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.
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 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.
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