Human Embryo Models and Drug Discovery.

For obvious reasons, such as, e.g., ethical concerns or sample accessibility, model systems are of highest importance to study the underlying molecular mechanisms of human maladies with the aim to develop innovative and effective therapeutic strategies. Since many years, animal models and highly proliferative transformed cell lines are successfully used for disease modelling, drug discovery, target validation, and preclinical testing. Still, species-specific differences regarding genetics and physiology and the limited suitability of immortalized cell lines to draw conclusions on normal human cells or specific cell types, are undeniable shortcomings. The progress in human pluripotent stem cell research now allows the growth of a virtually limitless supply of normal and DNA-edited human cells, which can be differentiated into various specific cell types. However, cells in the human body never fulfill their functions in mono-lineage isolation and diseases always develop in complex multicellular ecosystems. The recent advances in stem cell-based 3D organoid technologies allow a more accurate in vitro recapitulation of human pathologies. Embryoids are a specific type of such multicellular structures that do not only mimic a single organ or tissue, but the entire human conceptus or at least relevant components of it. Here we briefly describe the currently existing in vitro human embryo models and discuss their putative future relevance for disease modelling and drug discovery.