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Mnemonic

  • Week 1
    • hCG secretion begins around the time of blastocyst implantation.
    • Blastocyststicks” 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


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Aortic Arch DerivativeAdult Derivative
FirstPart of maxillary artery
SecondHyoid artery, Stapedial artery
ThirdCommon carotid artery, Proximal internal carotid artery
FourthOn left → aortic arch, On right → proximal right subclavian artery
SixthProximal pulmonary arteries, On left → ductus arteriosus