In This Lesson:
Unit 7
Stages of
Development
(Lesson 1 of 3)
Today is Tuesday,
December 22nd, 2015
A
B
Pre-Class:
Today begins our Embryology unit – in other
words, more baby pictures.
Also, take a look at the embryos in the
upper right. Which of those three will
develop into a human?
One last thing: you’ll need a paper towel.
C
Today’s Agenda
• Challenge Questions
• Unit warm-up questions
• Stages of development
• Where is this in my book?
– P. 536 and following…
By the end of this lesson…
• You should be able to construct a timeline of
embryological development in humans.
• You should be able to describe the nature and
value of stem cells.
By the way…
• The answer to the pre-class question was the
one on the right (C):
Challenge Questions
• Whiteboards! Markers! Paper towels!
– Oh My!
Review
• We’ve all had some form of health class by now.
– Or some tremendously awkward conversation with
our parents about our beginnings.
• While this unit will be a bit more of the biological
side of things, it’s also good to know that we’ve
got a solid background from those previous
experiences.
• Visit Quia and complete the quiz entitled:
– Sexual Health and Reproduction Questions
– This is for credit!
Where to begin…
• We’ve seen in previous units that the zygote (the first
human diploid cell) forms when a sperm cell fuses with
an egg cell. This leads to pregnancy.
• Birds, reptiles, fish, amphibians, insects, et cetera, lay
their eggs outside their bodies.
– They will still do this even if the eggs are not fertilized.
• In humans, unfertilized eggs are essentially discarded
at a rate of (approximately) one every 28 days.
• There’s also one more thing. Ovoviviparous animals
(or animals that exhibit ovoviviparity) are those that
reproduce by eggs, but give birth to live young. Some
snakes and fish do this.
Also by the way…
• Just so we all know, human pregnancy length is 9 months.
– That’s nine full months, so 40 weeks (not 9x4=36).
– Babies are considered premature if they are born at 36 weeks
or less.
• “Pregnancy” is also known as the gestation period (there’s
a slight difference – gestation starts after implantation –
more on that later). Some fun gestation periods:
–
–
–
–
–
Kangaroo: 33 days
Dog: 61 days
Grizzly bear: 220 days (about 7 ¼ months)
Giraffe: 15 months
Elephant: 22 months
• In other words, a human could squeeze almost two and a half babies
into the time it takes an elephant to have one.
Because I had to…
Philadelphia Zoo
Ontogeny and Phylogeny
Human Development Photos
• 36 Weeks (8 months)
http://www.webmd.com
Human Development Photos
• 28 Weeks (7 months)
http://www.webmd.com
Human Development Photos
• 24 Weeks (6 months)
http://www.webmd.com
Human Development Photos
• 20 Weeks (5 months)
http://www.webmd.com
Human Development Photos
• 16 Weeks (4 months)
http://www.webmd.com
Human Development Photos
• 12 Weeks (3 months)
http://www.webmd.com
Human Development Photos
• 10 Weeks (2.5 months)
Human Development Photos
• 8 Weeks (2 months)
http://www.webmd.com
Human Development Photos
• 4 Weeks (1 month)
• 7 Weeks (<2 months)
Development Photos
• Dog
http://www.nationalgeographic.com
Development Photos
• Dolphin
http://www.nationalgeographic.com
Development Photos
• Elephant
http://www.nationalgeographic.com
Embryology
• Embryology is the branch of biology that
studies the formation and early development
of living organisms.
• During the first two to three weeks of
development, most organisms look pretty
much the same, from fish to people to rabbits
and cuter things.
Recapitulation Theory
• Ernst Haeckel (19th century)
Fish
Salamander
Turtle Chicken
Pig
Cow RabbitHuman
Embryo Development
• Embryo – Fertilization to 8 weeks in humans.
• Fetus – After 8 weeks because all organ
systems have begun to form.
• After 3 months, most of the major organs and
tissues have begun to develop.
• The muscular system is developed so the fetus
can start to move!
Early Embryonic
Developmental Processes
• Be ready to make a timeline of these in your
notebooks:
1.
2.
3.
4.
Fertilization
Implantation
Gastrulation
Neurulation, Organogenesis, Morphogenesis
Pre-Step 1
• First there’s ovulation.
– Release of an ovum (or two) from the ovary.
• If ovulation hasn’t happened at the right time
before, uh, a “fertilization attempt,” no fertilization
will occur.
• Note: The egg is released prior to having fully
completed meiosis – it still needs to release a polar
body.
• Also note: The egg is encased in a protein shell
called the zona.
1. Fertilization
• Egg and sperm come together to form a
fertilized egg (called a zygote).
