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"All in all, we are coming closer to understanding how a fused neck develops normally or under stressful conditions (as is the case for farmed salmon) in a range of vertebrates at the base of our ancestry," says Catherine Boisvert. (Credit: iStockphoto)

bones

Can shark eggs clarify how human necks develop?

A new look at the fused neck vertebrae of elephant sharks could help researchers better understand how neck development can go awry in humans.

For a study in PLOS ONE, researchers investigated how the fused neck develops in elephant sharks, which is also known as the Australian ghostshark.

In people with the disease known as Klippel-Feil syndrome, the vertebrae of the neck becomes fused, but in living sharks and rays, and in some fossil armored fish called placoderms, having a neck encased in bone is normal.

“In some animal species, part of the animal’s body mimics what we see in a human disease. These species are known as ‘evolutionary mutants,’ and analyzing them provides unprecedented access to information in a healthy individual,” says lead researcher Catherine Boisvert of Monash University’s Australian Regenerative Medicine Institute.

“We are gaining a better understanding on how these morphologies develop and what developmental pathways (genes and their networks) are involved in producing them. This knowledge may help us better understand the disease in humans.”

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An elephant shark. (Credit: Alpha)

The researchers grew elephant sharks collected from eggs laid in captivity on the Mornington Peninsula, Victoria. They stained them to study the fused neck, called the synarcual, and reveal cartilage and muscle development. They also analyzed the fossils of placoderms, which offered data about how this fusion occurred for comparison to living animals.

Using microscopic imaging at the Australian Synchrotron, the researchers found that neck vertebrae in the elephant shark and placoderm developed normally, and only later became fused after emerging from their egg. Skates and rays appeared to show a similar pattern, suggesting this may be a normal condition for vertebrate animals in general.

This contrasts the belief that a fused neck forms because individual vertebrae fail to form in early development.

[Elephant shark genome offers a few surprises]

Boisvert says the next step is to look further into the genes that are responsible for this fusion in the shark species and apply them to diseases of the human spine.

The way the synarcual develops in placoderms and sharks is most similar to human disease fibrodysplasia ossificans progressiva, which slowly turns soft tissues to bone—patients are born with a normal skeleton, with fusion occurring subsequently.

“Sharks don’t have true bone—instead they have a hard kind of cartilage called prismatic calcified cartilage—and we don’t fully understand yet if the vertebra fusion is due to overdevelopment of cartilage, or if the soft tissue between the vertebrae becomes transformed into cartilage, resulting in fusion,” says Boisvert.

“These sorts of ‘metaplastic’ transformations of the spine have been observed in farmed salmon, and exciting new research is beginning to unravel the genes involved in these transformations. Our goal is to do the same for elephant sharks, rays, and skates.

“All in all, we are coming closer to understanding how a fused neck develops normally or under stressful conditions (as is the case for farmed salmon) in a range of vertebrates at the base of our ancestry.”

Source: Monash University

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