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A Wealth of Information. A World of Ideas.
Instructor’s Guide
The Human Body: How It Works
HUMAN DEVELOPMENT AND THE REPRODUCTIVE SYSTEM
Introduction
This program is part of the nine-part series The Human Body: How It Works. The series uses physiologic animations and illustrations, microscopic imaging, expert commentary, and footage of the body
in motion to provide a thorough overview of the amazing human machine. The series includes:
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Cells, Tissues, and Skin
The Immune System
Human Development and the Reproductive System
The Respiratory System
The Circulatory System
The Skeletal and Muscular Systems
Digestion and Nutrition
The Endocrine System
The Nervous System and the Senses
Topics
Chapter 1: Development Begins
The program begins by describing ovulation and conception, then moves on to a detailed discussion
of the development mechanisms of cell division, differentiation, and morphogenesis, up to the formation of the embryonic disc. Viewers will learn about chromosomes, diploid and haploid cells, and
mitosis and meiosis. Also covered: The structure of the blastocyst.
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HUMAN DEVELOPMENT AND THE REPRODUCTIVE SYSTEM
INSTRUCTOR’S GUIDE
Chapter 2: The Developing Embryo
This section focuses on gastrulation, and the ectoderm, endoderm, and mesoderm. The embryo grows
more complex with the formation of these layers; animations help viewers visualize this stage of
growth, and the anatomical parts the germ layers will eventually become. Also covered: The primitive
streak and the primitive groove.
Chapter 3: Establishing the Axes
The three axes of the human body and their role in “patterning” is the subject of this section. Also
covered: Limb buds.
Chapter 4: Embryonic Sexual Development
The program now traces the development of an embryo into either a male or female fetus. How
X and Y chromosomes influence the sex of the fetus, the evolution of primordial germ cells into
spermatogonia or oogonia, and the part played by androgens are all addressed. Also covered:
Gametes; the formation of male and female gonads; primordial follicles.
Chapter 5: Development of the Reproductive System
Unipotential and bipotential tissues are featured here — specifically, how the Müllerian and Wolffian
ducts form the fetus’ internal genitalia, and the urethral fold, genital swelling, and genital tubercle
form the external genitalia. Also covered: The role of androgens.
Chapter 6: Puberty in the Male
This section explains how the maturation of the hypothalamus in the male starts a chain of processes
that ends with the production of testosterone. The delivery of GnRH to the pituitary, the release of
gonadotropins, and spermiogenesis and other effects of testosterone are all covered. Also covered:
The epididymis, vas deferens, and semen.
Chapter 7: Puberty in the Female
This section explains how the maturation of the hypothalamus in the female triggers a chain of
processes stimulated by estrogen. The action of GnRH on the pituitary and gonadotropins in the
ovary, and the initiation of the menstrual cycle and other effects of estrogen are all covered.
Animations provide a detailed presentation of the menstrual cycle, with emphasis on the corpus
luteum. Also covered: Granulosa cells; Graafian follicles.
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The Human Body: How It Works
HUMAN DEVELOPMENT AND THE REPRODUCTIVE SYSTEM
INSTRUCTOR’S GUIDE
Learning Objectives
Students will learn…
• The three stages of development in the womb — pre-embryonic, embryonic, and fetal — and how
they are generated by the development mechanisms of cell division, differentiation, morphogenesis,
patterning, and growth
• The process of gastrulation, the formation of the germ layer, and the anatomical parts
the germ layers will eventually become
• How the axes are established
• How X and Y chromosomes influence the sex of the fetus, and how gonads, spermatogonia,
and oogonia are developed
• How bipotential and unipotential tissues form the internal and external genitalia of the fetus
• The effects of estrogen and testosterone in puberty, including the role of the hypothalamus
and the pituitary glands, and of gonadotropins
• The details of spermiogenesis
• The details of the menstrual cycle
Vocabulary
amniotic cavity: The fluid-filled cavity that surrounds a developing embryo.
androgens: Male hormones. Androgens comprise a group of steroid hormones that includes
testosterone and dihydrotestosterone. The presence of these hormones directs male sexual
differentiation throughout the body. Without androgen, a fetus will develop into a female.
anterior-posterior axis: The top-bottom axis, defined by the position of the node along the
primitive streak, in the developing embryo.
autosome: Any chromosome other than a sex chromosome. Forty-four of the chromosomes are
autosomal, carrying the genetic information for all of the characteristics that make up an individual
except for sexual determination.
axes: Plural of “axis,” a central line or structure around which objects are arranged.
bilaminar: Formed of, or having, two layers.
bipotential: The potential for a cell or organ to develop in one of two ways, especially as either
a male or female, depending on the presence or absence of male hormones.
