Toward engineering of vascularized three-dimensional liver tissue equivalents possessing a clinically significant mass

In this review, we focus on how to develop and rear liver tissue equivalents that can be finally used as liver tissues as a substitute for the original liver. The size should be over 500 cm3 and its per-volume-based functionalities should be those comparable to the in vivo liver. As can easily be imagined, it will necessitate continuous efforts and we cannot predict when it becomes feasible at present. However, we need to set up an appropriate road map based on the latest knowledge concerning various related areas and to make efficient and integrative efforts to address the issues. The efforts that are currently required include design and fabrication of scaffolds, procurement of large mass of mature hepatocytes, rearing of the liver tissue equivalents in vitro and proof-of-concept studies in large animals such as pigs. Through the establishment of fundamental methodologies in such preclinical studies, we will know whether we can proceed to human clinical trials of such tissue equivalents. According to the possible road map, we summarized latest related approaches, with consistently stressing the two important but sometimes conflicting standpoints, that is, optimization of oxygenation supply to the cells in both micro- and macro-scale and three-dimensional (3D) culture of hepatocyte progenitors or stem cells toward hepatic lineages. In addition, we tried to clear up the remaining issues and the clues to overcome them.