How the precursor of the spinal column develops in the embryo

somites

somites generated using stem cells. © Ebisuya Group/EMBL

The spine is the supporting structure of our body. Researchers have now been able to understand for the first time in the laboratory how this complex structure consisting of 33 vertebrae and adjacent structures develops in the early embryo. To do this, they observed how the precursors of the spinal column segments, the somites, are formed from stem cells. The experiments confirm that certain clock genes set the pace in the embryo: They use messenger substances to determine when which segment is formed and how large it is. Only the interplay of this “segmentation clock” with cell division and cell interactions allows our spine to develop in its typical human form.

Our backbone not only provides the support for our body and limbs, it also protects our spinal cord and is the central attachment point for our core muscles. In the embryo, this central component of our body develops early: the precursors of the vertebral segments, the somites, begin to form around 20 days after fertilization. These cell blocks bud sequentially in pairs from the mesoderm, the middle germ layer of the embryo. The sequence and time intervals of this differentiation process are regulated by a kind of internal clock: The segmentation clock, a molecular oscillator controlled by genes, showed an activity that fluctuated every five to six hours and triggered the formation of new somite segments, as cell culture experiments had already suggested.

From Stem Cells to Vortex Progenitors

So far, however, there has been no in-vitro model that can reproduce gene-controlled somite development in its entirety in the laboratory. A research team led by Marina Sanaki-Matsumiya from the European Molecular Biology Laboratory (EMBL) in Barcelona has now developed such a 3D model. As a starting point, they used human pluripotent stem cells that had been generated from adult cells. By adding a special cocktail of growth factors and other messenger substances, the scientists stimulated the differentiation of the stem cell clumps. After three days, it was already evident that the first cells were lengthening and aligning in one axis. After adding another protein-rich nutrient supplement, the first somites developed: “By day seven, all 210 batches had formed at least three somites, with an average of seven,” reports the team.

As in the embryo, the somites developed one after the other – first at the front end of the embryo-like cell clump, then successively towards the rear end. Irrespective of the number of stem cells initially present, all the somites were the same size. “This suggests that the somites have a preferential, species-specific size that is controlled by local cell interactions, the segmentation clock, or other mechanisms,” explains Sanaki-Matsumiya. At the same time, the formation of these cellular vortex precursors in laboratory experiments demonstrates that somitogenesis is an autonomous process that runs independently of adjacent tissues or a higher-level control system. The cells themselves, their interactions and genes initiate the processes that lead to the formation of the approximately 40 somites in the embryo.

oscillating genes

In order to be able to observe the role played by the genetic clocks in this process, the researchers marked the central gene of the segmentation clock, HES7, with a fluorescent marker. This showed that this gene became active as soon as the first messenger cocktail was added and showed regular cycles of activity. These waves of increased gene activity propagated forward from the most recent, freshly formed posterior somites. “The timing of the HES7 oscillations coincided with that of the somite formation: In each oscillation cycle, one somite or one somite pair was formed,” report Sanaki-Matsumiya and her colleagues. The three-dimensional in-vitro model thus confirms the previous assumptions, but at the same time opens up new possibilities for investigating the formation of these structures, which are essential for our body. “These cell models also provide us with a new platform to study hereditary malformations in the segmentation of the spine, including hereditary scoliosis,” the scientists write.

However, Sanaki-Matsumiya’s team is also planning to conduct comparative studies based on their method: they want to observe how somite formation takes place in different animal species and how it is regulated. To do this, they have already started to grow stem cells from cell samples from rabbits, cattle and rhinos. “In the next project, we will then generate somitoids of the different species from these stem cells in order to measure their cell proliferation and cell migration and to find out how somitogenesis differs in the different species,” reports Miki Ebisuya from EMBL Barcelona.

Source: Marina Sanaki-Matsumiya (European Molecular Biology Laboratory, Barcelona) et al., Nature Communications, doi: 10.1038/s41467-022-29967-1

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