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blastocoel: One of the three distinct regions of a blastocyst, it is the fluid-filled, central cavity
of a blastula.
blastocyst: A blastula in which some differentiation of cells has occurred.
blastula: An early stage of fetal development consisting of a layer of cells enclosing a hollow,
central cavity.
chromosomes: The small bodies in the nucleus of a cell that direct the development of our individual characteristics, such as hair color and facial features. We have 46 chromosomes, 44 of which are
autosomal; the remaining two are the sex chromosomes.
corpus luteum: A mass of progesterone-secreting endocrine tissue that forms from a ruptured
Graafian follicle after the release of a mature egg. If the egg is not fertilized, the corpus luteum
degenerates, triggering a process that ends 14 days later with menstruation.
development: The process by which a single cell becomes a multicellular organism. There are five general
mechanisms of development: cell division, differentiation, morphogenesis, patterning, and growth.
differentiation: The development mechanism occurring after the morula forms by which cells
become physically and functionally different.
diploid: Indicating a cell that contains two sets of chromosomes.
ectoderm: The outer layer of the three germ layers, it will develop into the epidermis, the hair
and nails, the nervous system, and the surfaces of the sensory organs.
embryo: In humans, the development stage up to the end of the second month after conception.
embryonic disc: A flat area in a blastocyst in which the first traces of the embryo are seen.
The embryonic disc is made up of two layers of cells, called the epiblast and the hypoblast.
endoderm: The innermost of the three germ layers, it will eventually become the digestive and
respiratory tracts, the tympanic cavity and Eustachian tube, the tonsils, parathyroid and thymus
glands, liver, pancreas, and gall bladder. The enteron, located within the endoderm, will form the
gut cavity.
epididymis: A convoluted tubule in each testis that carries sperm to the vas deferens.
estrogen: The primary sex hormone in females. In females, estrogen controls the development
of primary sex characteristics and secondary sex traits, and initiates the menstrual cycle.
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fallopian tubes: The pair of long, slender ducts in the female through which ova pass from
the ovary to the uterus and, in fertilization, through which sperm cells pass from the uterus to
the released ova.
fetus: In humans, the development stage from the end of the eighth week after conception
up to the moment of birth.
follicle stimulating hormone (FSH): A hormone of the pituitary gland that stimulates
the growth of follicles in the ovary and induces the formation of sperm in the testis.
gamete: A mature ovum or sperm cell that unites with another cell (i.e., a sperm cell unites with
an ovum) to produce a fertilized egg. Gametes develop from germ cells at about the fifth and sixth
weeks after fertilization.
gastrulation: The process during embryonic development in which the cells of the embryonic
disc rearrange themselves to lay out the general body plan and establish the basic tissue types
that will generate the organs. Gastrulation happens between two and three weeks after fertilization.
genital swelling: Bipotent tissue that helps form the labia in the female embryo and the scrotum
and shaft of the penis in the male embryo.
genital tubercle: Bipotential tissue that forms the clitoris in the female embryo and the glans
penis in the male embryo.
germ cells: Also called primordial germ cells, they are the sperm and ova. The term also refers
to the cells which will eventually mature into gametes.
germ layers: The three layers (ectoderm, endoderm, and mesoderm) making up the embryo after
formation of the primitive groove. The germ layers represent the three different cell types that will
continue to differentiate during development, to eventually become the more than 200 different
kinds of cells of the human body.
gonad: A sex gland (such as the testis or ovary) that produces gametes. The gonads play an active
role in directing sexual development during embryonic development, and after birth.
gonadotropin releasing hormone (GnRH): A hormone secreted by the hypothalamus and
then delivered to the anterior pituitary gland. GnRH stimulates the pituitary to synthesize and release
luteinizing hormone and follicle stimulating hormone, which in turn travel the testes to stimulate the
production of testosterone (in males) and to the ovaries to stimulate the production of estrogen
(in females).
