A 5-week-old human embryo is being studied in a developmental biology laboratory. The notochord is identified as a critical structure during early development. Which of the following is the PRIMARY role of the notochord during embryogenesis?

## Notochord: Structure and Function The notochord is a transient mesodermal structure that plays a crucial **inductive role** in early embryonic development. ### Primary Role: Neural Induction - The notochord secretes **signaling molecules** (particularly sonic hedgehog, SHH) that induce the overlying ectoderm to differentiate into the neural plate - This is the critical **organizer signal** that initiates neurulation and neural tube formation - Without notochordal signaling, the ectoderm would default to epidermis formation ### Fate of the Notochord - Most of the notochord **degenerates** during fetal development - Small remnants persist as the **nucleus pulposus** of intervertebral discs (derived from notochord, but the notochord itself does NOT form the vertebral bodies) - The vertebral bodies are derived from **somites** (mesodermal origin), not the notochord ### Key Point: **The notochord is an INDUCER, not a BUILDER.** Its primary function is signaling, not tissue formation. ### Clinical Pearl: Notochordal remnants can give rise to **chordomas** — rare malignant tumors arising from remnant notochordal tissue along the midline (sacrococcygeal or clival regions).