Stepwise enforcement of the notochord and its intersection with the myoseptum: an evolutionary path leading to development of the vertebra?

• Published in: Journal of Anatomy
• Main Authors: Grotmol, Sindre, Kryvi, Harald, Keynes, Roger, Krossøy, Christel, Nordvik, Kari, Totland, Geir K
• Format: Text
• Language: English
• Published: Blackwell Science Inc 2006
• Subjects: Original Articles; Atlantic salmon
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2100326; http://www.ncbi.nlm.nih.gov/pubmed/16928203; https://doi.org/10.1111/j.1469-7580.2006.00618.x

The notochord constitutes the main axial support during the embryonic and larval stages, and the arrangement of collagen fibrils within the notochord sheath is assumed to play a decisive role in determining its functional properties as a fibre-wound hydrostatic skeleton. We have found that during early ontogeny in Atlantic salmon stepwise changes occur in the configuration of the collagen fibre-winding of the notochord sheath. The sheath consists of a basal lamina, a layer of type II collagen, and an elastica externa that delimits the notochord; and these constituents are secreted in a specific order. Initially, the collagen fibrils are circumferentially arranged perpendicular to the longitudinal axis, and this specific spatial fibril configuration is maintained until hatching when the collagen becomes reorganized into distinct layers or lamellae. Within each lamella, fibrils are parallel to each other, forming helices around the longitudinal axis of the notochord, with a tangent angle of 75–80° to the cranio-caudal axis. The helical geometry shifts between adjacent lamellae, forming enantiomorphous left- and right-handed coils, respectively, thus enforcing the sheath. The observed changes in the fibre-winding configuration may reflect adaptation of the notochord to functional demands related to stage in ontogeny. When the vertebral bodies initially form as chordacentra, the collagen lamellae of the sheath in the vertebral region are fixed by the deposition of minerals; in the intervertebral region, however, they represent a pre-adaptation providing torsional stability to the intervertebral joint. Hence, these modifications of the sheath transform the notochord per se into a functional vertebral column. The elastica externa, encasing the notochord, has serrated surfaces, connected inward to the type II collagen of the sheath, and outward to type I collagen of the mesenchymal connective tissue surrounding the notochord. In a similar manner, the collagen matrix of the neural and haemal arch cartilages is tightly ...