1. Portland Community College
Lab Activity 35
Embryology
Portland Community College
BI 233

2. Fertilization
A sperm can survive for hours in the female reproductive tract
It takes about 10 hours to navigate into the ampulla of the fallopian tube where fertilization occurs. 

3. Zygote
A zygote is formed when the nucleus of the sperm and ovum combine to create a cell with 23 pairs of chromosomes. (prior to the start of cleavage)
Fertilization occurs in the ampulla of the uterine tube within 24 hours after ovulation (oocyte dies after 24 hours)

4. From Fertilization to Implantation

5. Formation of the Morula
Cleavage: Rapid mitotic cell division (without cell growth) that subdivides the cytoplasm of the zygote
Cleavage produces daughter cells called blastomeres
The zona pellucida remains around the entire group of cells.

6. Morula
After 3 days of cleavage, a solid ball of cells has formed that resembles a mulberry (similar to a Marion berry)
This Morula contains 16 or more cells

7. Blastocyst
By the fourth or fifth day the morula develops into a a fluid-filled hollow sphere called a blastocyst
Blastocyst : contains 3 parts
Trophoblast: The outer layer of cells
Take part in placenta formation
Inner cell mass: A group of cells clustered at one side of the blastocyst
Becomes the embryo
Blastocoele: The fluid filled cavity

8. Blastocyst

9. Implantation
By day six or seven the trophoblast will adhere to the endometrium with the inner cell mass side facing the uterine wall
The trophoblast cells proliferate and form two distinct layers
Syncytiotrophoblast: Cells on the perimeter touching the endometrium.
Cytotrophoblast: Cells on the interior of the trophoblast that retain their cell boundaries

10. Implantation

11. Implantation

12. Syncytiotrophoblast
At the point of contact between the trophoblast and the endometrium, the dividing cells lose their plasma membranes
This creates a syncytium in which there is a layer of cytoplasm containing multiple nuclei
The syncytiotrophoblast erodes a path through the uterine endometrium by secreting hyaluronidase
The implanted blastocyst will eventually be covered over by endometrial cells

13. Implantation
Viability of the corpus luteum is maintained by human chorionic gonadotropin (hCG) secreted by the trophoblast
hCG prompts the corpus luteum to continue to secrete progesterone and estrogen
Between the second and third month, the placenta assumes the role of progesterone and estrogen production

14. Chorionic Villi
Chorionic villi are extensions of the cytotrophoblast covered with syncytiotrophoblast that extend into the maternal blood filled spaces (lacunae)
Maternal blood will exchange nutrients through these villi that will eventually contain embryonic vessels.

15. Embryonic Disc
The inner cell mass organizes into an oval sheet with two layers (the embryonic disc)
Ectoderm: The layer closest to the attachment of the trophoblast
Endoderm: The layer closest to the blastocoele

16. Amniotic Cavity
The inner cell mass will separate from the trophoblast during implantation
The ectoderm cells will migrate and form a fluid-fill chamber (the amniotic cavity)
Amnion: Mesodermal cells will soon create the outer second layer of the amnion
The fluid and membranes will surround and cushion the developing embryo and fetus

17. Yolk Sac
Begins as the endodermal cells spread around the outer edges of the blastocoele.
The yolk sac is completed as the mesoderm cells migrate around the pouch.
Site of embryonic blood cells and gonadal stem cells

18. Gastrulation
This is the term for the process by which a third layer of cells called the mesoderm, forms between the ectoderm and endoderm.
Primitive streak : raised dorsal groove that establishes the longitudinal axis of the embryo
Gastrulation sets the stage for organogenesis; the formation of body organs

19. Gastrulation

20. Gastrulation

21. Primary Germ Layers
Serve as primitive tissues from which all body organs will be derived
Endoderm:
Epithelial lining of GI & lower respiratory tract
All ducts entering the GI tract
Urinary bladder
Ectoderm:
Nervous system
Epidermis
Lining of mouth, and anus
Sense organs such as eyes

22. Primary Germ Layers Mesoderm: Muscle Bone Cartilage Blood
Dermis and hypodermis
Kidneys, ovaries, testes
Lining of body cavities

23. Ectoderm
Mesoderm
Endoderm
Approximately 13 days after fertilization

24. 21-23 Days After Fertilization
Actual Size 1.0 – 1.5 mm

25. 28 Days After Fertilization
Actual Size: 4-6 mm
By the end of week 4 there’s significant development of the nervous system, heart, body form and placenta.
The embryo starts receiving nutrients and oxygen through the placenta.

26. Embryonic Membranes: Chorion and Allantois
Allantois: a small outpocketing at the caudal end of the yolk sac
Structural base for the umbilical cord
Becomes part of the urinary bladder
Chorion
Becomes the embryonic contribution to the placenta
Derived from trophoblast & mesoderm lining it
Contains the chorionic villi

28. Umbilical Cord Connects the embryo/fetus to the placenta
Contains 2 arteries and 1 vein

29. 2 Arteries
1 Vein

30. Placenta
The placenta is fully formed and functional by the 3rd month of pregnancy
Chorion of embryo & stratum functionalis layer of uterus
Chorionic villi extend into maternal blood filled intervillous spaces: maternal & fetal blood vessels do not join & blood does not mix
Diffusion of O2, nutrients, wastes
Stores nutrients & produces hormones
Barrier to microorganisms, except some viruses
AIDS, measles, chickenpox, poliomyelitis, encephalitis

31. Endocrine Placenta
Human Chorionic Gonadotropin (hCG): Maintains the integrity of the corpus luteum and promotes the continued secretion of progesterone
Human Placental Lactogen (hPL) and Placental Prolactin: Prepare the mammary glands for milk production

32. Endocrine Placenta
Relaxin: which relaxes connective tissue of pelvis and cervix
Suppresses the release of oxytocin by the hypothalamus
Progesterone and Estrogens: Maintain the endometrial lining

33. Placenta
This side faces the baby
This side touches the uterus

34. Gestation Fetal period: Begins at week 9
All major organs have been established
This is the time of growth
Gestation: 40 weeks measured from the first day of the uterine cycle (day one of menstruation)
Actually only 38 weeks from fertilization

35. Fetal Period
56 days after fertilization
Actual Size: 30 mm

36. The End
The End