1. Survey of Embryonic Development
Exercise 44
Survey of Embryonic Development

2. Objectives:
Fertilization, zygote, morula, blastocyst, gastrula, fetus, chorion, chorionic villi, placenta, amnion, yolk sac, umbilical cord
Embryonic structures and functions

3. Fertilization
Fusion of two haploid (1n) gametes, occurs in uterine tube usually within a day after ovulation
Sperm
Delivers paternal chromosomes to fertilization site
Egg
Provides all the cellular organelles, nourishment, genetic programming for development AND maternal chromosomes
~2000x the volume of the sperm cell
Secondary oocyte has been suspended in Metaphase II until now
Fig. 29-1

4. Zygote Product of fertilization (union of gametes)
Contains 46 chromosomes (normal # for a somatic cell)
Fig. 29-1

5. Prenatal Development **Atlas Diagrams
1st trimester
Embryological & early fetal development
Beginnings of organ systems appear
2nd trimester
Organs and systems nearly complete
3rd trimester
Rapid fetal growth
Most organ systems functional

6. Embryonic Development: MORULA
Day 3-4 of development/cleavage (cell divisions)
Solid multicellular (> 4-cell stage) ball
Morula = “mulberry”
Reaches uterus on day 4
Fig. 29-2

7. Embryonic Development: BLASTOCYST
Over next 2 days
Hollow ball with inner cavity
Outer cellular layer (“trophoblast”, provides nutrients)
Inner layer of cells (clustered at one end, will form the embryo)
Implants in uterine wall
Fig. 29-2

8. Embryonic Development: GASTRULA
Day 12
Third layer forms—cells on surface migrate to central line: “primitive streak”
Migrating cells move between the 2 layers
Now have 3 embryonic layers:
Ectoderm
Mesoderm
Endoderm
Fig. 29-4

9. Embryonic Germ Layers Ectoderm Mesoderm Endoderm Neural tissue
Connective tissues
Epithelia
&
glands
Muscle tissue
Fig. 29-4

10. Embryo to Fetus
Fetus:
Start of month 3 to delivery
Fig. 29-5

11. Extra-embryonic membranes
Yolk sac
Endoderm & mesoderm
Amnion
Ectoderm & mesoderm
Allantois
Chorion
Mesoderm & trophoblast

12. Yolk sac Endoderm + mesoderm Pouch
Important site of blood cell formation
Fig. 29-5

13. Amnion Ectoderm + mesodermal cells form this two layered structure
Enlarges throughout development
Amniotic cavity contains amniotic fluid
Surrounds, cushions developing embryo/fetus
Fig. 29-5

14. Chorion Mesoderm and trophoblast cells form this vascular layer
Provides transport of nutrients to the growing embryo/fetus (multicellular now, so diffusion alone won’t suffice)
Chorionic villi: branches in contact with maternal tissues
Embryonic blood vessels in each
Supplies embryonic heart (beating by week 3)
Gases and nutrients diffuse between embryonic and maternal circulations—no mixing of actual blood (separated by trophoblast)

15. Chorion
Fig. 29-6

16. Placenta
Appearance of blood vessels in the chorion = 1st step in creation of a functional placenta
Temporary structure in uterine wall that allows the diffusion between fetal and maternal circulatory systems
Fetal side
Maternal side
Fig. 29-6

17. Umbilical cord Connection between fetus and placenta
Contains the allantois, placental blood vessels, & yolk stalk of the yolk sac
Fig. 29-6

18. Fig. 29-8

19. Fig

20. Video:
The Miracle of Life