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. 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 nucleus pulposus of the intervertebral discs.
In the adult human, a notochord remnant is the apical ligament of the atlanto-axial joint.
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 as a pouch from the mesoderm.
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.