Outside of direct in vivo experiments, most biomedical research processes involve some level of metaphor. Whether it's a cluster of liver cells standing in for an entire liver or a mouse brain taking the place of a human brain, the goal of these models is to reduce the impact of the metaphor as much as possible while enabling further research.
That quest to create better and more accurate representations of organs and organisms led to major developments of newer and better forms of cell and tissue cultures — namely 3D cell cultures. 3D cell cultures can provide a more representative platform for drug development and other applications, and they're well on their way to becoming the standard step between traditional 2D cell culturing and full-scale animal testing.