Harvey's Garden

Embryogenesis

3 min read

  • Overall Timeline
    • Pre-embryonic Period (Weeks 1-2): Fertilization to implantation; formation of blastocyst.
    • Embryonic Period (Weeks 3-8): Organogenesis. Highest susceptibility to teratogens.
    • Fetal Period (Weeks 9-Birth): Growth and maturation of established organs.
  • Key Developmental Milestones
    • Week 1: Fertilization (in ampulla) → Zygote → Morula → Blastocyst. Implantation begins ~day 6. hCG secretion starts (syncytiotrophoblast).
    • Week 2: Implantation complete. Bilaminar disc forms (Epiblast, Hypoblast).
      • Clinical Note: hCG detectable in blood ~day 8, in urine ~day 14.
    • Week 3: Gastrulation → Trilaminar disc forms (Ectoderm, Mesoderm, Endoderm) from primitive streak. Notochord forms and induces neurulation.
      • Mnemonic: Week 3 = 3 layers.
    • Week 4: Heart begins to beat. Neural tube closes (failure → neural tube defects). Upper and lower limb buds form.
      • Mnemonic: Week 4 = 4 limbs, 4 heart chambers.
    • Week 8: End of embryonic period. Fetal movement (“gait at 8”). Embryo has a human-like appearance.
    • Week 10: Genitalia begin to show male/female characteristics.
    • Weeks 9-12: Red blood cells produced in the liver. Fetus can make a fist.
    • Weeks 13-16: Fingernails and toenails develop. Fetal movements can be felt by the mother (quickening).
    • Weeks 20-24: Surfactant production begins. Fetus is considered viable.
    • Weeks 36-40: Fetus is “full term.” Sufficient surfactant for lung function.

Mnemonic

  • Week 1
    • hCG secretion begins around the time of blastocyst implantation.
    • Blastocyst “sticks” on day six.
  • Week 2
    • Formation of bilaminar embryonic disc; two layers = epiblast, hypoblast.
  • Week 3
    • Formation of trilaminar embryonic disc via gastrulation (epiblast cell invagination through primitive streak); three layers = endoderm, mesoderm, ectoderm.
    • Notochord arises from midline mesoderm and induces overlying ectoderm (via SHH) to become neural plate, which gives rise to the neural tube via neurulation.
  • Week 4
    • Heart begins to beat (four chambers). Cardiac activity visible by transvaginal ultrasound.
    • Upper and lower limb buds begin to form (four limbs).
  • Week 8
    • Genitalia have male/female characteristics (pronounce “geneightalia”).

Abnormalities of morphogenesis

  • Agenesis: Absent organ due to absent primordial tissue.
  • Aplasia: Absent organ despite presence of primordial tissue.
  • Association: a collection of malformations that are often seen together and do not have a known, common cause
  • Hypoplasia: Incomplete organ development; primordial tissue present.
  • Disruption: 2° breakdown of tissue with normal developmental potential (eg, amniotic band syndrome).
  • Deformation: Extrinsic mechanical distortion (eg, congenital torticollis); occurs during fetal period.
  • Malformation: Intrinsic developmental defect (eg, cleft lip/palate); occurs during embryonic period.
    • Arteriovenous malformation (an abnormal, high-flow connection between arteries and veins bypassing capillaries which develops due to disrupted angiogenesis)
  • Sequence: Abnormalities result from a single 1° embryologic event (eg, oligohydramnios → Potter sequence).
  • Field defect: Disturbance of tissues that develop in a contiguous physical space (eg, holoprosencephaly).

Aortic arches

  • 1st: Maxillary artery.
  • 2nd: Stapedial artery.
  • 3rd: Common Carotid and proximal Internal Carotid arteries.
  • 4th: on the Left, the Aortic Arch; on the Right, the proximal Right Subclavian artery.
  • 5th: Degenerates.
  • 6th: Proximal Pulmonary arteries and Ductus arteriosus (on the left).

Graph View

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