1. PART 1
Basic Embryology

2. Embryology – study of the origin and development of single individual
Prenatal period
Embryonic period – first 8 weeks
Fetal period – remaining 30 weeks
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3. Embryonic Period Figure 3.1 (1 of 2)
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4. Fetal Period Figure 3.1 (2 of 2)
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5. Skin – dermis and epidermis
The Basic Body Plan
Skin – dermis and epidermis
Outer body wall – trunk muscles, ribs, vertebrae
Body cavity and digestive tube (inner tube)
Kidneys and gonads – deep to body wall
Limbs
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6. The Basic Body Plan Figure 3.2
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7. Week 1 – from zygote to blastocyst
The Embryonic Period
Week 1 – from zygote to blastocyst
Conception – in lateral third of uterine tube
Zygote (fertilized oocyte) moves toward the uterus
Blastomeres – daughter cells formed from zygote
Morula – solid cluster of 12–16 blastomeres
“Mulberry”
Blastocyst – fluid-filled structure – ~ 60 cells
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8. The Embryonic Period Stages of first week Zygote 4-cell Morula
Early blastocyst
Late blastocyst (implants at this stage)
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9. Fertilization and the Events of the First 6 Days of Development
Figure 3.3
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10. Week 2 – The Two-Layered Embryo
Bilaminar embryonic disc – inner cell mass divided into two sheets
Epiblast and the hypoblast
Together they make up the bilaminar embryonic disc
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11. Week 2 – The Two-Layered Embryo
Amniotic sac – formed by an extension of epiblast
Outer membrane forms the amnion
Inner membrane forms the amniotic sac cavity
Filled with amniotic fluid
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12. Week 2 – The Two-Layered Embryo
Yolk sac – formed by an 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
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13. Implantation of the Blastocyst
Figure 3.4 (1 of 3)
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14. Implantation of the Blastocyst
Figure 3.4 (2 of 3)
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15. Implantation of the Blastocyst
Figure 3.4 (3 of 3)
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16. Week 3 – The Three-Layered 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
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17. Week 3 – The Three-Layered 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
*All layers derive from epiblast cells!
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18. The Primitive Streak Figure 3.5e–h
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19. Primitive node – a swelling at one end of primitive streak
The Notochord
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
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20. Formation of the Mesoderm and Notochord
Figure 3.6
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21. Neurulation – ectoderm starts forming brain and spinal cord
Neural plate – ectoderm in the dorsal midline thickens
Neural groove – ectoderm folds inward
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22. Neurulation (continued)
Neural tube – a hollow tube pinches off into the body
Cranial part of the neural tube becomes the brain
Maternal folic acid deficiency causes neural tube defects
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23. Neurulation Neural crest Induction
Cells originate from ectodermal cells
Forms sensory nerve cells
Induction
Ability of one group of cells to influence developmental direction of other cells
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24. The Mesoderm Begins to Differentiate
Somites – our first body segments
Paraxial mesoderm
Intermediate mesoderm – begins as a continuous strip of tissue just lateral to the paraxial mesoderm
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25. The Mesoderm Begins to Differentiate
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
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26. PART 2
Basic Embryology

27. Changes in the Embryo Figure 3.7a, b
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28. Changes in the Embryo Figure 3.7c, d
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29. Week 4 – The Body Takes Shape
Folding of embryo laterally and at the head and tail
Embryonic disc bulges; growing faster than yolk sac
“Tadpole shape” by day 24 after conception
Primitive gut – encloses tubular part of the yolk sac
Site of future digestive tube and respiratory structures
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30. Week 4 – The Body Takes Shape
Figure 3.8
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31. Week 4 – The Body Takes Shape
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
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32. Week 4 – The Body Takes Shape
Mesoderm – forms
Muscle
Bone
Dermis
Connective tissues (all)
Mesoderm differentiates further and is more complex than the other two layers
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33. Derivatives of Germ Layers
Figure 3.10
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34. Week 4 – The Body Takes Shape
Mesoderm (continued)
Somites divides into
Sclerotome
Dermatome
Myotome
Intermediate mesoderm forms
Kidneys and gonads
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35. Week 4 – The Body Takes Shape
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
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36. PART 3
Basic Embryology

37. The Germ Layers in Week Four
Figure 3.9a–d
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38. Week 5-8 – The Second Month of Embryonic Development
Limb buds form
Embryo first looks recognizably human (week 8)
Head is disproportionately large
All major organs are in place
Figure 3.11
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39. A time of maturation and rapid growth
The Fetal Period
A time of maturation and rapid growth
Cells are differentiating during the first half of the fetal period
Normal births occur 38 weeks after conception
Premature birth is one that occurs before 38 weeks
PLAY
Ultrasound of Fetus
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40. Developmental Events of the Fetal Period
Table 3.1 (1 of 3)
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41. Developmental Events of the Fetal Period
Table 3.1 (2 of 3)
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42. Developmental Events of the Fetal Period
Table 3.1 (3 of 3)
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