**Core Concept**
The nucleus pulposus is a critical component of the intervertebral disc, playing a crucial role in spinal flexibility and shock absorption. It is composed of a gel-like substance surrounded by a fibrocartilaginous matrix.

**Why the Correct Answer is Right**
The nucleus pulposus is derived from the notochord, a precursor to the vertebral column. During embryonic development, the notochord elongates and induces the formation of the vertebral bodies, which eventually surround the notochordal tissue. The notochordal cells differentiate into the nucleus pulposus, while the surrounding tissue differentiates into the annulus fibrosus. The notochordal cells secrete proteoglycans, which accumulate and create the gel-like substance characteristic of the nucleus pulposus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the nucleus pulposus is not derived from the annulus fibrosus, which is a fibrocartilaginous ring surrounding the intervertebral disc.
**Option B:** This option is incorrect because the nucleus pulposus is not derived from the vertebral body, which is a bony structure that forms the vertebral arches.
**Option C:** This option is incorrect because the nucleus pulposus is not derived from the spinal cord, which is a complex structure composed of neurons, glial cells, and a myelin sheath.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the notochordal cells in the nucleus pulposus have a limited lifespan and are gradually replaced by fibrocartilaginous cells, leading to changes in the intervertebral disc's composition and function over time.

**Correct Answer: D. Notochord. Notochordal cells differentiate into the nucleus pulposus.**