Notochord
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 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.
Structure
The notochord is a long, rodlike structure that develops dorsal to the gut 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. The angle between these fibers determines whether increased pressure in the core will result in shortening and thickening versus lengthening and thinning.Contraction of these muscle fibers result in a side-to-side motion resembling swimming. The stiffened notochord prevents movement through telescoping motion, such as that of an earthworm.
Role in signaling & development
The notochord plays a key role in signaling and coordinating development. Embryos of modern vertebrates form transient notochord structures during gastrulation. The notochord is found ventral to the neural tube.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, synchronizing the development of the neural tube. On the ventral aspect of the neural groove an axial thickening of the endoderm takes place.. This thickening appears as a furrow the margins of which anastomose, and so convert it into a solid rod of polygonal-shaped cells which is then separated from the endoderm.
In vertebrates, it extends throughout the entire length of the future vertebral column, and reaches as far as the anterior end of the midbrain, where it ends in a hook-like extremity in the region of the future dorsum sellae of the sphenoid bone. Initially, it exists between the neural tube and the endoderm of the yolk-sac; soon, the notochord becomes separated from them by the mesoderm, which grows medially and surrounds it. From the mesoderm surrounding the neural tube and notochord, the skull, vertebral column, and the membranes of the brain and medulla spinalis are developed.
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.
In amphibians and fish
During development of amphibians and fish, the notochord induces development of the hypochord through secretion of vascular endothelial growth factor. The hypochord is a transient structure ventral to the notochord, and is primarily responsible for correct development of the dorsal aorta.In humans
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 or Tornwaldt cyst may form. The cells are the likely precursors to a rare cancer called chordoma.Neurology
Research into the notochord has played a key role in understanding the development of the central nervous system. By transplanting and expressing a second notochord near the dorsal neural tube, 180 degrees opposite of the normal notochord location, one can induce the formation of motor neurons in the dorsal tube. Motor neuron formation generally occurs in the ventral neural tube, while the dorsal tube generally forms sensory cells.The notochord secretes a protein called sonic hedgehog, a key morphogen regulating organogenesis and having a critical role in signaling the development of motor neurons. The secretion of SHH by the notochord establishes the ventral pole of the dorsal-ventral axis in the developing embryo.
Evolution in chordates
The notochord is the defining feature 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.The Ordovician oceans included many diverse species of Agnatha and early Gnathostomata which possessed notochords, either with attached bony elements or without, most notably the conodonts, placoderms, and ostracoderms. Even after the evolution of the vertebral column in chondrichthyes and osteichthyes, these taxa remained common and are well represented in the fossils record. Several species have reverted to the primitive state, retaining the notochord into adulthood, though the reasons for this are not well understood.
Scenarios for the evolutionary origin of the notochord were comprehensively reviewed by Annona, Holland, and D'Aniello. 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. Deciding between these two scenarios should be facilitated by much more thorough studies of gene regulatory networks in a wide spectrum of animals.
Post-embryonic retention
In most vertebrates, the notochord develops into secondary structures. In other chordates, the notochord is retained as an essential anatomical structure.The following organisms retain a post-embryonic notochord:
- Acipenseriformes
- Lancelet
- Tunicate
- Hagfish
- Lamprey
- Coelacanth
- African lungfish
- Tadpoles
- Ostracoderms
Within ''Amphioxus''