combination of improved diet, exercise, and
sometimes drugs.
Video: Gene Therapy for Diabetes
 This
video clip is available in CNN Today
Videos for Genetics, 2005, Volume VII.
Instructors, contact your local sales
representative to order this volume, while
supplies last.
Useful References for Section 8
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 American
Diabetes Association
 CDC: Diabetes
 InfoTrac: Pancreas and Islet Transplantation
in Type 1 Diabetes. Diabetes Care, April
2006.
 InfoTrac: Preventing Diabetes. Kathy
Doheny. Natural Health, April 2004.
Section 9
Some Final Examples of
Integration and Control
Some Final Examples of
Integration and Control
Light/dark cycles influence the pineal gland,
which produces melatonin.


Located in the brain, the pineal gland is a
modification of a primitive “third eye” and is
sensitive to light and seasonal influences; this
gland secretes the hormone melatonin.
•
•
Melatonin is secreted in the dark, and levels change
with the seasons.
The biological clock seems to tick in synchrony with
day length and is apparently influenced by
melatonin.
Some Final Examples of
Integration and Control


Seasonal affective disorder
(SAD) affects persons during
the winter and may result
from an out-of-sync biological
clock; melatonin makes it
worse; exposure to intense
light helps.
Melatonin levels may
potentially be linked to the
onset of puberty.
Some Final Examples of
Integration and Control
Hormones also are produced in the heart
and GI tract.



Atrial natriuretic peptide (ANP) produced by
the heart atria regulates blood pressure.
Gastrin and secretin from the GI tract stimulate
release of stomach and intestinal secretions.
Some Final Examples of
Integration and Control
Prostaglandins have many effects.


More than 16 prostaglandins have been
identified in tissues throughout the body.
•
•

When stimulated by epinephrine and norepinephrine,
prostaglandins cause smooth muscles in blood
vessels to constrict or dilate.
Allergic responses to dust and pollen may be
aggravated by the effects of prostaglandins on
airways in the lungs.
Prostaglandins have major effects on
menstruation and childbirth.
Some Final Examples of
Integration and Control
Growth factors influence cell division.


Hormonelike proteins called growth factors
influence growth by regulating the rate of
cellular division.
•
•
Epidermal growth factor
(EGF) influences the growth
of many cell types, as does
insulinlike growth factor (IGF).
Nerve growth factor (NGF)
promotes growth and survival
of neurons in the developing
embryo.
Some Final Examples of
Integration and Control

The current list of growth factors is expanding
rapidly; many of these factors may have
applications in medicine.
Pheromones may be important
communication molecules in humans.



Pheromones are released outside of the body
by several animals to serve as sex attractants,
territory markers, and communication signals.
Recent studies suggest that humans also may
communicate using pheromones.
Some Final Examples of
Integration and Control
Are endocrine disrupters at work?



Endocrine disrupters are proposed to be
environmental substances that interfere with
reproduction or development.
Sperm counts in
males in Western
countries declined
about 40% between
the years 1938 and
1990, possibly due
to exposure to estrogens in the environment.
Figure 15.13
Useful References for Section 9
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.

National Research Center for Women & Families:
Are Pretty Products Causing Early Puberty?
 Howard Hughes Medical Institute: The Matters:
Biological Clockworks
 InfoTrac: Drug of Darkness: Can a Pineal Hormone
Head Off Everything from Breast Cancer to Aging?
Science News, May 13, 1995.
 InfoTrac: The Chemistry of Love. Sanjay Gupta.
Time, Feb. 18, 2002.