Embryology Mnemonics for USMLE Step 1

A ValueMD community study resource, originally shared on our forums and preserved here for students reviewing embryology.

Educational note: these are student-built memory aids for exam review, not a substitute for your course material or a clinical reference. A few entries below have been corrected or reorganized from the original forum thread for accuracy.

Early Development Timeline

Key Events by Week

• Week 2: bilaminar germ disc forms.
• Week 3: trilaminar germ disc forms (gastrulation).
• Week 4: limb buds begin to appear (the limbs themselves aren’t fully formed yet at this point, but week 4 marks the start of limb development).

Teratogenesis: When It Occurs

The embryo is most vulnerable to structural teratogenic effects during organogenesis, roughly between the third and eighth weeks of gestation. This is the window when major organ systems are actively forming and disruption is most likely to cause structural malformations.

Vitelline Duct Closure

The vitelline duct (omphalomesenteric duct) normally obliterates by around the seventh week of intrauterine life. Persistence beyond this point can lead to conditions such as a Meckel diverticulum.

Germ Layer Derivatives

Neuroectoderm Derivatives

Neuroectoderm gives rise to structures with “neuro” in the name:

• Neurons
• Neuroglia (astrocytes and oligodendrocytes; note that microglia are the exception, they’re derived from mesoderm, not neuroectoderm)
• Neurohypophysis (posterior pituitary)
• Pineal gland

Cranial and Spinal Neural Crest: Major Derivatives (“GAMES”)

• Glial cells of peripheral ganglia
• Arachnoid and pia mater
• Melanocytes
• Enteric ganglia
• Schwann cells

Mesoderm Derivatives

Mesoderm gives rise to a wide range of tissues, including:

• Muscle (striated, smooth, and cardiac)
• Connective tissue, bone, and cartilage
• Serous linings of body cavities (peritoneal, pleural, pericardial) and mesothelium
• The spleen
• The adrenal cortex (the adrenal medulla, by contrast, is neural crest-derived)
• The gonads and the ducts of the internal genitalia
• Endothelium of blood vessels
• The kidneys and the renal system
• Dura mater

Foregut Derivatives

The foregut gives rise to the lower respiratory tract (lungs and trachea), the esophagus, the stomach, the proximal duodenum (up to the ampulla of Vater), the liver, the gallbladder, and the pancreas. The spleen is often grouped with foregut structures because it shares the same blood supply (the celiac trunk), even though the spleen itself develops from mesenchyme, not endoderm.

Wolffian Duct (Mesonephric Duct) Derivatives: “Gartner’s SEED”

• In females, the mesonephric duct largely regresses, leaving a remnant called Gartner’s duct cyst.
• In males, it develops into:
  • Seminal vesicles
  • Epididymis
  • Ejaculatory duct
  • Ductus deferens (vas deferens)

Congenital and Structural Anomalies

Potter Syndrome: Features (“POTTER”)

• Pulmonary hypoplasia
• Oligohydramnios
• Twisted skin (wrinkled appearance)
• Twisted face (Potter facies)
• Extremity defects
• Renal agenesis (bilateral)

Oligohydramnios from bilateral renal agenesis is the underlying driver of this sequence: without adequate amniotic fluid, the fetus is compressed, producing the facial and limb findings, while pulmonary hypoplasia results from the lack of amniotic fluid needed for normal lung development.

Tetralogy of Fallot: Four Components (“PROVe” or “DROP”)

• Pulmonary stenosis (right ventricular outflow obstruction)
• Right ventricular hypertrophy
• Overriding aorta
• Ventricular septal defect

The same four components are sometimes remembered with the letters rearranged as “DROP” (Defect/VSD, Right ventricular hypertrophy, Overriding aorta, Pulmonary stenosis), it’s the same four findings either way, just a different letter order.

Aortic Arch Derivative: “Aorta from the Fourth Arch”

The fourth pharyngeal arch gives rise to part of the aortic arch on the left side and the proximal right subclavian artery on the right side, a useful anchor point when working through the pharyngeal arch derivatives as a group.

Other Concepts

Placenta-Crossing Substances (“WANT My Hot Dog”)

• Wastes
• Antibodies (maternal IgG specifically, which crosses the placenta, unlike IgM)
• Nutrients
• Teratogens
• Microorganisms (including agents like Toxoplasma, syphilis, rubella, CMV, and HIV)
• Hormones
• Drugs

Vasculogenesis vs. Angiogenesis

Vasculogenesis is the formation of new blood vessels de novo from mesenchymal precursor cells (angioblasts). Angiogenesis is the formation of new vessels by sprouting from existing vessels. A simple way to keep them apart: vasculogenesis builds vessels from scratch, angiogenesis extends vessels that already exist.

Lung Development Phases: “Every Premature Child Takes Air”

• Embryonic period
• Pseudoglandular period
• Canalicular period
• Terminal sac period
• Alveolar period

Frequently Asked Questions

What’s the difference between vasculogenesis and angiogenesis? Vasculogenesis is the de novo formation of blood vessels from mesenchymal angioblasts, occurring early in development. Angiogenesis is the formation of new vessels by sprouting from vessels that already exist, and it continues throughout life, including in wound healing and tumor growth.

Why is microglia the exception among “neuro” structures? Most CNS glial cells (astrocytes, oligodendrocytes) are neuroectoderm-derived, matching the naming pattern of other neuroectoderm derivatives. Microglia are the exception: they originate from mesodermal (yolk sac-derived) precursor cells and function as the resident immune cells of the CNS, more closely related to macrophages than to true glial cells.

What causes the features of Potter syndrome? Bilateral renal agenesis (or another cause of significantly reduced fetal urine output) leads to oligohydramnios. The resulting lack of amniotic fluid compresses the fetus, producing the characteristic facial appearance, wrinkled skin, and limb abnormalities, in addition to pulmonary hypoplasia from impaired lung development.

During which weeks of pregnancy is the embryo most vulnerable to teratogens? The third through eighth weeks of gestation, the period of organogenesis, carry the highest risk of major structural malformations from teratogen exposure, since this is when organ systems are actively forming.

Want to Know More?

Working through a tricky embryology topic or want to compare mnemonics with other students? Visit our forum to connect with students and residents reviewing the same material and share what’s helped it stick.