A 32-year-old pregnant woman undergoes routine second-trimester ultrasound. The radiologist identifies the notochord in the developing embryo. Which of the following structures is primarily derived from the notochord?

Explanation

## Notochord and Its Derivatives The **notochord** is a mesodermal structure that appears early in embryonic development (around week 3) and plays a crucial role in organizing the developing embryo. ### Primary Derivative: Nucleus Pulposus - The notochord gives rise to the **nucleus pulposus** of the intervertebral discs - This is the only major structure directly derived from the notochord - The nucleus pulposus is the gelatinous central core of the intervertebral disc - It is surrounded by the annulus fibrosus (derived from sclerotomes) ### Other Notochord Functions: - Acts as an **organizer** for surrounding mesodermal tissues - Induces formation of vertebral bodies (but does not form them directly) - Regresses in most areas except where it persists as the nucleus pulposus ### Clinical Pearl: **Notochordal remnants** can persist and form: - Chordomas (malignant tumors, usually in sacrococcygeal region) - Chordomas are the most common primary malignancy of the sacrum **Key Point:** While the notochord organizes vertebral development, it does NOT form the vertebral bodies themselves—those are derived from sclerotomes (somitic mesoderm).