Morphogen gradients applied basally to human embryonic stem cells to control and dissect tissue patterning

Morphogen gradients are used repeatedly during development to pattern embryonic tissues. Absolute concentrations, duration or even temporal derivative of morphogen concentration have all been proposed to carry positional information. However, establishing the causal relationship between the spatio-temporal profile of the gradient and the resulting cellular diversity and tissue patterning is difficult to address in live embryos because of lack of tools to take control of those variables. Here, we developed microfluidics devices able to apply spatiotemporally well-defined morphogen landscapes on the basal side of human embryonic stem cells colonies, thus mimicking how BMP4 is delivered to the pluripotent epiblast during mouse gastrulation. Combining live imaging and theoretical modelling, we show that in this configuration the absolute concentration of BMP4 provides positional information and the cell identities emerging during differentiation can vary according to the shape of the gradient. Our toolbox provides a powerful means to dissect the logic of patterning and to engineer tissues precisely.