The research group of Meritxell Huch, a director at the MPI-CBG, has been working to bridge this gap for several years. In a foundational study conducted in 2021, Huch’s team developed a liver organoid consisting of two cell types: cholangiocytes (cells that line the bile ducts) and mesenchymal cells (connective tissue cells). While that model was a step forward in understanding cell-to-cell interactions, it lacked the most critical component: the hepatocytes that make up the vast majority of the liver’s mass and perform its primary metabolic functions.
In the current study, Huch and her colleagues, including fellow MPI-CBG directors Marino Zerial and Heather Harrington, took a "LEGO-like" approach to bioengineering. They set out to build a more complete model by assembling three key cell types in a precise, stepwise fashion.
"To achieve our goal, we first created organoids only consisting of hepatocytes that formed working bile channels and maintained key features of real hepatocytes in the tissue," explains Aleksandra Sljukic, one of the study’s first authors and a doctoral student in the Huch group. Once these functional hepatocyte units were established, the researchers added cholangiocytes and fibroblast (mesenchymal) cells to the mix.
The result was the "periportal assembloid." Unlike previous models, these assembloids demonstrated the ability to consistently drain bile from the internal canaliculi into the bile duct, mirroring the physiological flow found in a living liver. This functional validation proved that the researchers had successfully recapitulated the accurate tissue architecture necessary for the liver’s drainage system to operate.
A New Lens for Liver Disease and Fibrosis
The ability to create a functional, multi-cellular model has immediate implications for disease modeling. The research team focused specifically on the periportal region because of its central role in bile transport and its vulnerability in many liver pathologies.
"We chose this region in particular since it plays a key role in bile transport and is often disrupted in liver diseases when the connection of cells responsible for bile transport is blocked," says Anna Dowbaj, a postdoctoral researcher in the Huch group and first author of the study. Dowbaj, who will transition to an assistant professor role at the Technical University of Munich (TUM) in 2025, emphasizes that the model allows researchers to see exactly how different cell types contribute to the breakdown of this system.
One of the most striking applications of the assembloid was the simulation of liver fibrosis. By manipulating the number of mesenchymal cells within the model, the scientists were able to trigger a pathological response similar to the scarring seen in human patients. Furthermore, the modular nature of the assembloid allows for "mix-and-match" experiments. Researchers can combine healthy cells with mutated cells, or use gene-editing tools to turn off specific genes in one cell type while leaving others untouched. This level of precision enables scientists to pinpoint which specific cell interaction is responsible for a disease state, a task that is nearly impossible in complex animal models or simplified 2D cultures.
