Vertebral compression involves alterations of the growth plates, and is but one component of a range of changes with complex aetiology that result in spinal deformity.

A striking feature of compressed salmon vertebrae is the coincidence of metaplastic synchondrosis and the shape alterations of the vertebral end plates, raising the possibility that either bone alterations induce the conversion of the notochord tissue or that changes of the notochord induce bone alterations. The current state of knowledge suggests the latter, since the notochord plays an important role in the induction of vertebral formation and in maintaining vertebral morphogenesis and growth.

Unlike other vertebrate species with limited growth (e.g. human), in salmon, the notochord should fulfil its regulating role for vertebral body differentiation throughout life, since salmon and other fish species do not stop growing.

Some authors propose that an altered mechanical load could have caused the transformation of the bone growth zones in the ‘short tails’ and the concomitant replacement of the intervertebral (notochord) tissue by cartilaginous tissues. This hypothesis is supported by the role that notochord cells are known to play in vertebral development and for maintaining the integrity of the intervertebral disk.

It is not clear if intervertebral tissue damage under farming conditions can be caused by excessive short-term pressure, excessive mechanical stress in the long term, or even disuse.

Histologically, in the final stages of vertebral fusion, the vertebral endplates and the cartilage of the intervertebral space are replaced by intravertebral bone spongiosa. Vertebral compressions could related to the notochord tissue. A metaplasia (notochordal tissue converting into cartilage), possibly as a result of an altered mechanical load regime, leads to alteration of vertebral shape. Transformation of notochordal tissue into cartilage has been reported only in old fish.

REFERENCES

• Hall, B. K. (2005). Bones and cartilage: developmental and evolutionary skeletal biology. Elsevier.
• Vagsholm, I., & Djupvik, H. O. (1998). Risk factors for spinal deformities in Atlantic salmon, Salmo salar L. Oceanographic Literature Review, 7(45), 1235.
• Witten, P. E., Gil-Martens, L., Hall, B. K., Huysseune, A., & Obach, A. (2005). Compressed vertebrae in Atlantic salmon Salmo salar: evidence for metaplastic chondrogenesis as a skeletogenic response late in ontogeny. Diseases of aquatic organisms, 64(3), 237-246.