Mineralization of the vertebral bodies inAtlantic salmon (Salmo salarL.) is initiated segmentally in the form of hydroxyapatite crystal accretions in the notochord sheath

Abstract

We performed a sequential morphological and molecular biological study of the development of the vertebral bodies in Atlantic salmon (Salmo salar L.). Mineralization starts in separate bony elements which fuse to form complete segmental rings within the notochord sheath. The nucleation and growth of hydroxyapatite crystals in both the lamellar type II collagen matrix of the notochord sheath and the lamellar type I collagen matrix derived from the sclerotome, were highly similar. In both matrices the hydroxyapatite crystals nucleate and accrete on the surface of the collagen fibrils rather than inside the fibrils, a process that may be controlled by a template imposed by the collagen fibrils. Apatite crystal growth starts with the formation of small plate-like structures, about 5 nm thick, that gradually grow and aggregate to form extensive multi-branched crystal arborizations, resembling dendritic growth. The hydroxyapatite crystals are always oriented parallel to the long axis of the collagen fibrils, and the lamellar collagen matrices provide oriented support for crystal growth. We demonstrate here for the first time by means of synchroton radiation based on X-ray diffraction that the chordacentra contain hydroxyapatite. We employed quantitative real-time PCR to study the expression of key signalling molecule transcripts expressed in the cellular core of the notochord. The results indicate that the notochord not only produces and maintains the notochord sheath but also expresses factors known to regulate skeletogenesis: sonic hedgehog (shh), indian hedgehog homolog b (ihhb), parathyroid hormone 1 receptor (pth1r) and transforming growth factor beta 1 (tgfb1). In conclusion, our study provides evidence for the process of vertebral body development in teleost fishes, which is initially orchestrated by the notochord.

Loss of col8a1a function during zebrafish embryogenesis results in congenital vertebral malformations

“…In teleost fish, the formation of the axial centra (chordacentra) provides a segmented matrix for subsequent development of vertebral bodies (Grotmol et al, 2003). In that, centra are formed by the direct mineralization of the extracellular notochord sheath (Wang et al, 2013) and ultimately the elaboration of the vertebral bodies is carried out by the actions of sclerotome-derived osteoblasts on these segmented templates (Grotmol et al, 2003). We suggest that vertebral defects observed in lev/col8a1a mutants are directly attributed to the function of the notochord as a linear “vertebral body template”.…”

Transcriptome sequencing of Atlantic salmon (Salmo salar L.) notochord prior to development of the vertebrae provides clues to regulation of positional fate, chordoblast lineage and mineralisation

“…One of the most highly expressed genes within the notochord is the major collagen of hyaline cartilage, col2a1 [40]. This collagen type displays a much higher level of expression than the major type I collagen ( colI ) found in bone, further confirming the chondroblast-like nature of chordoblasts [13]. Since the notochord appeared early in chordate evolution, as a key structure characterising the clade, and anticipating by far the emergence of hyaline cartilage, which probably first appeared in jawed fish, it may be fair to say that chondroblasts are chordoblast-like, having co-opted morphogenic pathways that evolved together with the notochord in the pre-chordate lineage, early in the Cambrian (see discussion in [12]).…”

Building the backbone: the development and evolution of vertebral patterning

“…This process is similar to that occurring in elasmobranchs, in which the cartilage itself is calcified rather than being replaced by bone. Indeed, there is evidence that the teleost chordacentrum mineral is hydroxyapatite (Wang et al, 2013), matching both bone and calcified cartilage in this respect (Hall, 2005).…”