Scientists at EMBL Barcelona, Europe’s leading laboratory for life sciences, have developed an in vitro 3D model that mimics how the spinal column forms during human embryonic development.
The vertebral column is the central support structure of the skeleton. It not only provides fixation for the muscles, but also protects the spinal cord and nerve roots. Defects in their development are known to cause rare hereditary diseases.
The spinal column consists of 33 vertebrae, which are formed from pairs of precursor structures called somites: they give rise not only to the vertebrae, but also to the ribs and muscles of the skeleton.
To ensure proper formation of these structures, somite development is tightly regulated, with each pair of somites emerging at a particular time in embryonic development.
This process is controlled by the segmentation clockwhich is a group of genes that creates oscillating waves, each of which gives 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 cleavage clock in the laboratory,” explains Marina Sanaki-Matsumiya, first author of the study published in Nature Communications.
To achieve this, the team cultured human induced pluripotent stem cells (hiPSCs) with a cocktail of signaling molecules that induce cell differentiation.
Three days later, the cell clusters began to lengthen and create anterior (top) and posterior (bottom) axes. At that point, the scientists added Matrigel to the culture mix.
Matrigel is what some scientists call the magic dust: a mixture of proteins that is crucial for various developmental processes. This process ultimately led to the formation of somitoids, which would be the in vitro equivalents of the precursor structures of human somites.
To test whether the cleavage clock regulates somitogenesis in these virtual somitoids, the researchers monitored the expression patterns of HES7, the central 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 epithelialization, an important step in their maturation.
The research group that has developed this initiative, led by Dr. Miki Ebisuyastudies how and why humans are different from other species when it comes to embryonic development.
One of the model systems of differences between species that they use is the segmentation clock. In 2020, the group found that the oscillation period of the human segmentation clock is longer than that of the mouse. The current study also shows a relationship between somite size and segmentation clock.
“The somitoids we created, regardless of the initial cell number, had a somite size that was constant. It did not increase, even though the number of initial cells did,” explains Sanaki-Matsumiya.
“This suggests that somites have a preferred size, which could be determined by local cell-cell interactions, the cleavage clock, or other mechanisms,” he adds.
To further study, Ebisuya and his group now plan to grow somitoids from different species and compare them. The researchers are already working with several species of mammals, such as rabbits, cattle and rhinos, creating a “stem cell zoo” in the laboratory.
“Our next project will focus on creating somitoids from different species, measuring their cell proliferation and cell migration speed to establish what and how somitogenesis is different between species,” concludes Ebisuya.
Periodic formation of epithelial somites from human pluripotent stem cells. Marina Sanaki-Matsumiya et al. Nature Communications, Volume 13, Article number: 2325 (2022). DOI:https://doi.org/10.1038/s41467-022-29967-1