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HUMAN DEVELOPMENT AND THE REPRODUCTIVE SYSTEM
INSTRUCTOR’S GUIDE
gonadotropins: Hormones that stimulate the growth and activity of the gonads. Luteinizing
hormone and follicle stimulating hormone are gonadotropins.
Graafian follicle: One of the small vesicles containing a developing ovum in the ovary. When
ovulation occurs, the Graafian follicle ruptures, ejecting the egg from the ovary.
granulosa cells: Cells in the ovary that surround the egg and are stimulated by LH to produce estrogen.
haploid: Indicating a cell that contains only one set of chromosomes, such as a sperm or unfertilized
egg cell.
inner cell mass: One of the three distinct regions of a blastocyst, it is a collection of cells which
eventually forms into the body of the embryo.
left-right axis: The line established by the position of cells along either side of the embryonic
disc in the developing embryo.
limb buds: Structures which appear along the trunk of the body during its fourth week of development,
eventually growing into arms and legs.
luteinizing hormone (LH): A hormone produced by the pituitary gland that stimulates ovulation and
the development of the corpus luteum in the female and the production of testosterone in the male.
meiosis: The process of division in sex cells resulting in four “daughter” cells.
menstrual cycle: The sequence of changes that occur in the ovary and the endometrial lining
of the uterus, preparing the lining for pregnancy. If pregnancy does not occur, the lining is shed.
The menstrual cycle recurs approximately every 28 days.
mesoderm: The middle layer of the three germ layers, it will develop into bone, muscle, cartilage
and connective tissue, blood, blood vessels, heart, kidneys, and spleen. The celom, located within
the mesoderm, will become the body cavity.
mitosis: The dividing of a somatic cell into two identical “daughter” cells.
morphogenesis: The development mechanism occurring after the morula forms by which the
shape and structure of the organism changes.
morula: The mass of cells resulting from division of the ovum before the formation of a blastula.
The morula becomes the blastocyst about four days after fertilization.
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HUMAN DEVELOPMENT AND THE REPRODUCTIVE SYSTEM
INSTRUCTOR’S GUIDE
Müllerian ducts: Unipotential tissue that will form the internal genitalia of the developing
embryo. In the female embryo, the absence of androgens causes the Müllerian ducts to develop
into the fallopian tubes, uterus, and cervix. In the male embryo, Müllerian ducts degenerate.
oocyte: An immature egg cell that develops into an ovum.
oogonium: A cell that arises from a PGC and differentiates into an oocyte in the ovary. The oogonia
are stem cells for egg production.
ovary: The female reproductive gland in which ova are produced.
ovulation: The production and release of ova from the ovaries.
ovum (plural is ova): Egg; female reproductive cell.
patterning: The development mechanism by which cells arrange themselves along axes to lay
out the body plan. Patterning occurs during fetal development to ensure that all parts of the body
develop in the correct place.
placenta: An organ that forms in the uterus during pregnancy that connects the embryo or fetus to
the mother’s circulatory system. The placenta provides nutrients to the developing embryo or fetus.
pre-embryo: A fertilized ovum up to 14 days old, before it becomes implanted in the uterus.
primitive groove: A groove that appears on the surface of the primitive streak.
primitive streak: A line of cells that forms a ridge along the midline of the embryonic disc.
primordial follicle: An oogonium surrounded by a layer of cells. Primordial follicles are formed
from the outer layers of the female gonad at about 16 weeks after fertilization.
primordial germ cells (PGCs): Also called germ cells, they are the sperm and ova. The term also
refers to the cells which will eventually mature into gametes.
semen: A mixture of sperm and secretions from the seminal vesicles, prostate gland, and
bulbourethral glands.
seminiferous tubules: Tightly coiled structures within the testes in which spermatozoa develop.
sex cells: Also called germ cells, these are the ovum and sperm.
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HUMAN DEVELOPMENT AND THE REPRODUCTIVE SYSTEM
INSTRUCTOR’S GUIDE
sex chromosomes: The two chromosomes, designated as either X or Y, which determine our sex.
