notochord

The notochord is a crucial embryological structure that forms during the third week of human development from the notochordal process. It extends from the primitive pit to the prochordal plate and serves multiple critical functions: (1) It acts as the primary organizing center for the developing embryo, secreting signaling molecules like Sonic hedgehog (Shh) that pattern surrounding tissues; (2) It induces the overlying ectoderm to form the neural plate and subsequently the neural tube through a process called neurulation; (3) It serves as the primitive axial skeleton, providing structural support before vertebrae develop. During the sixth week of development, the notochord begins to degenerate as the vertebral bodies form around it through endochondral ossification. However, notochordal remnants persist as the gelatinous nucleus pulposus within intervertebral discs. Clinically, notochordal remnants can give rise to chordomas (rare malignant tumors) typically found at the skull base or sacrococcygeal region. The notochord is also evolutionarily significant as it defines the phylum Chordata and represents the most primitive form of an axial skeleton.