The notochord is a flexible, rod-like scaffold that forms in the earliest stages of embryonic life. It serves as the primitive backbone, providing the primary axial support for the developing embryo long before the spinal column is formed. Made from an embryonic tissue called the mesoderm, the notochord is fundamental to the organized growth of the early human form.
Role in Early Human Development
During the first few weeks of development, the notochord fulfills two main functions. The first is structural, as it provides a rigid yet flexible axis that maintains the shape and organization of the embryo. This physical support is necessary for the coordinated folding and shaping of the embryonic body.
Its second role is as a signaling center. The notochord releases molecules that instruct nearby tissues to form specialized structures. One of its most significant signaling tasks is to induce the overlying ectoderm—the outermost embryonic layer—to thicken and form the neural plate. This plate then folds to create the neural tube, the precursor to the brain and spinal cord. This process is directed by proteins it secretes, such as Sonic hedgehog (SHH), which influences cell differentiation based on its concentration.
The notochord also orchestrates the formation of somites, which are blocks of mesoderm arranged on either side of it. These somites are building blocks that later develop into the vertebrae, ribs, and the skeletal muscles of the back. The notochord’s signals ensure these structures form correctly and in their proper locations.
Fate After Embryonic Development
As the embryo continues to mature, the notochord’s prominence begins to fade. Its structural role is gradually taken over by the developing vertebral column, the bony structure that will become the permanent spine.
Once surrounded by the forming bones of the spine, the majority of the notochordal cells are programmed to die off in a process known as apoptosis. This programmed cell death is a normal part of development, allowing for the transition from a primitive support structure to a more complex and permanent one.
However, the notochord does not vanish completely. Small clusters of its cells persist in specific locations within the newly formed spinal column. These remnants become trapped within the center of the intervertebral discs, the cartilaginous cushions that sit between each vertebra. Here, the notochordal cells form the core of a new structure: the nucleus pulposus.
The nucleus pulposus, a gel-like substance, is the direct descendant of the embryonic notochord. While the embryonic notochordal cells are eventually replaced by other cell types as a person ages, their initial presence lays the foundation for this component of the adult spine.
Medical Significance of Notochord Remnants
The remnants of the notochord, primarily the nucleus pulposus within each intervertebral disc, have lifelong implications for spinal health. The nucleus pulposus is a gelatinous material that gives the disc its ability to absorb shock and withstand compression and twisting forces. This allows the spine to be flexible and resilient, protecting the vertebrae from grinding against each other during movement.
A common medical issue involving this structure is a herniated disc, sometimes called a slipped or ruptured disc. This occurs when the tough outer layer of the intervertebral disc, the annulus fibrosus, tears, allowing the soft, gel-like nucleus pulposus to bulge or leak out. If this extruded material presses on a nearby spinal nerve, it can cause significant pain, numbness, or weakness in the parts of the body served by that nerve.
While the nucleus pulposus serves a beneficial function, other notochordal remnants can lead to a rare form of cancer. Notochordal cells that persist into adulthood can become malignant, giving rise to a tumor called a chordoma. These are slow-growing bone cancers that occur at the base of the skull or the bottom of the spine, areas where notochordal remnants are most likely to persist.