In the egg, the sex chromosome will always be an X, while in the sperm, it can be either an X or a Y.
somatic cells: Any cell other than a germ cell.
spermatogonium: A type of stem cell for sperm production that develops from primordial germ cells.
spermatozoa: Sperm cells.
spermiogenesis: A process during which spermatids (the precursors of sperm cells) develop into
spermatozoa.
testosterone: An androgenic hormone secreted by the testes that stimulates the development
of male sex organs, secondary sex traits, and sperm.
trophoblast: One of the three distinct regions of a blastocyst, it is the blastocyst’s outer layer;
the trophoblast attaches to the uterine wall and aids in nourishing the embryo.
unipotential: Capable of developing into only one type of cell or tissue.
urethral fold: Bipotential tissue that helps form the labia in the female embryo and the shaft of
the penis in the male embryo.
uterus: Also called a womb, it is the hollow organ in which the fertilized egg implants and develops.
vas deferens: The duct that transports the sperm from the epididymis to the penis.
ventral-dorsal axis: The front-back axis. The cells that migrate through the primitive groove into
the embryonic disc become the dorsal, or back side of the embryo, establishing the ventral-dorsal axis.
Wolffian ducts: Unipotential tissue that will form the internal genitalia of the developing embryo.
In male embryos the presence of androgens causes the Wolffian ducts to develop into the epididymis,
vas deferens, and seminal vesicles. In female embryos the Wolffian ducts will degenerate.
zygote: The single cell that results from the fertilization of an egg by a sperm cell; a fertilized egg.
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INSTRUCTOR’S GUIDE
Student Projects
• Create a human development timeline, starting with ovulation and ending with birth. The timeline
can be in the form of a labeled poster, naturalist’s sketchbook with labeled drawings, or any other
medium that conveys detailed, accurate information. Using the video program and the vocabulary list
provided in this guide, be sure to correctly label and explain each stage and process of development.
• There has been much debate about the ethical, legal, and social implications of Assisted
Reproductive Technology, or ART. In addition to in vitro fertilization, there are three other main
categories of ART, various subcategories, several types of artificial insemination, as well as fertility
drugs available to potential parents. Research the basics of some of these options, and create a
chart to identify them. (Use the video program and the vocabulary list provided in this guide to
help understand terms such as gamete, zygote, blastocyst, oocyte, gonadotropins, and GnRH.)
Include on the chart any significant risks (to mother or child) involved with these procedures.
Share the information you’ve gathered with the class, then open the floor to discussion and debate.
Some issues that may come up include:
— the rights of the sperm donor
— the rights of the egg donor or surrogate mother
— who should have ownership of previously fertilized, then frozen eggs in the event of death
or divorce of the parents
— whether there should be restrictions on how old a potential mother can be
— whether the number of births (e.g., implanted fertilized eggs) should be restricted
— various philosophical and religious views about the creation of life, selective reduction, etc.
• There are many opportunities for discussion and debate in the field of bioethics! After gathering the
appropriate information from solid, reputable sources, have the class tackle one of the following:
— Genetic testing: How is genetic testing done? Would you want to be tested to learn
if you had a predisposition to an incurable disease? Why or why not?
— Cloning: Has a human ever been cloned? Would it be okay to clone human organs for use
in transplantation? How would you feel about cloning endangered or extinct animals?
The USFDA announced in 2008 that meat and milk from cloned cattle, pigs, and goats is
safe to eat. Would you eat meat from cloned animals?
— Designer babies: Do parents have the right to use reproductive and genetic technologies
to choose their child’s gender, appearance, skills, and personality? What might be the outcome if this practice were common? If only the wealthy could afford it?
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HUMAN DEVELOPMENT AND THE REPRODUCTIVE SYSTEM
INSTRUCTOR’S GUIDE
• According to the 2007 report “The Falling Age of Puberty in U.S. Girls” (financed by the Breast
Cancer Fund), girls are entering puberty earlier now than they have in the past. Scientists call this
“precocious puberty,” and many believe it is due to the use of polychlorinated biphenyls (BPAs) —
a plastic found in items such as water bottles and baby bottles — as well as to the rise in childhood obesity. What are the factors, both natural and environmental, that trigger puberty? Using
the library or some of the Web sites listed below, investigate and then write a report on the biological reasons for this trend.
• Do a mother’s health and activities affect her unborn child? There’s no question that good nutrition
and prenatal vitamins have positive effects on the fetus — and according to a Pennsylvania State
University study, a child’s food preferences can be influenced by what its mother ate while pregnant.
