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In This Lesson: Unit 7 Stages of Development (Lesson 1 of 3). Today is Monday, January 2 nd , 2014 (!). A B C. Pre-Class: Today begins our Embryology unit – in other words, more baby pictures.

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Today is monday january 2 nd 2014

In This Lesson:

Unit 7

Stages of Development

(Lesson 1 of 3)

Today is Monday,January 2nd, 2014 (!)



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.

Today s agenda
Today’s Agenda

  • Challenge Questions

  • Unit warm-up questions

  • Stages of development

  • Where is this in my book?

    • P. 1016 and following…

By the way
By the way…

  • The answer to the pre-class question was the one on the right (C):

Challenge questions
Challenge Questions

  • Whiteboards! Markers! Paper towels!

    • Oh My!


  • 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
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
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
Because I had to…

Philadelphia Zoo

Human development photos
Human Development Photos

  • 36 Weeks (8 months)

Human development photos1
Human Development Photos

  • 28 Weeks (7 months)

Human development photos2
Human Development Photos

  • 24 Weeks (6 months)

Human development photos3
Human Development Photos

  • 20 Weeks (5 months)

Human development photos4
Human Development Photos

  • 16 Weeks (4 months)

Human development photos5
Human Development Photos

  • 12 Weeks (3 months)

Human development photos6
Human Development Photos

  • 8 Weeks (2 months)

Development photos
Development Photos

  • Dog

Development photos1
Development Photos

  • Dolphin

Development photos2
Development Photos

  • Elephant


  • 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
Recapitulation Theory

  • Ernst Haeckel (19th century)









Embryo development
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
Early EmbryonicDevelopmental Processes

  • Be ready to make a timeline of these in your notebooks:

    • Fertilization

    • Implantation

    • Gastrulation

    • Neurulation, Organogenesis, Morphogenesis

Pre step 1
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.

1 fertilization
1. Fertilization

  • Egg and sperm come together to form a fertilized egg (called a zygote).

    • Occurs in the fallopian tube.

Aside college sports
Aside: College Sports

  • Allow me to ruin a logo for you:


  • 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” – zonapellucida modified.

    • Others (especially sea urchins): “Fast block” – egg cell charge changes to positive; sperm are positively charged as well, preventing entry.

Cell divisions
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
Early Embryonic Image

  • This is 2-3 days past fertilization, so it’s between the zygote and morula stages:

Regarding twins
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
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…

Chimerism vs mosaicism
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.


2 implantation
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 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
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 works on this dynamic.

Aside birth control pills
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


  • Ovulation

Ectopic pregnancy
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.

3 gastrulation
3. Gastrulation

  • The process in which cells of theblastocyst (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
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

Stem cells
Stem Cells

  • Stem cells are unspecialized cells with the ability to differentiate (get different jobs).

    • Differentiation starts during gastrulation.

What can we do with stem cells
What can we do with stem cells?

  • Well, let’s find out.

  • NOVA – Artificial Organ Growth

4 neurulation
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

  • Morphogenesis – limbs begin to assume shape.

    • 5th week- arms, legs develop

Stages of development
Stages of Development

  • Morula to Blastula

  • Blastula to Gastrula

Zebra fish development
Zebra Fish Development

  • Courtesy the Exploratorium in San Francisco:

    • Video!

  • Zebrafish Embryo Photo:

Mammals vs non mammals
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
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 membranes1
Extraembryonic Membranes

  • The chorionforms the outermost membrane and the connection to the mother.

    • Also forms the placenta, which includes the umbilical cord.


  • The placenta is the organ that allows the mother to exchange materials with the embryo/fetus:

    • Nutrients

    • Gas (O2/CO2)

    • Waste


  • Things of note:

    • 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
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
Trimesters(1-3 months, 4-6 months, 7-9 months)


  • Head to Quia and try the quiz called:

  • Embryology Practice Problems

    • This will give you feedback and answers!


  • 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.


4. Neurulation, Organogensis, Morphogenesis

1. Fertilization

2. Implantation

3. Gastrulation


Three-Layered Differentiated

Ball of Cells

Single Cell

Hollow Ball of Cells

Solid Ball of 64 Cells