A gradient of opposing signals regulates embryonic development

Embryonic development is a complex process where multiple signals interact in time and space to determine the identity and fate of individual cells.  The group of reNEW Leiden Associate Investigator Pascale Dijkers and Principal Investigator Niels Geijsen uses gastruloids, structures made from embryonic stem cells to mimic specific stages of embryonic development and investigate these signals in detail. In their latest study, published in Development, the team discovered how a gradient of two signaling molecules dictates cellular composition and elongation in these embryo models.

Retinoic Acid (RA), a metabolite from Vitamin A, is a signaling molecule crucial for healthy embryonic development. Nevertheless, at higher doses, it can lead to birth defects. Others therefore previously studied the role of this molecule in early embryonic development. They did so by directly supplementing RA, which has teratogenic effects and impairs normal embryonic development. The role of RA at physiological levels therefore remains elusive.

From Retinoic Acid to Retinol
The researchers thus took an innovative approach to study the role of RA. They removed all RA from the culture medium and added different concentrations of its precursor – Retinol.

First author and PhD candidate Mehmet Yildiz explains: “Not all cells during embryonic development are equally capable to process Retinol and produce RA from it. We therefore preserved a gradient of RA signaling in our gastruloids, unlike the global RA activation induced by direct addition of RA to the medium.”

Signals matter. Dosing decides.
The new approach allowed Yildiz and his colleagues to study the effect of RA in great detail. They first confirmed findings by others that RA negatively regulates FGF signaling. Furthermore, they found that RA-signaling has several dose-dependent effects on the gastruloids.

Yildiz: “We found that cellular composition of our organoids was highly dependent on RA signaling. At high levels, our gastruloids almost exclusively develop into cells that form the nervous system. Low RA levels lead to development of cells that form our internal organs and muscle.”

The researchers concluded that proper gastruloid development, accompanied by their elongation, could only be achieved when they added retinol at moderate levels. This condition preserves a signaling gradient of opposing FGF and RA molecules, highlighting the importance of this signaling axis for embryonic development. 

Cell fate commitment
“Altogether we developed a method that allows us to precisely direct cell fate decisions in gastruloids,” said Dijkers. “This opens possibilities to explore lineage commitment during embryonic development further in follow-up studies. We thereby hope to advance fundamental insights that hold promise for application in stem cell differentiation and regenerative medicine.”