During the third week of embryonic development, which of the following structures is formed by the epiblast?

Explanation

## Epiblast Derivatives in Week 3 During the **third week of embryonic development**, the epiblast undergoes **gastrulation** and gives rise to the three primary germ layers: ### Key Point: The epiblast is the source of: - **Intraembryonic mesoderm** — formed via ingression through the primitive streak - **Notochord** — derived from the primitive node (cranial end of primitive streak) - **Ectoderm** — epiblast cells that do not ingress through the primitive streak ### High-Yield Fact: The epiblast is the **only embryonic layer** (not extraembryonic) that contributes to the three primary germ layers. This is the hallmark of gastrulation. ### Clinical Pearl: Abnormalities in epiblast migration during week 3 can lead to: - Notochord defects → spinal dysraphism - Mesodermal defects → vertebral anomalies (VATER association) ## Embryonic vs. Extraembryonic Origins | Structure | Origin | Timing | |-----------|--------|--------| | Intraembryonic mesoderm | Epiblast (via primitive streak) | Week 3 | | Notochord | Epiblast (via primitive node) | Week 3 | | Trophoblast | Outer blastomere cells | Week 1–2 | | Yolk sac endoderm | Hypoblast | Week 2 | | Amnion | Epiblast (amniogenic cells) | Week 2 | | Chorion | Trophoblast + extraembryonic mesoderm | Week 2 |