Bottom-up Approach to Mammalian Gastrulation – Gastrulation

One of the most striking features of embryonic development is that differentiation is happening in a spatially ordered fashion: tissue self-organize to form well-defined patterns that pre-figure the body plan. During gastrulation, the cells of the embryo are allocated into three germ layers: ectoderm, mesoderm and endoderm. During the last decades, signaling pathways responsible for the initiation of gastrulation in mammalian embryos have been identified. However, the physical rules governing the tissue spatial patterning and the extensive morphogenetic movements occurring during that process are still elusive.

Studying the spatio-temporal dynamics of pattern formation is difficult in live embryos, because of their inherent lack of observability (especially in organisms that develop in utero) but also because it is not possible in an embryo to control in a quantitative manner the parameters that are likely to be relevant for the establishment of the multi cellular pattern such as the size and shape of the tissue and its physical and chemical environment.

Recently, we took the first step toward in vitro recapitulation of early embryonic patterning by showing that human Embryonic Stem Cells (ESCs) confined to circular disks comparable in size to mammalian embryos using micro-patterning technology and treated with the gastrulation inducing signal BMP4 differentiate to an outer trophectoderm-like ring followed by the three embryonic germ layers in an ordered, reproducible radial pattern. Quite notably, at the level of the Brachyury positive ring of mesodermal cells, hESC colonies on micro-patterned glass substrates show the molecular (up regulation of Snail) and morphological the (Epithelial-Mesenchymal Transition) signature of gastrulating cells. These results demonstrate that geometric confinement is sufficient to trigger self-organized patterning in ESCs and that the intrinsic tendency of stem cells to make patterns can be harnessed by controlling colony geometries. ESC culture on micro-patterned substrates thus provides a quantitative assay to study early development.

The observed pattern doesn’t however recapitulate all the features of the patterning observed in embryos, suggesting that the imposed boundary conditions are too stringent or incorrectly defined. By taking advantage the micro-fabrication and the micro-fluidics toolbox, we propose here to study how the physical (stiffness of the substrate, size and shape of the tissue) and chemical (spatio-temporal profile of differentiating signals) properties of the cells micro-environment is affecting the final pattern, in order to define what is the minimal set of boundary conditions allowing for proper gastrulation. With this series of experiments, we expect to better understand the physical rules governing the self-organization of differentiating tissues. This knowledge is necessary if we want to be able one day to grow organs in a dish.