1. Today is Tuesday, December 22nd, 2015
In This Lesson:
Unit 7 Stages of Development
(Lesson 1 of 3)
Today is Tuesday, December 22nd, 2015
A B C
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.

2. Today’s Agenda Challenge Questions Unit warm-up questions
Stages of development
Where is this in my book?
P. 536 and following…

3. 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.

4. By the way…
The answer to the pre-class question was the one on the right (C):

5. Challenge Questions
Whiteboards! Markers! Paper towels!
Oh My!

6. 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!

7. 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.

8. 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.

9. Because I had to…
Philadelphia Zoo

10. Ontogeny and Phylogeny

11. Human Development Photos
36 Weeks (8 months)

12. Human Development Photos
28 Weeks (7 months)

13. Human Development Photos
24 Weeks (6 months)

14. Human Development Photos
20 Weeks (5 months)

15. Human Development Photos
16 Weeks (4 months)

16. Human Development Photos
12 Weeks (3 months)

17. Human Development Photos
10 Weeks (2.5 months)

18. Human Development Photos
8 Weeks (2 months)

19. Human Development Photos
4 Weeks (1 month)
7 Weeks (<2 months)

20. Development Photos
Dog

21. Development Photos
Dolphin

22. Development Photos
Elephant

23. 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.

24. Recapitulation Theory
Ernst Haeckel (19th century)
Fish
Salamander
Turtle
Chicken
Pig
Cow
Rabbit
Human

25. 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!

26. Early Embryonic Developmental Processes
Be ready to make a timeline of these in your notebooks:
Fertilization
Implantation
Gastrulation
Neurulation, Organogenesis, Morphogenesis

27. 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.

28. 1. Fertilization
Egg and sperm come together to form a fertilized egg (called a zygote).
Occurs in the fallopian tube.

29. Aside: College Sports Allow me to ruin a logo for you:

30. 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.

31. 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

32. Early Embryonic Image
This is 2-3 days past fertilization, so it’s between the zygote and morula stages:

33. 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).

34. 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.

35. 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…

36. 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.

37. Mosaicism

38. 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.

39. 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.

40. Aside: Birth Control Pills
An incomplete list of George Carlin’s (fake) Birth Control pill names:
Junior Miss
Inconceivable!
Mommy Not
Fetus Fail
Preg-Not
Poppa Stopper
Embry-No
Womb Broom
Nary-a-Carry
Humpty Dumpty
Nay, Family Way!
Baby-Maybe

41. Video!
Ovulation

42. 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.

43. 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.

44. 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

45. 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

46. Another Look at Gastrulation

47. Stem Cells
Stem cells are unspecialized cells with the ability to differentiate (get different jobs).
Differentiation starts during gastrulation.

48. 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.

49. What can we do with stem cells?
Well, let’s find out.
NOVA – Artificial Organ Growth

50. 4. Neurulation Development of nervous system.
Occurs soon after gastrulation is complete.
By now the embryo is 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.

51. Stages of Development
Morula to Blastula
Blastula to Gastrula

52. Zebra Fish Development
Courtesy the Exploratorium in San Francisco:
Video!
Zebrafish Embryo Photo:

53. 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.

54. 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.

55. Extraembryonic Membranes
The chorion forms the outermost membrane and the connection to the mother.
Also forms the placenta, which includes the umbilical cord.

56. Placenta
The placenta is the organ that allows the mother to exchange materials with the embryo/fetus:
Nutrients
Gas (O2/CO2)
Waste

57. 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!

58. Et Cetera Other interesting things: Oxytocin In-Vitro Fertilization
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.

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

61. Closure Head to Quia and try the quiz called:
Embryology Practice Problems
This will give you feedback and answers!

62. 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.

63. Three-Layered Differentiated
Timeline
4. Neurulation, Organogensis, Morphogenesis
Zygote
Morula
Blastula
Gastrula
1. Fertilization
2. Implantation
3. Gastrulation
Cleavages
Three-Layered Differentiated
Ball of Cells
Single Cell
Hollow Ball of Cells
Solid Ball of 64 Cells