Scientists at the Institute for the Advanced Study of Human Biology (ASHBi) at Kyoto University say they used a petri dish and induced pluripotent stem cells (iPSCs) to learn how the early stages of the human body plan are established.

In their study “Reconstituting human somitogenesis in vitro,” published in Nature, Cantas Alev, MD, PhD, and his team created a cocktail consisting of human iPSC-derived cells and Matrigel, a viscous gel compound enriched with extracellular matrix components, to generate a 3D model that reportedly can recapitulate the development of our early body plan in a dish, which they coined “axioloids.”

“Our (bottom-up) approach of generating axioloids has not only allowed us to uncouple fundamental biological processes, such as cell morphology and cell states, but it allowed us to determine how mutations contribute to spine disease,” explains Alev. “We also anticipate similar strategies will become increasingly necessary in order to understand better the etiology and pathology of other diseases.”

Immunofluorescence staining of human pluripotent stem cell-derived axioloids. [Alev Lab, ASHBi/Kyoto University]

The segmented body plan of vertebrates is set up during somitogenesis, a well-studied process in model organisms, but remains elusive in humans due to ethical and technical limitations, note the investigators. “Despite recent advances with pluripotent stem cell (PSC)-based approaches, models that robustly recapitulate human somitogenesis in both space and time are still largely missing,” they write.

“Here, we introduce a PSC-derived mesoderm-based 3D model of human segmentation and somitogenesis, which we termed ‘axioloid’, that captures accurately the oscillatory dynamics of the segmentation clock and the morphological and molecular characteristics of sequential somite formation in vitro. Axioloids show proper rostrocaudal patterning of forming segments and robust anterior-posterior FGF/WNT signaling gradients and retinoic acid (RA) signaling components.

Unexpected critical role of retinoic acid signaling

“We identify an unexpected critical role of RA signaling in the stabilization of forming segments, indicating distinct, but also synergistic effects of RA and extracellular matrix (ECM) on the formation and epithelialization of somites. Importantly, comparative analysis demonstrates striking similarities of axioloids to the human embryo, further validated by the presence of a HOX code in axioloids.

“Lastly, we demonstrate the utility of axioloids to study the pathogenesis of human congenital spine diseases, by using patient-like iPSCs with mutations in HES7 and MESP2. These results suggest that axioloids represent a promising novel platform to study axial development and disease in humans.”

“(Our) axioloids capture, not only the oscillatory nature of the segmentation clock, but also the molecular as well as the 3-D morphological and structural characteristics observed during the process of segmentation and somitogenesis” notes Alev, whose team identified a previously unappreciated functional role for retinoids during somite formation.

“Our [study] was critical to unraveling the role of retinoids during somitogenesis. It is likely that many researchers missed this essential role because vitamin A is a common supplement that usually gets included into culture media.”

When the  axioloids were compared to actual human embryos, they revealed “remarkable similarities to Carnegie Stage 9-12 human embryos, which is known to be a critical stage during human development where organs such as the brain and heart start forming,” he remarks.

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