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Embryonic Period Timeline

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Topic updated on 08/17/17 3:10pm

 
Time
Landmark
Day 0 
  • Embryogenesis begins
    • sperm fertilized ovum in ampulla
    • all mitochondria are maternal in origin
Week 1
  • Cleavage and blastocyst formation
    • zygote undergoes mitotic divisions to form blastomeres → morula → blastocyst
    • blastocyst cavity forms two cell masses
      • inner cell mass = embryoblast which will become embryo
      • outer cell mass = trophoblast which becomes placenta
  • hCG secretion
    • begins after implantation of blastocyst
    • syncytiotrophoblast invades uterine wall and secretes hCG
    • hCG detected in blood at day 8 and in urine at day 10 (pregnancy test)
      • ↓ hCG: ectopic pregnancy or a sign for spontaneous abortion
      • ↑ hCG: multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia
Week 2
  • Bilaminar disk formation
    • consists of 2 germ layers: epiblast and hypoblast,
    • 2 cavities are present: amniotic cavity and yolk sac 
    • placenta has 2 components: cytotrophoblast and synctiotrophoblast
    • notochord forms
Week 3
  • Gastrulation
    • establishment of three embryonic germ layers (ectoderm, mesoderm, and endoderm)
    • initiated by formation of primitive streak
  • Formation of notochord and neural plate
    • notochord induces its overlying ectoderm to become neuroectoderm
      • neuroectoderm will form neural plate, fully closes by   week 4
      • neural plate forms neural tube and neural crest cells
    • notochord will become adult nucleus pulposus in the vertebral column
Week 3-8
  • Embryonic period
    • major organs systems have begun development
    • period in embryo is most susceptible to teratogens
    • hematopoiesis begins around yolk sac at week 3 and is taken over by liver, thymus, spleen, and final and lifetime production is done in bone marrow
    • week 7 embryo begins sexual differentiation
Week 4 
  • Initiation of heart beat
  • Formation of upper and lower limb buds
  • Closure of the neural plate
Week 8
  • Fetal period
    • fetus begins to move and has appearance of a baby
Week 10-12
  • Sex of fetus becomes recognizable
  • Phenotypic differentiation completed by week 20


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