Our spinal discs evolved from worm muscle, new study shows

Suddenly, the term "worming your way out" doesn't sound so spineless anymore.

 

A new study has shown that our spinal discs may have evolved from the muscles of an ancient marine worm, the European Molecular Biology Laboratory said.

 

"We identify a population of mesodermal cells in a developing invertebrate, the marine annelid Platynereis dumerilii, that converges and extends toward the midline and expresses a notochord-specific combination of genes. These cells differentiate into a longitudinal muscle, the axochord, that is positioned between central nervous system and axial blood vessel and secretes a strong collagenous extracellular matrix. Ancestral state reconstruction suggests that contractile mesodermal midline cells existed in bilaterian ancestors. We propose that these cells, via vacuolization and stiffening, gave rise to the chordate notochord," the researchers said in an abstract.

 

They noted the emergence of the notochord is one key issue in the debate on the origin of chordates for more than a century.

 

In vertebrates, the notochord develops by convergence and extension of the chordamesoderm, midline cells of unique molecular identity.

 

Citing the study, EMBL said the notochord - the first vertebrate skeleton - probably evolved from muscle.

 

It added the marine worm P. dumerilii has muscle with the same genetic signature in the same place.

 

The EMBL noted humans are part of a group of animals called chordates, which have a rod of cartilage that runs lengthwise along the middle of their body, under their spinal chord.

 

The structure - the notochord - was the first vertebrate skeleton and is present in human embryos, and is replaced with the backbone as we develop.

 

“People simply haven’t been looking beyond our direct relatives, but that means you could be fooled, if the structure appeared earlier and that single group lost it. And in fact, when we looked at a broader range of animals, this is what we found,” says Detlev Arendt from EMBL, who led the study.

 

The researchers said muscle may have shifted to cartilage because a stiffened central rod would make swimming more efficient.

 

Genetic signature

 

Antonella Lauri and Thibaut Brunet in Arendt’s lab identified the notochord's genetic signature.

 

They found that the larva of P. dumerilii has a group of cells with that same genetic signature. They then worked with Philipp Keller’s group at Janelia Farm to use microscopy to follow those cells as the larva developed.

 

"They found that the cells form a muscle that runs along the animal’s midline, precisely where the notochord would be if the worm were a chordate. The researchers named this muscle the axochord, as it runs along the animal’s axis. A combination of experimental work and combing through the scientific literature revealed that most of the animal groups that sit between Platynereis and chordates on the evolutionary tree also have a similar, muscle-based structure in the same position," EMBL said. — Joel Locsin/TJD, GMA News