No, the notochord does not become the spinal cord. These two structures have different embryonic origins and different fates. The notochord is a temporary rod of tissue that acts as a signaling center during early development, while the spinal cord forms from a completely separate structure called the neural tube. What the notochord actually becomes is a small, squishy core inside each of your spinal discs.
Where the Confusion Comes From
The notochord and the spinal cord sit in nearly the same location in the developing embryo, running along the midline of the back. They also develop around the same time, during the third and fourth weeks after conception. So it’s natural to assume one turns into the other. But they come from different cell layers entirely. The notochord forms from the mesoderm, the middle layer of embryonic tissue. The spinal cord forms from the ectoderm, the outer layer, which folds inward to create the neural tube.
What the Notochord Actually Does
The notochord’s main job is to act as a scaffold and a signaling hub. Physically, it works like a hydrostatic skeleton: pressurized internal vacuoles push against a surrounding membrane, creating stiffness that drives the embryo to elongate along its head-to-tail axis. This happens before a spine exists, so the notochord essentially defines the body’s central axis.
Its signaling role is arguably more important. The notochord releases a protein called Sonic hedgehog (yes, named after the video game character) along with several other molecules that block competing signals. Together, these chemicals tell the ectoderm cells sitting directly above the notochord to change shape, stack into a plate of neural tissue, and then fold inward to form the neural tube. Without the notochord sending those signals, the neural tube, and therefore the spinal cord and brain, would not form properly. So while the notochord doesn’t become the spinal cord, it is the reason the spinal cord develops where it does.
How the Spinal Cord Actually Forms
During the third week of development, signals from the notochord and nearby tissue cause a strip of ectoderm cells to thicken into what’s called the neural plate. These cells elongate into tall, column-shaped cells and then begin to fold. The edges of the plate rise up, curl toward each other, and eventually fuse to create a hollow tube, the neural tube, which pinches off from the surface ectoderm.
This process, called neurulation, divides the original outer cell layer into three distinct populations: the neural tube on the inside (which becomes the brain and spinal cord), the epidermis on the outside (which becomes skin), and a group of migratory cells called neural crest cells (which go on to form parts of the peripheral nervous system, pigment cells, and facial structures). The spinal cord is therefore ectodermal in origin, not mesodermal like the notochord.
What the Notochord Becomes in Adults
Once the notochord has done its signaling work, it doesn’t simply disappear. Over the following weeks, cells from nearby somites (blocks of mesoderm tissue flanking the neural tube) migrate inward and surround both the notochord and the neural tube. These cells, collectively called the sclerotome, condense into a repeating pattern that becomes the vertebrae and the outer rings of the intervertebral discs.
As the vertebral column forms, the notochord gets squeezed out of the regions where bone develops. But it persists in the spaces between vertebrae, becoming the nucleus pulposus: the gel-like center of each intervertebral disc. If you’ve ever heard of a “slipped” or herniated disc, that’s the nucleus pulposus bulging out of position. So every time you bend or twist your spine, you’re relying on what’s left of your embryonic notochord to cushion the movement.
Timeline of Notochord Development
The human notochord develops remarkably fast. Its precursor, the notochordal process, first appears around day 15 to 17 after conception as a cluster of cells migrating forward from the primitive node (a key organizing region at the back of the early embryo). Between days 17 and 23, this process transforms into the notochordal plate, a flat strip of cells integrated with the embryo’s inner lining. By days 26 to 30, the plate detaches and rolls into the definitive notochord, a solid rod running the length of the embryo. The entire sequence from first appearance to finished structure takes roughly two weeks.
Meanwhile, the neural tube is closing during roughly the same window, days 22 to 28, which is why these structures are so closely linked in both timing and location. But they remain separate tissues with separate destinies.
In Other Animals, the Notochord Plays a Bigger Role
In simpler chordates like lancelets (small, fish-like marine animals), the notochord persists throughout life as the primary structural support for the body. These animals never develop a bony spine, so the notochord remains their “backbone.” In vertebrates, the evolution of a true vertebral column made the notochord’s structural role temporary, but its signaling role became even more critical for organizing the complex nervous system that vertebrates depend on.
When Notochord Remnants Cause Problems
In most people, leftover notochord cells sit quietly inside the nucleus pulposus for a lifetime. But in rare cases, these remnants can give rise to a tumor called a chordoma. Chordomas are slow-growing but malignant, and they tend to appear at the base of the skull or in the sacrum (the triangular bone at the base of the spine), locations that correspond closely to where notochord remnants are most commonly found. Notochordal remnants are actually present in far more people than those who develop chordomas, which suggests that some additional trigger, like a genetic mutation, is needed for the cells to become cancerous. Benign notochordal cell tumors also exist and are considered potential precursors to chordoma, though most never progress.