notochords

The notochord is a flexible rod that supports the body of a chordate animal. It is a characteristic feature of the phylum Chordata, which includes vertebrates, tunicates, and lancelets. The notochord is a mesodermal derivative, meaning it arises from the middle germ layer of the embryo. In many vertebrates, the notochord is a temporary structure that plays a crucial role during embryonic development. It provides skeletal support and acts as an organizer, influencing the development of surrounding tissues, particularly the nervous system.

During the development of vertebrates, the notochord typically serves as a scaffold around which the vertebral column forms. The vertebrae eventually replace the notochord, although remnants of the notochord can persist in the adult as the nucleus pulposus of the intervertebral discs. In some primitive chordates, such as lancelets, the notochord is retained throughout life, providing persistent axial support. In contrast, in tunicates, the notochord is present only in the larval stage and is lost upon metamorphosis. The presence and fate of the notochord are key diagnostic features used in classifying chordate animals. Its signaling functions are essential for neurulation, the process by which the neural tube, the precursor to the central nervous system, forms.