| Summary: | 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 ...
|
|---|