Compression and Fusion of Vertebral Bodies

In salmonids the most common types of anomalies observed in the vertebral bodies are fusion (ankylosis) and compression, which can progress to the adjacent vertebral bodies, severely affecting the movement of the fish and making a reduction in fish length.

There is an ongoing discussion about the causes for vertebral body compression and fusion. Hoever, compression-related tissue alterations are foremost alterations of the tissue in the intervertebral space rather than damages to the vertebral bodies themselves. Some authors propose that altered mechanical load (in the intervertebral spaces) could cause the transformation of intervertebral tissues (notochord and notochord sheath) into cartilaginous tissues (metaplastic chondrogenesis). This leads to compression and subsequent fusion of the vertebral bodies. It is not clear whether the damage of the intervertebral tissue under productive conditions is caused by short-term mechanical overload, by excess long-term mechanical load or even disuse,

Fusion of vertebral bodies can progress according to two different scenarios, which we will refer to as “aggravation” and “containment“. In the aggravation scenario the initial fusion of two vertebral bodies leads to a vertebral compression center through the continuous amalgam of additional vertebrae. This process of fusion of the vertebrae causes shortening of the spine and has obvious implications for the functioning of the animal. This type of malformation is well known for salmon and other species of fish. Furthermore, fish are able to contain the progress of vertebral body fusion by reshaping the fused vertebrae into an undeformed vertebra (i.e. the second stage called containment).

Compression and / or fusion of vertebral bodies are divided in:

One-sided compressed vertebral bodies.

Although no sign of vertebral fusion is yet visible, it has been observed that could represent an early stage of vertebral fusion.

Compression and fusion (ankylosis)

With possible mineralisation of intervertebral space (radiodense vertical lines in the intervertebral space). It is possible to observe inward bending of the edges of vertebral end plates, flattening of anterior or posterior vertebral end plates and compression of vertebral bodies.

O. kisutch, Adulto (2,8 k), agua de mar.
O. kisutch, Adult (2.8 k), seawater. Note compression and fusion of vertebral bodies. Latero-lateral view.

Complete fusion (ankylosis) of two (or three) vertebral bodies

Followed by remodeling of the fused vertebrae into one, regularly shaped vertebra, albeit with supernumerary neural and haemal arches.

O. kisutch, Alevín (4g), agua dulce.
O. kisutch, Fry (4g), freshwater. Note complete fusion of vertebral bodies. Lateral-lateral view.

Vertebral body fusion centre (ankylosis)

Here the fusion centre consists of five vertebral bodies with adjacent unilateral flattened vertebrae.

O. kisutch, Adulto (3 k), agua de mar.
O. kisutch, Adult (3 k), seawater. Note central fusion of vertebral bodies. Latero-lateral view.
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REFERENCES

Davie, P.S., Walker, S.P., Perrott, M.R., Symonds, J.E., Preece, M., Lovett, B.A., Munday, J.S. 2018. Vertebral fusions in farmed Chinook salmon (Oncorhynchus tshawytscha) in New Zealand. Fish Dis. 42:965–974.

Witten, P.E., Martens, L.G., Huysseune, A., Takle, H., Hjelde, K. 2009. Towards a classification and an understanding of developmental relationships of vertebral body malformations in Atlantic salmon (Salmo salar L.). Aquaculture 295: 6–14.

Witten, P. E., Obach, A., Huysseune, A., Baeverfjord, G. 2006. Vertebrae fusion in Atlantic salmon (Salmo salar): Development, aggravation and pathways of containment. Aquaculture, 258, 164–172. 

Kvellestad, A., Høie, S., Thorud, K., Tørud, B., Lyngøy, A., 2000. Platyspondyly and shortness of vertebral column in farmed Atlantic salmon Salmo salar in Norway— Description and interpretation of pathological changes. Dis. Aquat. Org. 39, 97–108.

Lewis-McCrea LM, Lall SP. 2004. Development of the axial skeleton and skeletal abnormalities of Atlantic halibut (Hippoglossus hippoglossus) from first feeding through metamorphosis. Aquaculture 257: 124–13.

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