In anatomy, the notochord is a flexible rod formed of a material similar to cartilage. If a species has a notochord at any stage of its life cycle, it is, by definition, a chordate. In vertebrates, the notochord develops into the vertebral column. The notochord lies along the anteroposterior ("head to tail") axis, is usually closer to the dorsal than the ventral surface of the animal, and is composed of cells derived from the mesoderm.
The most commonly cited functions of the notochord are: as a site of muscle attachment, as a vertebral precursor, and as a midline tissue that provides directional signals to surrounding tissue during development. Notochords are thought to be advantageous through providing a rigid yet flexible structure for muscle attachment. In some chordates, it persists throughout life as the main structural support of the body, while in most vertebrates it develops into the nucleus pulposus of the intervertebral disc.
Notogenesis is the development of the notochord by epiblasts that form the floor of the amnion cavity. The progenitor notochord is derived from cells migrating from the primitive node and pit. The notochord forms during gastrulation and soon after induces the formation of the neural plate (neurulation), synchronizing the development of the neural tube. On the ventral aspect of the neural groove an axial thickening of the endoderm takes place. (In bipedal chordates, e.g. humans, this surface is properly referred to as the anterior surface). This thickening appears as a furrow (the chordal furrow) the margins of which anastomose (come into contact), and so convert it into a solid rod of polygonal-shaped cells (the notochord) which is then separated from the endoderm.
A postembryonic vestige of the notochord is found in the nucleus pulposus of the intervertebral discs. Isolated notochordal remnants may escape their lineage-specific destination in the nucleus pulposus and instead attach to the outer surfaces of the vertebral bodies, from which notochordal cells largely regress.
By the age of 4, all notochord residue is replaced by a population of chondrocyte-like cells of unclear origin. Persistence of notochordal cells within the vertebra may cause a pathologic condition: persistent notochordal canal. If the notochord and the nasopharynx do not separate properly duning embryonic development, a depression (Tornwaldt bursa) or Tornwaldt cyst may form. The cells are the likely precursors to a rare cancer called chordoma.
The notochord is the defining feature (synapomorphy) of chordates, and was present throughout life in many of the earliest chordates. Although the stomochord of hemichordates was once thought to be homologous, it is now viewed as a convergence. Pikaia appears to have a proto-notochord, and notochords are present in several basal chordates such as Haikouella, Haikouichthys, and Myllokunmingia, all from the Cambrian.
Scenarios for the evolutionary origin of the notochord were comprehensively reviewed by Annona, Holland, and D'Aniello (2015). They point out that, although many of these ideas have not been well supported by advances in molecular phylogenetics and developmental genetics, two of them have actually been revived under the stimulus of modern molecular approaches (the first proposes that the notochord evolved de novo in chordates, and the second derives it from a homologous structure, the axochord, that was present in annelid-like ancestors of the chordates). Deciding between these two scenarios (or possibly another yet to be proposed) should be facilitated by much more thorough studies of gene regulatory networks in a wide spectrum of animals.