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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