Researchers that have now created the first lab-dish models of the segmentation clock using stem cells derived from adult human tissue.
The achievements not only provide the first evidence that the segmentation clock ticks in humans but also give the scientific community the first in vitro systems enabling the study of very early spine development in humans.
"We know virtually nothing about human development of somites, which form between the third and fourth week after fertilization, before most people know they're pregnant," said the senior author. "Our system should be a powerful one to study the underlying regulation of the segmentation clock."
"Our innovative experimental system now allows us to compare mouse and human development side by side," said the co-first author of the paper, published in Nature. "I am excited to unravel what makes human development unique."
Both models open new doors for understanding developmental conditions of the spine, such as congenital scoliosis, as well as diseases involving tissues that arise from the same region of the embryo, known as the paraxial mesoderm. These include skeletal muscle and brown fat in the entire body, as well as bones, skin and lining of blood vessels in the trunk and back.
Although scientists have derived many kinds of tissue by reprogramming adult cells into pluripotent stem cells and then coaxing them along specific developmental paths, musculoskeletal tissue proved stubborn. In the end, however, the authors discovered that they could facilitate the transformation by adding just two chemical compounds to the stem cells while they were bathed in a standard growth culture medium.
The researchers were surprised to find that the segmentation clock began ticking in both the mouse and human cell dishes and that the cells didn't first need to be arranged on a 3D scaffold more closely resembling the body.
"It's pretty spectacular that it worked in a two-dimensional model," said the senior author. "It's a dream system."
The team found that the segmentation clock ticks every 5 hours in the human cells and every 2.5 hours in the mouse cells. The difference in frequency parallels the difference in gestation time between mice and humans, the authors said.
Human segmentation is similarly regulated by FGF, WNT, Notch and YAP signalling. Single-cell RNA sequencing reveals that mouse and human presomitic mesoderm (PSM) cells in vitro follow a developmental trajectory similar to that of mouse PSM in vivo. Furthermore, we demonstrate that FGF signalling controls the phase and period of oscillations, expanding the role of this pathway beyond its classical interpretation in ‘clock and wavefront’ models.
Among the next projects for lab are investigating what controls the clock's variable speed and, more ambitiously, what regulates the length of embryonic development in different species.
A third group publishing in the same issue of Nature uncovered new insights into how cells synchronize in the segmentation clock using mouse embryos engineered to incorporate fluorescent proteins.
Cellular clock regulating human spine development
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