1. Topic 5: Fertilization to Embryonic Development

2. The Human Body with Dr. Winston (BBC)
The Human Body: Part 2 An Every Day Miracle

3. First Stages of Development
Fertilization: Occurs: in FALLOPIAN TUBE (within 24 h of ovulation) Only a few dozen of the best swimmers make it!!
When the sperm reaches the egg, the acrosome releases enzymes to digest the jelly-like coating surrounding the egg

4. Fertilization Gametes (n) fuse to form a zygote (2n)
Zygote = first SINGLE cell of new life
Sperm nucleus 1n
Egg nucleus
ZYGOTE 2n

5. Early Stages of Development
ZYGOTE

6. Cell Cleavage
Zygote (2n)
Zygote undergoes mitotic divisions (cleavage) to form ball of cells called morula (16-32 cells)
Morula develops into blastocyst (day 6)
Blastocyst
(2n)
Morula (2n)

7. First Stages of Development
Blastula (aka Blastocyst): a cavity opens up in the morula (space inside called blastocoel)
Chorion (trophoblast): outer layer of blastocyst
Chorion forms placenta and the amnion
inner cell mass: will develop into embryo
blastopore: indentation on blastula which allows gastrulation

8. From Ovulation to Implantation
cells
Trophoblast begins to secrete enzymes to digest some of the tissues and blood vessels of the endometrium; blastocyst slowly sinks into the uterine wall = implantation

9. From Ovulation to Implantation
Day 1: First Cleavage – cell divides by mitosis
Day 4: 16 – 32 cell stage. Ball of cells is called a morula.
Day 5: the cells of the morula begin to move around to form an inner and outer layer of cells. The outer layer of flattened cells (chorion) are important for implantation in the uterine lining.
Day 6: The two layers of cells arrange themselves around a hollow fluid filled cavity called the blastocoel, the actual cell mass is called a blastocyst.
Trophoblast begins to secrete enzymes to digest some of the tissues and blood vessels of the endometrium; blastocyst slowly sinks into the uterine wall = implantation

10. From Ovulation to Implantation
Day 7: Implantation - The blastocyst, by means of villi and enzymes secreted by the chorion (a membrane that forms around it), implants itself in the uterine wall, resulting in pregnancy (gestation). The chorion secretes hCG, a hormone which stimulates the corpus luteum to produce progesterone and estrogen for the first 3 months.
Stem cells from the blastula are undifferentiated (totipotent) and can be used for stem cell research.
Trophoblast begins to secrete enzymes to digest some of the tissues and blood vessels of the endometrium; blastocyst slowly sinks into the uterine wall = implantation

11. zygote morula blastocyst hCG Helps to implant chorion
- Inner cell mass becomes the embryo itself
Corpus luteum produces progesterone and estrogen for 3 months
Secretes hCG for 3 months

12. Layers of the Blastocyst
Outer layer of blastocyst forms 2 membranes:
chorion- produces hCG
amnion - fluid filled sac protecting fetus

13. Zebras Make Better Guacamole Every Friday
How to remember the stages?
Zebras Make Better Guacamole Every Friday
Zygote – Day 0 (Sperm + egg)
Morula – Day 4 (16 – 32 cells)
- Inner cell mass becomes the embryo itself
Blastocyst – Day 6 **Implantation Outer layer (chorion) helps the implantation process, secretes hCG and forms placenta

14. Zebras Make Better Guacamole Every Friday
How to remember the stages?
Zebras Make Better Guacamole Every Friday
Gastrula – Day germ layers – ectoderm, mesoderm, endoderm. Neurulation – formation of neural tube which will form CNS.
Embryo – Day 10 - chorion starts to form the placenta
- Inner cell mass becomes the embryo itself
Fetus – week 8 – called fetus because all major organ systems have started to develop

15. Ectopic Pregnancy: Faulty Implantation
In an ectopic pregnancy, a fertilized egg has implanted outside the uterus, usually in the fallopian tube.
Severe bleeding and possible death of the mother can result from this type of pregnancy

16. Pregnancy Tests
Outer layer of the blastocyst (the chorion) starts to secrete hCG when it implants in the endometrium on ~day 7. This causes morning sickness & is the hormone measured in a pregnancy test!
Pregnancy test: tests for the presence of hCG in the urine (sometimes the blood).
Remember hCG: similar to LH – keeps corpus luteum secreting progesterone & estrogen for 3 months!!
hCG = human chorionic gonadotropin  has the same effects as LH; maintains the corpus luteum past the time when it would otherwise start to degenerate (secretion of estrogen and progesterone continues; maintains endometrium and preventing mensturation)
Later on, the placenta secretes sufficient estrogen and progesterone

