Got your back – the origins of the human spine in development

The spinal column, also called the backbone, spine, and vertebral column, is the central supporting structure of the skeleton in all vertebrates. Made of 33 vertebrae in humans, the spine provides a place for muscles to attach while also protecting the spinal cord and nerve roots.

Now, for the first time, molecular biologists have created a 3D model in vitro (outside of a living organism) that mimics how the precursor structures that give rise to the spinal column form during human embryonic development, a process called somitogenesis. Their new study has been published in Nature Communications.

Human vertebrae form from pairs of precursor structures called somites – repeated segments of cells which also give rise to our ribs and skeletal muscles – and their development is tightly regulated to ensure that these structures are formed correctly.

Each pair of somites forms at a particular sequential time point in embryonic development; the process starts around day 20 after fertilisation.

Called somitogenesis, this process is controlled by the segmentation clock – a group of genes that creates molecular oscillatory waves that peak in activity every 5–6 hours in humans. Every wave gives rise to a new pair of somites; a total of about 40 pairs are formed.

“For the first time, we have been able to create periodic pairs of human mature somites linked to the segmentation clock in the lab,” says first author Marina Sanaki-Matsumiya, a postdoctoral researcher at the European Molecular Biology Laboratory (EMBL) in Barcelona, Spain.

The team cultured human induced pluripotent stem cells (hiPSC) – which have the ability to differentiate into any cell type of the human body – in the presence of a cocktail of signalling molecules that induce cell differentiation.

Three days later, as the cells started to elongate and create anterior (top) and posterior (bottom) axes, they added Matrigel into the mix. Matrigel contains a combination of proteins that are critical to many developmental processes.

This eventually led to the formation of somitoids – in vitro equivalents of human somite precursor structures.

To test whether the segmentation clock regulates somitogenesis in these somitoids, the researchers monitored the expression patterns of HES7, the core gene involved in the process. They found clear evidence of the gene being expressed in an oscillatory cycle, especially when somitogenesis was about to start.

The study also shows a link between the size of somites and the segmentation clock.

“The somites that were generated had a constant size, independently of the number of cells used for the initial somitoid,” explains Sanaki-Matsumiya. “The somite size did not increase even if the initial cell number did.

“This suggests that the somites have a preferred species-specific size, which might be determined by local cell-cell interactions, the segmentation clock, or other mechanisms,” she says.

To study this further, the researchers are now planning to grow somitoids of different species and compare them.

“Our next project will focus on creating somitoids from different species, measure their cell proliferation and cell migration speed to establish what and how somitogenesis is different among species,” says senior author Miki Ebisuya, a synthetic developmental biologist at EMBL.