Notochord

In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consists of inner, vacuolated cells covered by fibrous and elastic sheaths, lies along the anteroposterior axis (front to back), is usually closer to the dorsal than the ventral surface of the embryo, and is composed of cells derived from the mesoderm. The most commonly cited functions of the notochord are: as a midline staple that provides directional reference to surrounding tissue during embryonic development, as an axial endoskeleton (structural element) and vertebral precursor, and as an elastic spring that allows more efficient tail motion when swimming. In lancelets, the notochord persists throughout life as the main structural support of the body. In tunicates the notochord is present only in the larval stage, being completely absent in the adult animal. In these invertebrate chordates, the notochord is not vacuolated. In all vertebrates other than the hagfish, the notochord is integrated into the vertebral column, with its original structure being (nearly) retained in the intervertebral discs as the nucleus pulposus. The notochord is a long, rod-like midline structure that develops dorsal to the gut tube and ventral to the neural tube. The notochord is composed primarily of a core of glycoproteins encased in a sheath of collagen fibers wound into two opposing helices. The glycoproteins are stored in vacuolated, turgid cells, which are covered with caveolae on their cell surface. The angle between these fibers determines whether increased pressure in the core will result in shortening and thickening versus lengthening and thinning. Alternating contraction of muscle fibers attached to each side of the notochord result in a side-to-side motion resembling stern sculling, which allows tail swimming and undulation. The stiffened notochord prevents movement through telescoping motion such as that of an earthworm.