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Notochord
Transverse section of a chick embryo of forty-five hours’ incubation.
Gray's	subject #8 52
Precursor	chordamesoderm
Gives rise to	nucleus pulposus
MeSH	Notochord
Dorlands/Elsevier	n_10/12579301

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 lower vertebrates, it persists throughout life as the main axial support of the body, while in higher vertebrates it is replaced by the vertebral column. The notochord is found on the ventral surface of the neural tube.

Notochords were the first "backbones", as well, serving as support structures in chordates that lacked a bony skeleton. The very first vertebrates, such as Haikouicthys, had only a notochord. Embryos of vertebrates have notochords today, as embryonic development often happens to follow a pattern similar to the ancestral evolution of the modern animal's traits [this idea that 'ontogeny recapitulates phylogeny' is something of an old-fashioned concept in embryology]. 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. In humans, they eventually develop into the disks between the vertebrae.

Notogenesis is the development of the notochord by the epiblasts that make up the floor of the amnion cavity (Human Embryology). The notochord arises as a pouch from the mesoderm.

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^[1]. The secretion of SHH by the notochord establishes the ventral pole of the dorsal-ventral axis in the developing embryo.

Surface view of embryo of Concolor gibbon (Hylobates concolor).

Diagram of a transverse section, showing the mode of formation of the amnion in the chick.

Section through the head of a human embryo, about twelve days old, in the region of the hind-brain.

Transverse section of human embryo eight and a half to nine weeks old.