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The notochord is a long, rod-like midline structure that develops dorsal to the gut tube and ventral to the neural tube. The notochord is composed primarily of a glycoproteins core that is encased in a sheath of collagen fibers. This is wound into two opposing helices.
The outer sheath resists changes in fluid pressure in the notochord, allowing for flexibility while simultaneously withstanding contraction. In this way the notochord can be characterized as a hydrostatic organ. [1] During muscle contraction, the notochord also plays an important role in withstanding shortening of the body. [10]
A tunicate is an exclusively marine invertebrate animal, a member of the subphylum Tunicata (/ ˌ tj uː n ɪ ˈ k eɪ t ə / TEW-nih-KAY-tə).This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates).
These cell types are specified by the secretion of the Shh from the notochord (located ventrally to the neural tube), and later from the floor plate cells. [6] Shh acts as a morphogen, meaning that it acts in a concentration-dependent manner to specify cell types as it moves further from its source. [7]
The upper tube is formed from the vertebral arches, but also includes additional cartilaginous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification. [26]
The dorsal nerve cord is an anatomical feature found in chordate animals, mainly in the subphyla Vertebrata and Cephalochordata, as well as in some hemichordates.It is one of the five embryonic features unique to all chordates, the other four being a notochord, a post-anal tail, an endostyle, and pharyngeal slits.
HNF-3 is needed for development of the notochord and node. [14] The gene Apolipoprotein B, which is involved in transporting and metabolizing fat soluble molecules in the blood, is expressed in the yolk sac and fetal liver. [6] Within the neurula in Xenopus laevis, development genes Xwnt-3 and Xwnt-4 are present. [15]
However, they are now recognized to not share histological composition to that of the notochord found in chordates, lacking the fibrous sheath characteristic of a notochord. [ 1 ] In addition, gene expression studies have failed to provide any evidence for homology between the notochord and the stomochord, or between the notochord and any ...