But can a woman’s diet at conception affect the unborn baby’s gender? Will playing it classical music
really boost the baby’s IQ? Does its mother having the flu while pregnant increase a baby’s chances of
developing schizophrenia later in life? And what about the father — can tobacco use, pollution, diet,
or age affect sperm count and quality? Use the library or Web sites below to come up with a chart,
poster, or other visual display of information you gather on this topic. Be sure your data is backed up
by scientific research; if research is inconclusive or contradictory, indicate that on your chart.
The following Web sites will be helpful for students researching topics in embryology
and reproductive health:
Science : Physics : Tech : Nano : News
www.PhysOrg.com
Scientific American
www.sciam.com
News & Articles in Science, Health, Environment & Technology
www.sciencedaily.com
Biology News Net
(click on “Biology Links” for a list of science news Web sites)
www.biologynews.net
How Stuff Works
www.howstuffworks.com
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INSTRUCTOR’S GUIDE
Clear Answers for Common Questions
www.wisegeek.com
Centers for Disease Control and Prevention
www.cdc.gov
National Institutes of Health
www.nih.gov
Center for the Evaluation of Risks to Human Reproduction
http://cerhr.niehs.nih.gov
TEDX — The Endocrine Disruption Exchange
www.endocrine.disruption.com
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HUMAN DEVELOPMENT AND THE REPRODUCTIVE SYSTEM
INSTRUCTOR’S GUIDE
Quiz
1. Place the following in the correct developmental order by putting a number before each term:
___ blastocyst
___ development of placenta
___ morula
___ ovulation
___ start of differentiation and morphogenesis
___ zygote
2. The _____ is the development stage from conception to the end of the second month; after that,
the developing human is called a _____.
a) zygote; fetus
b) fetus; embryo
c) embryo; fetus
d) blastocyst; embryo
3. _____ is the process during which the cells rearrange themselves to lay out the general body
plan, and form the basic foundation for what will eventually become the organs, tissues, etc.
a) Gastrulation
b) Ovulation
c) Meiosis
d) Differentiation
4. _____ is an early developmental stage in which cells become physically and functionally different.
a) Morphogenesis
b) Differentiation
c) Gastrulation
d) Fertilization
5. The process of arranging cells to lay out the body plan after formation of the axes is called _____.
a) cell migration
b) positioning
c) parturition
d) patterning
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6. The main mechanism of development during the fetal stage is _____.
a) differentiation
b) morphogenesis
c) growth
d) cell division
7. In the egg, the sex chromosome is
a) X
b) Y
c) X or Y
d) XX
e) YY
f) XY
8. In the sperm, the sex chromosome is
a) X
b) Y
c) X or Y
d) XX
e) YY
f) XY
9. A fertilized egg with _____ chromosomes will develop into a female, while a fertilized egg with
_____ chromosomes will develop into a male.
a) XX; YY
b) XY; YY
c) YY; XX
d) XX; XY
e) XY; XX
f) YY; XY
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INSTRUCTOR’S GUIDE
10. True or False? Until about the second month after fertilization, there is no difference between
the male and female gonads of the fetus.
11. The Wolffian and Müllerian ducts are unipotential, meaning that _____.
a) they secrete androgens
b) they are capable of developing into only one type of cell or tissue
c) like stem cells, they are capable of developing into a wide array of cells or tissues
d) they will not develop in the absence of hormones
12. _____ stimulates the male’s pituitary gland to synthesize and release hormones that in turn
stimulate the production of testosterone.
a) Gonadotropin releasing hormone
b) Luteinizing hormone
c) Follicle stimulating hormone
d) Androgen
13. Estrogen _____.
a) is not present in the female until after puberty
b) controls the female pattern of fat distribution on the hips and upper thighs
c) is responsible for the primary sex characteristics in women and the secondary sex traits in men
d) inhibits the growth of the endometrial lining during the menstrual cycle
14. Ovulation occurs _____ the menstrual cycle.
a) at the start of
b) in the middle of
c) at the end of
d) throughout
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Answers to Quiz
1. The correct sequence is:
1) ovulation
2) zygote
3) morula
4) start of differentiation and morphogenesis
5) blastocyst
6) development of placenta
2. c) embryo; fetus
3. a) Gastrulation
4. b) Differentiation
5. d) patterning
6. c) growth
7. a) X
8. c) X or Y
9. d) XX; XY
10. True
11. b) they are capable of developing into only one type of cell or tissue
12. a) Gonadotropin releasing hormone
13. b) controls the female pattern of fat distribution on the hips and upper thighs
14. b) in the middle of
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