notochord

The notochord is a flexible, rod-shaped structure that runs along the dorsal midline of chordate embryos. It is composed of large vacuolated cells encased in a collagen-rich sheath and surrounded by mesodermal tissue. In most vertebrates, the notochord is transient; during development it serves as primary axial support and acts as a critical signaling center that patterns surrounding tissues. It secretes morphogens, especially Sonic hedgehog (SHH), which influences the formation of the neural tube, somites, and other mesodermal structures. As the embryo develops, the vertebral column forms around the notochord, and in many species the notochord is largely replaced by bone and cartilage; remnants persist in adulthood as the nucleus pulposus of intervertebral discs. In cephalochordates (lancelets), the notochord remains the main axial support in adulthood, whereas in tunicates (urochordates) it is present mainly in the larval stage.

The notochord is a defining feature of the phylum Chordata and serves as an essential organizer of embryonic patterning. Its presence in the embryo allows coordinated elongation and proper body plan development. Evolutionarily, the notochord provided stiffness and locomotor flexibility and contributed to the evolution of a segmented spine in vertebrates. Modern humans retain notochord-derived tissue only as part of the nucleus pulposus within intervertebral discs.