– Occurs in the fallopian tube.
http://images.tutorvista.com/content/reproduction/female-reproductive-system.jpeg
Aside: College Sports
• Allow me to ruin a logo for you:
http://www.sportsgeekery.com/wp-content/uploads/2011/07/Texas-wp-2-1920.jpg
Polyspermy
• Polyspermy is the rare occurrence in which more
than one sperm fertilizes one egg cell.
• Typically, zygotes that form this way do not
survive.
• Under normal circumstances, upon fertilization,
the egg cell is modified and all subsequent sperm
are blocked from entry.
– Mammals: “Slow block” – zona pellucida modified.
– Others (especially sea urchins): “Fast block” – egg cell
charge changes to positive; sperm are positively
charged as well, preventing entry.
Cell Divisions
• After the zygote is formed, it begins
undergoing a series of mitotic divisions called
cleavages.
• The first stage is when the growing embryo
becomes a solid ball of cells called a morula.
– 64 cells
– Approximately 4 days old
• (that means each cell divided about 1-2 times a day)
– Actual size <1 mm
Early Embryonic Image
• This is 2-3 days past fertilization, so it’s
between the zygote and morula stages:
http://www.popularmechanics.co.za/multimedia/image-of-the-week/early-human-embryo/
Regarding Twins…
• Fraternal twins occur when two eggs are released
by Mom and two separate sperm fertilize them.
– Sometimes called dizygotic twins.
• Each develop normally and separately.
– Two different sets of DNA.
• Identical twins occur when one egg is released by
Mom and one sperm fertilizes it.
– Sometimes called monozygotic twins.
• The zygote then splits into two zygotes prior to
undergoing cleavages.
– One set of DNA, thus, identical twins (same sex).
Regarding…Anti-twins?
• Suppose, however, in a case of fraternal twins,
both developing zygotes fuse together and
develop as one organism.
– You’ve got yourself a chimera.
• A chimera is an organism with two different
sets of DNA as a result of two zygotes joining
and developing as one.
• People can be chimeras, too, which means
DNA testing isn’t 100% reliable.
Chimeras and Conjoined Twins
• Note that if the “chimera
process” begins but doesn’t
fully join the two cells,
conjoined twins may result.
– Also note: Some people feel
that conjoined twins may
actually start with a single
zygote that doesn’t completely
divide. Either way…
http://sansscience.files.wordpress.com/2012/08/venus.jpg
Chimerism vs. Mosaicism
• When two zygotes fuse to become one
organism, it’s chimerism.
• When one zygote experiences genetic
recombination and results in two different
populations of cells in one organism (just like
a chimera), it’s mosaicism.
– This one’s weird. Apparently, sometimes cells
doing plain ol’ mitosis will suddenly do some
crossing over. You know, that thing that’s only
supposed to happen in meiosis.
Mosaicism
https://upload.wikimedia.org/wikipedia/commons/0/03/Heterochromia_iridum_and_iridus_2013-09-30_14-15.jpg
2. Implantation
• The morula will transform into
a hollow ball of cells, called a
blastocyst (or blastula).
• The outer cells of the
blastocyst are called the
trophoblast.
• Then the blastocyst will shed
the zona and attach to the
uterine wall. We call this
implantation.
• 6-7 days after fertilization
– This is technically when
gestation begins. Pregnancy
has already started.
Implantation and Menstrual Cycle
• Normally, with no fertilized egg
to be implanted, the lining of the
uterus (the endometrium) is
shed as part of the menstrual
cycle.
– This is the “period” part of the
menstrual cycle.
• When there is an egg implanted,
however, the lining is not shed,
thus causing a missed period and
a sign of possible pregnancy.
– Sometimes the endometrium is
shed anyway, up to 50% of the
time an egg is fertilized.
• Birth control pills often prevent
ovulation or implantation.
http://upload.wikimedia.org/wikipedia/commons/2/2a/MenstrualCycle2_en.svg
Aside: Birth Control Pills
• An incomplete list of
George Carlin’s (fake)
Birth Control pill
names:
– Preg-Not
– Embry-No
– Nary-a-Carry
– Nay, Family Way!
– Junior Miss
– Inconceivable!
– Mommy Not
– Fetus Fail
– Poppa Stopper
– Womb Broom
– Humpty Dumpty
– Baby-Maybe
Video!
• Ovulation
Ectopic Pregnancy
• When implantation does not occur in the
uterine wall, an ectopic pregnancy may occur.
• Typically, the blastocyst will implant in the
fallopian tubes or, rarely, the ovary/elsewhere.
– As you might imagine, development cannot occur
successfully in these locations.
