Everything about the Notochord totally explained
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's 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 [thisidea that '
ontogeny recapitulates
phylogeny' is known as
Recapitulation theory]. 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.
Additional images
Image:Gray14.png|Surface view of embryo of Concolor gibbon (Hylobates concolor).
Image:Gray29.png|Diagram of a transverse section, showing the mode of formation of the amnion in the chick.
Image:Gray898.png|Section through the head of a human embryo, about twelve days old, in the region of the hind-brain.
Image:Gray1111.png|Transverse section of human embryo eight and a half to nine weeks old.
Further Information
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