A 25-year-old woman is evaluated at 6 weeks gestation. During embryological assessment, the clinician explains that the notochord has already begun to regress. Which of the following structures is the notochord responsible for inducing during early embryonic development?

Explanation

## Notochord Function in Early Embryology The **notochord** is a transient mesodermal structure that arises from the primitive node (Hensen's node equivalent in humans) during the third week of development. Its primary embryological significance lies in its **inductive role**. ### Key Inductive Functions: **Correct Answer: The neural tube and vertebral column** - The notochord secretes **sonic hedgehog (Shh)** and other signaling molecules (FGF, Wnt inhibitors) that induce the overlying ectoderm to form the **neural tube** (primary neurulation). - The notochord also induces surrounding mesenchyme to form the **vertebral bodies and intervertebral discs** (nucleus pulposus). - By 8–12 weeks, the notochord regresses, but its remnants persist as the **nucleus pulposus** of the intervertebral disc. ### Clinical Pearl: **Notochordal remnants** can give rise to **chordoma** (a rare malignant tumor of the spine and sacrococcygeal region), typically presenting in adults. ### High-Yield Mnemonic: **"Notochord = Neuro-inducer + Nucleus"** — it induces neural tube formation and becomes the nucleus pulposus.