[!Mnemonic]

• 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.

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