• Ectopic pregnancies, if not removed
immediately, can lead to death or the inability
to have subsequent children.
Pause.
• Just a heads-up:
– This whole “implantation” thing happens in
mammals only.
– Birds, reptiles, amphibians, et cetera? Go directly
to gastrulation.
• Do not pass Go.
3. Gastrulation
• The process in which cells of the blastocyst
(after implantation) develop into three layers
to form a gastrula.
– This is when stem cells start to specialize and
differentiate.
– Three germ layers are formed.
• “It is not birth, marriage, or death, but
gastrulation that is the most important time of
your life.”
– Lewis Wolpert
Germ Layers in Gastrulation
• Ectoderm – Outer-most layer
– Forms first
– Skin, hair, brain, nervous
system
• Endoderm – Inner-most layer
– Forms next
– Digestive system, lungs, liver
• Mesoderm – Middle layer
– Forms last
– Bones, blood, muscle, heart
http://www.bio.miami.edu/dana/pix/gastrulation.jpg
Another Look at Gastrulation
Stem Cells
• Stem cells are unspecialized cells with the
ability to differentiate (get different jobs).
– Differentiation starts during gastrulation.
Cell Differentiation
• Not all stem cells are equally able to differentiate:
– Totipotent stem cells can form all cell types, including
placental cells. Only the earliest post-zygotic cells are
totipotent.
– Pluripotent stem cells can form all cell types, not
including placental cells. Embryonic stem cells are
pluripotent.
– Multipotent stem cells can still differentiate, but are
limited. Adult stem cells and cells from umbilical cord
blood are multipotent.
http://stemcell.ny.gov/faqs/what-difference-between-totipotent-pluripotent-and-multipotent
What can we do with stem cells?
• Well, let’s find out.
• NOVA – Artificial Organ Growth
4. Neurulation
• Development of nervous system.
• Occurs soon after gastrulation is complete.
• By now the embryo is 18-26 days old.
• After Neurulation:
• Organogenesis – organs begin to form.
– Heart forms (3rd week) beats by the 4th.
– 5th week – eyes, ears, digestive system develop.
• Morphogenesis – limbs begin to assume shape.
– 5th week – arms, legs develop.
Stages of Development
• Morula to Blastula
• Blastula to Gastrula
Zebra Fish Development
• Courtesy the Exploratorium in San Francisco:
– Video!
• Zebrafish Embryo Photo:
http://www.popularmechanics.co.za/multimedia/image-of-the-week/zebrafish-embryo/
Mammals vs. Non-mammals
• When it comes to development, there’s a
structure some animals have that others don’t.
Know what it is?
– Hint: Look at the title of this slide…
• Because mammals do not lay eggs, there needs
to be direct contact between mother and fetus.
• Note: Many of these structures are also present in
egg-laying animals. They’re just a little different.
Extraembryonic Membranes
• Two main membranes form
to protect and nourish the
developing embryo.
• The amnion is a fluid-filled
sac that cushions and
moistens the embryo.
– Acts as an “air bag.”
– Contains amniotic fluid.
– It’s what breaks when the
“water” breaks.
Extraembryonic Membranes
• The chorion forms the
outermost membrane and
the connection to the
mother.
– Also forms the placenta,
which includes the
umbilical cord.
Placenta
• The placenta is the
organ that allows the
mother to exchange
materials with the
embryo/fetus:
– Nutrients
– Gas (O2/CO2)
– Waste
Placenta
• Things of note:
– Remember, blood is not directly shared with the baby.
• A little can leak through the placenta normally.
• If a lot leaks, it is a condition called hemolysis and can result in
hemolytic disease.
• RhoGAM® is a drug available to treat Rh incompatibility.
– The placenta is pushed out following the baby as part of
the “afterbirth.”
– Many animals (and people too) typically will eat the
placenta.
– Placenta Recipes!
Et Cetera
• Other interesting things:
– Oxytocin
• A hormone released in large amounts during the
childbirth process.
• Oxytocin leads to forming bonds, particularly between
mother and child.
– In-Vitro Fertilization
• Artificial methods of fertilization.
• “In-Vitro” means “in glass,” implying work done outside
the body and inside a lab.
Trimesters
(1-3 months, 4-6 months, 7-9 months)
Closure
• Head to Quia and try the quiz called:
• Embryology Practice Problems
– This will give you feedback and answers!
Closure
• Make sure your timeline is as complete as
possible.
• Draw a picture of a fetus with the three major
embryonic membranes around it. You’ll need:
– The placenta
– The amnion
– The chorion
• I will be coming around to check your
progress.
Timeline
Cleavages
Zygote
Morula
Blastula
Gastrula