Notochord
| 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 | |
Notochord is a flexible, rod-shaped supporting structure that is one of the distinguishing features of the phylum Chordatas, being found at some point in the life cycle of all chordates (vertebrates, tunicates, lancelets). Composed of cells derived from the mesoderm, the notochord defines the primitive axis of the chordate embryo and is retained as the main support in the lowest chordates. In higher vertebrates, it is replaced by the vertebral column.
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.
It is 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.
Overview
The notochord is one of the defining characteristics of the phylum Chordata, along with the presence of a hollow dorsal nerve cord and pharyneal slits. Lancelets (suphylum Cephalochordata) retain the notochord throughout their lives and it extends into the head region (unlike the vertebrate spine) in both the young and the adults. Tunicates (subphylum subphylum Urochordata) have a notochord only as larvae, not as adults, and it does not extend into the head. In vertebrates (subphylum Vertebrata) the notochord becomes surrounded by vertebrae and in higher vertebrates is found only in the embryhonic stage, not as adults, as it is replaced by the vertebrae. Members of the vertebrate class Agnatha (jawless fish), the hagfish and lampreys, have a notochord that remains throughout life. This notochord is the first primitive vertebral column. In the higher vertebrates (Gnathostomata (jawed vertebrates), the notochord is only present in the embryonic stage and is completely replaced by the vertebrae.
Hagfish, which have the presence of a notochord in both larvae and adults, are generally classifed in Aganatah (jawless fish) in the subphylum Vertebratae despite the lack of vertebrae
Unlike vertebrates, cephalochordates and tunicates lack a backbone or vertebral column.
In the adult human, a notochord remnant is the nucleus pulposus of the vertebral disks.
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.
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[1]. The secretion of SHH by the notochord establishes the ventral pole of the dorsal-ventral axis in the developing embryo.
Additional images
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.
Neural development/Neurulation - Neurula - Neural folds - Neural groove - Neural tube - Neural crest - Neuromere (Rhombomere) - Notochord - Neural plate
Eye development - Optic vesicles - Optic stalk - Optic cup - Auditory vesicle - Auditory pit
ReferencesISBN links support NWE through referral fees
- ↑ 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
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
