Fusion of vertebral bodies characterises many types of spine deformities in farmed Atlantic salmon and other bony fish. Little is known about development and progress of this condition in individual fish, knowledge that is required for prevention and control of spine malformations. To clarify the pathogenesis of vertebrae fusion, we describe the development and progress of the disease in farmed Atlantic salmon based on (a) tracing vertebrae fusion in individual fish at three different stages, (b) analyzing vertebrae fusion in animals 12 months after seawater transfer, and (c) histological examination of the fusion process. Vertebrae fusion was observed to develop both prior to and after smoltification; early and late fusion stages were detected in pre-smolts and in animals 12 months after seawater transfer. The process involves transformation of intervertebral notochord tissue into cartilage, shape alterations of vertebral body endplates, mineralisation of the intervertebral cartilage, and finally replacement of intervertebral cartilage by bone. Two fused vertebrae can develop into a centre of severe malformation through the continuous amalgamation of neighbouring vertebrae. Alternatively, animals have the capacity to contain the problem through reshaping and remodelling of two fused vertebral bodies into a single, regularly structured and jointed vertebra. Successfully reshaped vertebrae apparently do not inflict further spine malformations. We here demonstrate for the first time that the onset of vertebrae fusion must not inevitably lead to fish with deformed vertebral columns. Defining conditions that favour repair and prevent the spread of vertebrae fusion is a future task that could make a significant contribution to the control of spine deformities in farmed salmon.
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