17. How can a miscarriage occur?
For first 3 months progesterone comes from mother
Baby starts making it’s own after 3 months
If mom stops progesterone and baby does not start…
Miscarriage occurs

18. Gastrulation (~Day 7) embryo now called gastrula
Gastrulation is the process in which the inner cell mass of blastula turns into 3 germ layers (embryonic tissues) – the (ectoderm, mesoderm, endoderm)
embryo now called gastrula

19. Gastrulation (~Day 7)
Cells begin to differentiate (change) to form specific organ systems!

20. PBS Nova: Miracle of Life
NOVA: The First 2 Weeks
PBS Nova: Miracle of Life

21. NOVA: The Embryo Takes Shape

22. 3 Germ Layers Ectoderm (outer): skin(epidermis), nervous system
Mesoderm (middle): Skeleton, muscles, gonads
Endoderm (inner): lining of the digestive and respiratory systems and endocrine glands
How to remember?
Mesoderm: So Many Gonads
Endoderm: RED
Bozeman 6:50- 8:59

23. Neurulation: formation of neural tube  develops into brain and spinal cord
Becomes the primative curve of the spine

24. Stem Cells – The Song…

25. Summary of Events After Implantation
Days 7-10
Days 10-14
Days 15-21
Day 21
Week 4
Weeks 5 – 8
Gastrulation begins – major cellular reorganization into the three germ layers
“Gastrula” stage is when different genes will be turned on to express different organs in the later stages of the pregnancy
Pregnancy fully established
Amniotic cavity forms
Yolk sac forms
Embryo forms
Chorion starts to form the placenta
Emergence of the body plan
“Primitive streak” starts to form (at site of Gastrulation) becomes mesoderm
Neural Groove forms (future brain and spinal cord)
5th week – eyes open but the baby does not have eyelids of irises 1.3 cm long embryo
7th-8th week: eyes lids are closed shut, nostrils develop but at plugged up with mucous (baby doesn’t need to breath until birth)
Physical genitalia not yet formed.
9th week: size of a paper clip. Organs are developed and just need to be enlarged and mature
Heart begins to beat
Eyes, ears and lower limbs begin to develop
Teeth, palate, external genitalia begin to develop

26. Extra-Embryonic Membranes: formed after implantation
1. Chorion: outer membrane of blastula
secretes hCG until 2nd trimester (first 3 months)
fetal contribution to placenta
gas/nutrient/waste exchange
2. Amnion: inner membrane of outer layer of blastula
becomes fluid-filled sac that protects embryo from infection, impact and temperature fluctuations

27. Extra-Embryonic Membranes: formed after implantation
3. Allantois: forms the foundation of the umbilical cord
Becomes part of the bladder
4. Yolk sack: in other animals, provides nutrients
Small in humans (forms blood cells)

28. Extra-Embryonic Membranes
Implantation, hCG and placenta
Amniotic Fluid
Umbilical cord and bladder
Blood cells
Pool of blood

29. Extra-Embryonic Membranes Practice

30. Cat Fetus 1. umbilicus, 2 amnion, 3 allantois 4 Yolk sac
5 embryo part of
placenta
6 maternal part
of placenta
(endometrium)

31. Placenta
Placenta: allows exchange of some substances between mother and fetus
from mother: nutrients, oxygen, antibodies, viruses, drugs, alcohol
from fetus: metabolic wastes (CO2, urea)
NO exchange of blood cells

32. Placenta formed when villi from chorion extend into endometrium
not fully developed until the 2nd trimester
secretes estrogen & progesterone during the 2nd & 3rd trimesters
Progesterone prevents contractions
Progesterone and estrogen cause growth of the endometrium

33. The Placenta The placenta will function as the:
To Baby
Away
The placenta will function as the:
Lungs: exchange O2 and CO2
Small Intestine: provide nutrients
Kidneys: remove nitrogenous wastes (urine)

34. Umbilical Cord Rope-like structure that forms after 8 weeks
Runs from the belly button of the fetus to the placenta
Contains 2 arteries and 1 vein
2 arteries carry deoxygenated blood… from fetus to mother
Vein carries oxygenated blood… from mother to fetus
Normally arteries carry oxygenated blood
Only 2 exceptions exist…pulmonary artery & umbilical artery!!!

35. Umbilical Vein (1) carrying oxygenated blood
Uterus
Amniotic Fluid
Placenta
(amniocentesis)
Umbilical Cord
Umbilical Vein (1) carrying oxygenated blood
Chorion
Amnion
Umbilical Arteries (2)
carrying deoxygenated blood
Cervical plug (mucus) Helps to keep out pathogens
Cervix
Approximately 3 cm during pregnancy
Vagina

36. NOVA: Feeding the Growing Fetus