Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes the vertebral body of the vertebral column. The notochord is found ventral to the neural tube.

Notochords were the first "backbones" serving as support structures in chordates such as Haikouichthys. Notochords were advantageous to primitive fish-ancestors because they were a rigid structure for muscle attachment, yet flexible enough to allow more movement than, for example, the exoskeleton of the dominant animals of that time. Embryos of vertebrates have notochords today, as embryonic development often follows a pattern similar to the ancestral evolution of the modern animal's traits. In tetrapods, they eventually develop into the nucleus pulposus of the intervertebral discs.

Development of the notochord
Notogenesis is the development of the notochord by the epiblasts that make up the floor of the amnion cavity (Human Embryology). The notochord arises from the bilaminar embryonic disk.

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 bi-pedal 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.

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 sellæ of the sphenoid bone. Initially it exists between the neural tube and the endoderm of the yolk-sac, but soon 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.

Postembryonic vestige of the notochord is found in the nucleus pulposus of the intervertebral disks, but not in the vertebral bodies, from which notochordal cells usually regress entirely. 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. They are also found to persist in the nasopharyngeal space and, in such an unusual instance, may give rise to Tornwaldt's cyst.

The notochord in neural development
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 motoneurons in the dorsal tube. Motoneuron formation generally occurs in the ventral neural tube, while the dorsal tube generally forms sensory cells.

The notochord secretes a protein called sonic hedgehog homolog (SHH), a key morphogen regulating organogenesis and having a critical role in signaling the development of motoneurons. The secretion of SHH by the notochord establishes the ventral pole of the dorsal-ventral axis in the developing embryo.

Evolution of the notochord
The notochord is the defining feature of Chordates, and was present throughout life in many of the earliest chordates. Although the stomatochord 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 agnathan fish which possessed notochords, either which 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 (see list below) have reverted to the primitive state, retaining the notochord into adulthood, though the reasons for this are not well-understood.

The structure of the notochord
The notochord is composed primarily of a core of glycoproteins, encased in a sheath of collagen fibers wound into two opposing helices. The angle between these fibers determines whether increased pressure in the core will result in shortening and thickening versus lengthening and thinning.

Organisms which retain a post-embryonic notochord

 * Amphioxus
 * Tunicate larvae
 * Hagfish
 * Lamprey
 * Sturgeon
 * African lungfish
 * Tadpoles
 * Ostracoderms (extinct)