1. The First Three Weeks of Human Embryogenesis
Department of Histology, Cytology and Embryology
Kharkiv State Medical University

2. Week 1-3: Early Stages: 1. Fertilization 2. Cleavage 3. Gastrulation
4. Formation of the embryo body
Late stages:
Histogenesis, Organogenesis

3. Week 1
1.Fertilization – is the fusion of the sperm and ovum = Zygote formation

4. Fertilization. Cleavage
2 cells stage
3-5 cells stage
Morula
Blastula . .
uterus
uterine tube
Implantation
Day 6 - 8
Fertilization
(positive rheotaxis,
gyno-androgamones,
capacitation, acrosomal
reaction) = zygote

5. Week 1
2.Cleavage – is the division of the zygote in the uterine tube = Blastula formation (2 cell stage – 4 cell stage …16-32 cell stage (morula) – blastocyst)
During cleavage the germ is moved to the uterus

6. At the end of the cleavage outer cells (trophoblast) involve nutritive fluid, which forms cavity, moving inner cells to the periphery (embryoblast). Blastula is formed.
Inner Cell Mass
(embryoblast)
Trophoblast
Blastocoele

7. Week Gastrulation
Gastrulation by the delamination and migration leads to formation of three germ layers: ectoderm, endoderm and mesoderm,
and axial organs:
notochord, neural tube, somites

8. Early Gastrulation. At the beginning of gastrulation germ sinks into the uterine wall – implantation. Gastrulation begins from delamination – division of embryoblast into two germ layers - ectoderm and endoderm, forming embryonic disc and two sacs – ectoblast and endoblast

9. Delamination Trophoblast Ectoblast Embryonic disc: Ectoderm Endoderm
Endoblast

10. Migration - extraembryonic mesoderm formation From embryonic disc cells begin to migrate which form extraembryonic mesoderm, surrounding upper and lower sacs, and underlying trophoblast As a result appear amnion, yolk sac and chorion (extraembryonic organs)

11. Week 2 Extraembryonic Mesoderm Trophoblast Chorion Ectoderm Amnion
Endoderm
Yolk Sac

12. Late gastrulation – migration continues within embryonic disk
and
leads to formation of the embryonic mesoderm (third germ layer) and axial organs (neural tube, notochord and somites - 43, 44 pairs)

13. Transverse section. Moving ectodermal and endodermal cells form group of cells = primitive streak
Amniotic Cavity Embryonic mesoderm
Ectoderm
Primitive streak
Yolk Sac
Endoderm

14. Notochord is formed from invaginated ectoderm and primitive streak

15. Notochord formation (longitudinal section)

16. Development of the Neural Tube by the invagination of ectoderm

17. Development of the Neural Tube Neural groove
Thus, the cells can become pyramidal in shape, and this change in cell shape can alter a flat sheet of cells to become a hollow tube.

18. Development of the Neural Tube

19. Development of the Neural Tube
It is known that the notochord and the adjacent mesoderm around it can induce the ectoderm above to form the neural tube.

20. Development of the Neural Tube
Surface Ectoderm
During the formation of the neural tube, ectodermal cells that are at the junction of the neural plate with the surface ectoderm, form neural crest cells.
Neural Crest
Neural Tube

21. Neural tube formation

22. Neural tube formation

23. Somites locate between ectoderm and endoderm, arise from primitive streak and notochord and
consist of mesoderm

24. Axial organs –neural tube, notochord, somites (mesoderm)

25. Formation of the Body Axis Left – Right Sidedness
FGF Nodal Lefty PITX2

26. Left – Right sidedness Situs inversus 20% (Kartagener syndrome)
Dysfunctional cilia
Respiratory problems
Male fertility problems

27. Body flexion, head and tail folds formation.
4. Formation of the embryo body (20 days)
Body flexion, head and tail folds formation.
Amnion accumulates fluid and increases, yolk sac decreases.
Formation of a gut, allantois

29. Body flexion formation

30. Body flexion formation (Gut formation
Body flexion formation (Gut formation. The upper part of yolk sac forms tube inside the forming body = gut - primitive digestive tube
gut

