From notochord to vertebral column: studies on Atlantic salmon (Salmo salar L.)
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The notochord initially form a single-cell file stage of chordoblasts covered by a three layered sheath comprising a basal lamina, a main collagenous layer and an external elastic membrane. During early ontogeny, the collagenous layer of the sheath goes through changes in configuration from a circular orientation, with fibres oriented perpendicular to the long axis, and to a lamellar organization where alternating layers of parallel collagen fibres form helices, with a tangent angle of 75-80° to the long axis. The shift in collagen arrangement is taking place shortly after hatching and may reflect an adaptation of the notochord to functional demands related to stage of development. The serrated surfaces of the elastica externa encasing the notochord seem to provide an interconnection between the notochord and the myosepta, with a function related to transmission of force from the musculature. The subsequent segmentation process leading to formation of vertebral and intervertebral regions seems to be initiated within the notochord through formation of cellular chordoblast segments. Formation of the cellular segments is closely followed by a segmental pattern of ALP activity within the chordoblast layer and formation of mineralized chordacentra within the collagenous layer of the notochord sheath. Development of the vertebral column thus seem to involve a dual segmentation where notochord derived chordacentra define the vertebral segments and the sclerotome external to the notochord provide further growth to the vertebrae. The salmon vertebral column comprises notochord and sclerotome derived structures that tightly interconnect the vertebrae and the intervertebral regions. There are four distinct layers of bone and mineralized tissues that form the vertebral body. The two inner layers comprise the chordacentrum (mineralization of the notochord sheath) and the inner layer of the amphicoel (mineralization of the intervertebral ligament). The third layer is deposited by osteoblasts, forming the main portion of the amphicoelous centrum, and on its external surface, trabecular bone is deposited. The three inner layers have a lamellar organization, with collagen matrixes oriented perpendicular from layer to layer, while the fourth layer has a woven structure. The 6 layers also differ in osteocyte content. The notochord remains within the full length of the vertebral column and form the inner part of the intervertebral ligament. The sclerotomal external portion of the ligament is incorporated in the vertebrae as it is covered by bone during vertebral growth, while remaining unmineralized intervertebrally. The present studies on salmon shows that the notochord constitutes the main axial support during embryonic and larval stages and that the enforcing rearrangement of the collagen layer of the notochord sheath seems to be important for the properties of the notochord as a fibre-wound hydrostatic skeleton. The notochord further reinforces through formation of chordacentra, generating the segmental pattern of the vertebral column, while sclerotomal tissue external to the notochord provides further growth to the vertebrae and intervertebral structures.
Paper I: Anatomy and Embryology 207, Grotmol, S.; Kryvi, K.; Nordvik, K.; Totland, G. K.; Notochord segmentation may lay down the pathway for the development of the vertebral bodies in the Atlantic salmon, pp. 263-272. Copyright 2003 Springer-Verlag. Full-text not available in Bora due to publisher restrictions. <a href”http://dx.doi.org/10.1007/s00429-003-0349-y”target=_”blank”> http://dx.doi.org/10.1007/s00429-003-0349-yPaper II: Journal of Anatomy 206, Grotmol, S.; Nordvik, K.; Kryvi, H.; Totland, G. K.; A segmental pattern of alkaline phosphatase activity within the notochord coincides with the initial formation of the vertebral bodies, pp. 427-436. Copyright 2005 Blackwell Publishing. Full-text not available in Bora due to publisher restrictions. <a href”http://dx.doi.org/10.1111/j.1469-7580.2005.00408.x.”target=_”blank”> http://dx.doi.org/10.1111/j.1469-7580.2005.00408.xPaper III: Journal of Anatomy 206, Nordvik, K.; Kryvi, H.; Totland, G. K.; Grotmol, S.; The salmon vertebral body develops through mineralization of two preformed tissues that are encompassed by two layers of bone, pp. 103-114. Copyright 2005 Blackwell Publishing. Abstract only. Full-text not available in Bora due to publisher restrictions. <a href”http://dx.doi.org/10.1007/s00429-003-0349-y”target=_”blank”> http://dx.doi.org/10.1007/s00429-003-0349-yPaper IV: Journal of Anatomy 209, Grotmol, S.; Kryvi, K.; Keynes, R.; Krossøy, C.; Nordvik, K.; Totland, G. K.; Stepwise enforcement of the notochord and its intersection with the myoseptum: an evolutionary path leading to development of the vertebra?, pp. 339-357. Copyright 2006 Blackwell Publishing. Full-text not available in Bora due to publisher restrictions. <a href”http://dx.doi.org/10.1111/j.1469-7580.2006.00618.x”target=_”blank”> http://dx.doi.org/ 10.1111/j.1469-7580.2006.00618.x