1. Embryology Review

2. Embryology
Embryology – study of origin and development in utero—prenatal formation, growth and differentiation
Prenatal period
Embryonic period – first 8 weeks
Fetal period – remaining 30 weeks

3. Embryonic Period

4. Fetal Period

5. The Embryonic Period
Week 1 – from zygote to blastocyst
Fertilization (Conception) – in lateral third of uterine tube
Zygote (fertilized oocyte) moves toward the uterus
Blastomeres – daughter cells formed from zygote through mitotic division called cleavage
Morula – solid cluster of 12–16 blastomeres
Blastocyst – fluid-filled embryonic stage– ~ 60 cells

6. The Embryonic Period Events in first week Zygote 4-cell Morula
Early blastocyst
Late blastocyst— implantations occur at this stage

7. Fertilization and the Events of the First 6 Days of Development

8. Week 2 – Blastulation
Two-layered embryo formation
Bilaminar embryonic disc – inner cell mass divided into two sheets
Epiblast and the hypoblast
Together they make up the bilaminar embryonic disc

9. Week 2 – Blastulation
Amniotic sac – formed by an extension of epiblast
Outer membrane forms the amnion
Inner membrane forms the amniotic sac cavity
The cavity is filled with amniotic fluid

10. Digestive tube forms from yolk sac
Week 2 – Blastulation
Yolk sac – formed by an lateral extension of hypoblast
Digestive tube forms from yolk sac
NOT a major source of nutrients for embryo
Tissues around yolk sac gives rise to earliest blood cells and blood vessels

11. Implantation of the Blastocyst

12. Implantation of the Blastocyst

13. Implantation of the Blastocyst

14. Disorders of implantation
Ectopic pregnancy
Tubal— 95%
Peritoneal
Douglas pouch
Pracental previa
-Placenta forms at the inner cervical os
-Characterized with uterine bleeding

15. Week 3 – Trilaminar embryo
Primitive streak – raised groove on the dorsal surface of the epiblast
Gastrulation – a process of invagination of epiblast cells
Begins at the primitive streak
Forms the three primary germ layers

16. Week 3 – Tri-laminar embryo
Three Germ Layers
Endoderm – formed from migrating cells that replace the hypoblast
Mesoderm – formed between epiblast and endoderm
Ectoderm – formed from epiblast cells that stay on dorsal surface
Note that all layers derive from epiblast cells

17. Week 3: The Primitive Streak

18. Week 3 – Tri-laminar embryo

19. Week 3: Notochord formation
Primitive node – a swelling at one end of primitive streak
Notochord forms from primitive node and endoderm
Notochord – defines body axis
Is the site of the future vertebral column
Appears on day 16

20. Week 3: Notochord and Mesoderm

21. Week 3: Notochord and Mesoderm

22. Neural plate – ectoderm in the dorsal midline thickens
Week 3: Neurulation
Neurulation – ectoderm starts forming brain and spinal cord
Neural plate – ectoderm in the dorsal midline thickens
Neural groove – ectoderm folds inward

23. Cranial part of the neural tube becomes the brain
Week 3: Neurulation
Neurulation (continued)
Neural tube – a hollow tube pinches off into the body
Cranial part of the neural tube becomes the brain
Folic acid deficiency at this stage causes neural tube defects

24. Week 3: Neurulation
Neural crest
Cells originate from ectodermal cells
Forms sensory nerve cells
Induction
Ability of one group of cells to influence developmental direction of other cells

25. Week 3: Mesodermal differentiation
Somites – Body segementation
Paraxial mesoderm
Intermediate mesoderm – begins as a continuous strip of tissue just lateral to the paraxial mesoderm

26. Week 3: Mesodermal differentiation
Lateral plate – most lateral part of the mesoderm
Coelom – becomes serous body cavities
Somatic mesoderm – apposed to the ectoderm
Splanchnic mesoderm – apposed to the endoderm

27. Parts of the mesoderm

28. Neurulation and notocord

29. Neuralation and notocord

30. Week 4 – Embryonic folding
Folding of embryo laterally and at the head and tail
Embryonic disc bulges; growing faster than yolk sac
Primitive gut – encloses tubular part of the yolk sac
Site of future digestive tube and respiratory structures

31. Week 4 – The Body Takes Shape

32. Week 4 – Folding Derivatives of the germ layers Ectoderm forms
Brain, spinal cord, and epidermis
Endoderm forms
Inner epithelial lining of the gut tube
Respiratory tubes, digestive organs, and urinary bladder
Notochord – gives rise to nucleus pulposus within intervertebral discs

33. Week 4 – Folding and systemic development
Mesoderm – forms
Muscle
Bone
Dermis
Connective tissues
Mesoderm differentiates further and is more complex than the other two layers

34. Week 4 – System development
Mesoderm (continued)
Somites divides into
Sclerotome
Dermatome
Myotome
Intermediate mesoderm forms
Kidneys and gonads

35. Week 4 – System development
Mesoderm (continued)
Splanchnic mesoderm
Forms musculature, connective tissues, and serosa of the digestive and respiratory structures
Forms heart and most blood vessels
Somatic mesoderm – forms
Dermis of skin
Bones
Ligaments

36. Derivatives of Germ Layers
Figure 3.10

37. The Germ Layers in Week Four

39. Week 5-8 – Organogenesis Limb buds form
Embryo first looks recognizably human (week 8)
Head is disproportionately large
All major organs are in place

41. Fetal period- summary

42. Fetal period- summary

43. Fetal period- summary