A research team from the European Molecular Biology Laboratory at the University of Barcelona has demonstrated for the first time how the cellular structures that give rise to the human spine form sequentially.
Why is the human spine important?
The human spine is the central supporting structure of the skeleton in all vertebrates. Not only does it provide a place for muscles to attach, but it also protects the spinal cord and nerve roots. Thus, defects in the development of the spinal cord are known to cause rare hereditary diseases.
Researchers from the Ebisuya Group at EMBL Barcelona have created a 3D in vitro model that mimics how the precursor structures that give rise to the spinal column form during human embryonic development.
This study was recently published in the journal Nature Communications.
How is the spine constructed?
The spinal column consists of 33 vertebrae, which form pairs of precursor structures called somites. Somites give rise to not only our vertebrae but also our ribs and skeletal muscles. To ensure that these structures are formed correctly, somite development is tightly regulated, and each pair of somites arises at a particular sequential time and point in development. This process is controlled by the segmentation clock, which is a group of genes that creates oscillatory waves, every wave giving rise to a new pair of somites.
“For the first time, we have been able to create periodic pairs of human mature somites linked to the segmentation clock in the lab,” explained Marina Sanaki-Matsumiya, first author of the study.
Utilising this approach meant that the researchers were able to develop a 3D in vitro model of human somite formation, also known as ‘somitogenesis.’
How does the somitogenesis process work?
The team cultured human induced pluripotent stem cells (hiPSC) in the presence of a combination of signaling molecules that are known to induce cell differentiation. Three days later, the cells started to elongate and create anterior (top) and posterior (bottom) axes.
Furthermore, researchers then decided to add Matrigel to the sequence. Matrigel is what some scientists refer to as the ‘magic powder.’ It is a protein mixture that is critical to many developmental processes. This process 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, scientists monitored the expression patterns of HES7, which is the core gene involved in the process. They discovered clear evidence of oscillations, especially when somitogenesis was about to start. The somites that formed also had clear markers of epithelisation – an important step in their maturation.
How does somite size impact the results?
The Ebisuya group studies how and why humans are different from other species regarding embryonic development. One of the model systems employed to understand interspecies differences is the segmentation clock. In 2020, the group uncovered that the oscillation period of the human segmentation clock is longer than the mouse segmentation clock.
The current study also shows a link between the size of somites and the segmentation clock. Sanaki-Matsumiya noted: “The somites that were generated had a constant size, independently of the number of cells used for the initial somitoid. 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.”
To investigate this further, Miki Ebisuya and her group intend to grow somitoids of different species and compare them. The researchers are currently working on several mammalian species, including rabbits, cattle, and rhinoceroses, setting up a ‘stem cell zoo’ in the lab.
“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,” concluded Ebisuya.
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