31. (Gut. Allantois – is extention of hindgut)
Body flexion formation
(Gut. Allantois – is extention of hindgut)
Allantois
FUTURE PLACENTA

32. Body formation hindgut foregut midgut Cloacal Plate Oral Plate
Allantois
Heart
Yolk Sac

33. Body formation
Chorionic plate participates in the formation of placenta
Amniotic cavity
Uterine
Wall
Chorionic
Cavity
Yolk Sac and allantois participate in the formation of ambilical cord

34. What should we study by heart ?

35. EXTRAEMBRYONIC ORGANS
chorion
Amnion
Allantois
FUTURE PLACENTA
Yolk sac

36. Extraembryonic organs (supportive, nutritive)
Amnion – protective bag of water
Yolk sac – gut, germs of gametes
first blood vessels, cells
Allantois – urinary bladder
Chorion – protection, hormones, placenta
Placenta – main nutritive, protective, hormonal

37. Differentiation of GERM LAYERS:
1. Differentiation of Ectoderm
A. Surface Ectoderm
B. Neural Tube
2. Differentiation of Endoderm
A. G.I. Tract
B. Respiratory Tree
C. Endocrine glands
3. Differentiation of Mesoderm
A. Somites-dermatome, myotome, sclerotome
B. Intermediate- nephrotome
C. Lateral-splanchnotome
D. Mesenchyme

38. Surface Ectoderm differentiates to skin, its derivatives, oral, rectal epithelium, corneal epithelium, tooth enamel
amnion ectoderm
proctodeum
stomatodeum

39. Ectoderm forms neuroectoderm –neural tube (neurons and neuroglia of the brain and the retina) and neural crests (nerve ganglia, neuroglia, adrenal medulla, melanocytes, APUD-system) weeks
Surface Ectoderm
During the formation of the neural tube, ectodermal cells that are at the junction of the neural plate with the surface ectoderm, form neural crest cells.
Neural Tube
Neural Crest

40. Endoderm differentiates to epithelium of the stomach, intestine, liver, pancreas, respiratory, endocrine systems weeks - gut
endoderm gut

41. Endoderm (gut, allantois)
foregut
hindgut
midgut
Heart
Cloacal
Plate
Oral Plate
Stomatodeum
Allantois
Yolk Sac

42. Mesoderm - formation of the first blood vessels in the wall of yolk sac and allantois

43. Mesoderm Ectoderm (Somite) Amniotic Cavity Intermediate mesoderm
(nephrotome)
Lateral plate mesoderm
(somatopleuric,
splanchnopleuric
mesoderm)
Amniotic Cavity
During the third week of development the mesoderm on either side of the notochord thickens and is called the paraxial mesoderm. The paraxial mesoderm gives rise to somites.
Laterally, the mesoderm is thin and is called the lateral plate mesoderm. A cavity develops in the lateral plate mesoderm that is continuous with the extraembryonic coelom, called the intraembryonic coelomic cavity. This cavity divides the lateral mesoderm into two layers.
The mesoderm that lies dorsal to the intraembryonic coelomic cavity is called the somatic (parietal) mesoderm. The ectoderm and somatic mesoderm is called the somatopleure.
That mesoderm found ventral to the intraembryonic coelomic cavity is called the splanchnic (visceral) mesoderm. The splanchnic mesoderm and the endoderm is called the splanchnopleure.
The mesoderm between the paraxial and lateral plated mesoderm is called the intermediate mesoderm.
Yolk Sac
Notochord
Endoderm

44. Mesoderm dermatome - dermis of skin myotome - muscles, Somite
sclerotome - skeleton, except skull
Mesoderm
Somite
urogenital system
including kidneys, gonads,
ducts, and accessory glands
Intermediate Mesoderm.
Nephrotome
Lateral Mesoderm -
serous membranes of pleura, pericardium and peritoneum
Mesenchyme (loose part) –
connective tissue of viscera and limbs, blood and lymph cells, cardiovascular and lymphatic systems

45. Chorionic plate participates in the formation of placenta
Amniotic cavity
Uterine
Wall
Chorionic
Cavity
Yolk Sac and allantois participate in the formation of ambilical cord

46. Late embryonic stages
Histogenesis
Organogenesis