Notochord
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| Notochord | |
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| Transverse section of a chick embryo of forty-five hours’ incubation. | |
| Gray's | subject #8 52 |
| MeSH | 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 defining 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 Pikaia and Haikouicthys had only a notochord. This is the reason the embryos of vertebrates have them today; embryonic evolution often happens to follow a pattern similar to the ancestral development 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.
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[edit] Development
Notogenesis is the development of the notochord by the epiblasts that make up the floor of the amnion cavity (Human Embryology).
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. This thickening assumes the appearance of a furrow (the chordal furrow) the margins of which come into contact, and so convert it into a solid rod of polygonal-shaped cells (the notochord) which is then separated from the endoderm.
In higher vertebrates, 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.
[edit] Research
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 the protein sonic hedgehog (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.
[edit] Additional images
 Surface view of embryo of Hylobates concolor. |
 Diagram of a transverse section, showing the mode of formation of the amnion in the chick. |
 Transverse section of human embryo eight and a half to nine weeks old. |
[edit] Reference
- ↑ J. P. G. Urban , S. Roberts , and J. R. Ralphs. The Nucleus of the Intervertebral Disc from Development to Degeneration. Amer Zool. 2000;40(1):53-61. full text
- ↑ Christopherson LR, Rabin BM, Hallam DK, Russell EJ. Persistence of the notochordal canal: MR and plain film appearance. AJNR Am J Neuroradiol. 1999 Jan;20(1):33-6. PMID 9974055
- ↑ Echelard Y, Epstein DJ, St-Jacques B, Shen L, Mohler J, McMahon JA, McMahon AP. Sonic hedgehog, a member of a family of putative signaling molecules, is implicated in the regulation of CNS polarity. Cell 1993;75(7):1417-30. PMID 7916661
