1. Embryology day 6-20

2. First, Embryology in context
It will be covered again…
ESA1
Gastro: development of foregut, midgut, hindgut
ESA 2
Repro: fertilization, urogenital embryology, abnormal implantation
Musculoskeletal: muscles and bones
Cardiovascular: heart and circulation
ESA 3
Urinary: renal developmental abnormalities

3. Early Development in Brief…
Week 1: Egg travels down fallopian tube, dividing as it goes, until it reaches the uterus
Week 2: ‘The week of 2s’
Blastocyst divides in 2: ‘Baby’, Placenta
Week 3: Embryo now has 3 layers
Ectoderm, Endoderm, Mesoderm

4. Early Development in pictures

5. WEEK ONE

6. Week one Day 5 Blastocyst reaches uterus
Day 5-10 Implantation of blastocyst into uterine wall

7. Up to 50% of blastocysts are abnormal, and may be lost before the mother even knows she is pregnant.

8. Abnormalities in week one
Abnormal implantation:
Ectopic pregnancy
Abnormal blastocyst:
Abortion

9. WEEK TWO
PLACENTA
BABY
BABY IN 2 LAYERS
2 CAVITIES

10. Abnormalities in week two
Abnormal blastocyst:
Abortion
Possible molar pregnancy (see repro module)

11. WEEK 3
Baby develops 3 layers
…and things start to get
…complicated

12. Some words… Embryo = ? Fetus = ?
‘Embryo’ = ‘that which grows’ 0-8 weeks (not quite human-looking)
Fetus = ‘offspring’ 8 weeks – term (looks human)

13. Week 2 is the ‘week of 2s’

14. Some words ‘Blast’ = immature Blastocyst = ? ‘Tropho’ = nourish
Trophoblast = ?
Embryoblast = ?

15. More words Trophoblast divides in two: Embryoblast divides in two:
Cytotrophoblast
syncytiotrophoblast
Embryoblast divides in two:
Epiblast
Hypoblast

16. Week two: PLACENTA Blastocyst divided into placenta and baby
Placenta = Trophoblast
2 layers: cytotrophoblast has proliferating cells that move into syncytiotrophoblast
Syncytiotrophoblast invades maternal capillaries to form UTERO-PLACENTAL CIRCULATION

17. Week two: BABY Inner cell mass of blastocyst becomes embryoblast
Divides in two: epiblast and hypoblast
Epiblast from all 3 embryological layers
Hypoblast forms lining of future yolk sac

18. Week two: ANOTHER LAYER (not baby, not placenta)
A new layer of mesoderm forms outside between baby and placenta from the hypoblast
‘Extra-embryonic mesoderm’
A big space develops within it, so that mesoderm now lines a new cavity: chorionic cavity
The connecting stalk becomes the….

19. This mesoderm invaginates the primary villi of the cytotrophoblast, and forms capillaries
These now link up baby and mother via the connecting stalk to nourish the baby

20. More words Chorion = ‘skin’ = outer membrane
Amnion = ‘lamb’ = inner membrane which surrounds baby
Yolk sac = provides nourishment

21. Syncytiotrophoblast
(‘invading layer’)
Utero-placental circulation
Trophoblast (future placenta)
Blastocyst
Cytotrophoblast
(proliferating cell layer)
Primary villi
Epiblast
Ectoderm, mesoderm, endoderm
Embryoblast (future baby)
Hypoblast
Lining of secondary yolk sac,
Extra-embryonic mesoderm

22. WEEK 3
Baby develops 3 layers
…and things start to get
…complicated

23. So: Gastrulation The process by which 3 layers are formed
First, a streak appears on the surface of the epiblast
Cells start to move toward it, then invaginate underneath
First ones to invaginate become endoderm, next ones become mesoderm, ones on top are ectoderm

24. Week three: THE TRICKY BIT
Gastrulation occurs
Formation of 3 layers of
Neurulation begins
Formation of neural
tube

25. Endoderm forms…
Gut
Lung
Bladder

26. Mesoderm forms… Muscle Skeleton Connective tissue Mesentery (in gut)
Heart
One foregut structure (from dorsal mesentery)
Joins extra-embryonic mesoderm

27. Ectoderm forms…
Nervous tissue
Skin
Mouth and anus

28. Abnormalities week 3 onward
Abnormal development of three layers:
Neural tube defects
Lower limb abnormalities
Craniofacial abnormalities

29. Abnormalities associated with defective gastrulation
Often cause death
Toxins (alcohol) may kill off cells migrating along the midline: brain and facial deformities
Insufficient mesoderm may form caudally: sirenomelia (legs fused ‘mermaid-like’)

30. Are these good ESA 1 questions?
The gut is comprised of which embryological layer?
In which week of development does this layer begin to develop?
What is likely consequence of a significant chromosomal abnormality in the first weeks of development?
What function does the yolk sac have in the developing embryo?
How does a neural tube defect occur?

31. Note: Remember that exam questions must…
…examine clinically important material
…be clear and unambiguous
…not test irrelevant details
…ideally integrate with other subjects (therefore learn your GI embryology for ESA1)