Liver cell migration plays a significant role in liver organogenesis, disease, and cell therapy. During liver organogenesis (E8.5-9.0, mouse), the ventral foregut pre-hepatic epithelium begins to express liver genes due to inductive signals emanating from the surrounding mesenchyme and heart. At E9.0, the foregut epithelium thickens as the cells transition from a cuboidal to a pseudostratified columnar morphology, forming the liver diverticulum1,2. At this critical stage, the liver diverticulum is comprised of only ~1,500 cells. Next, the hepatic endoderm lining the liver diverticulum thickens, delaminates, and forms cords of migrating hepatoblasts (HBs). These HBs co-migrate with endothelial cells and mesenchymal cells, branching into the surrounding mesenchymal tissue, initiating three-dimensional collective cell migration to form the liver bud3,4. In fact, during this stage, cells collectively undergo (1) co-migration, or movement together with other cell types, (2) branching morphogenesis or formation of branching tube-like structures, and (3) interstitial migration, or migration on top of, or between, other cells. By E11.5, migration ceases, and the primitive liver has formed, expanding 103 fold3. Liver cell migration may also be required in later stages of liver organogenesis, as rat fetal HBs expression has shown evidence of highly upregulated genes associated with 3D collective cell migration, morphogenesis, and extracellular matrix remodeling5. In addition to its role in early liver organogenesis, 3D collective migration is intricately linked to the local spread and metastasis of advanced hepatocellular carcinoma (HCC), ultimately leading to worsened prognosis and increased treatment resistance6. Adult and fetal hepatocytes also employ collective migration when moving from the spleen to within the liver during liver repopulation. In vivo imaging studies have demonstrated that transplanted hepatocytes enter the portal vein and then the capillaries within hours, migrating across the liver sinusoids and through the liver tissue7,8,9. Finally, recent studies demonstrate that migrating HBs arise during murine and human liver regeneration, with some evidence of movement in sheets10. Overall, liver collective migration, capable of multiple modes of morphogenesis, plays a significant role in organogenesis, cancer, hepatocyte cell therapy, and liver regeneration.
Numerous genetic studies have explored the molecular pathways underlying 3D liver collective cell migration. These studies reveal that the absence of hepatic cords hinders liver formation, underscoring the necessity of hepatic cord formation and their interactions with supporting cells for liver formation1,11,12,13. These studies also demonstrate that liver growth is initiated by various factors, including FGF-2 secreted from the cardiac mesoderm, BMP4 secreted from the surrounding mesenchyme, HGF, and endothelial cell interactions. Additionally, migration-associated transcription factors, including HEX, PROX1, and TBX3, play crucial roles2,14. In conclusion, genetic studies provide evidence that soluble factor signaling, leading to transcription factor expression, drives migration, signaling, and molecular interactions between HBs and their surrounding mesenchyme.
While cell migration in early liver organogenesis has been extensively investigated, current in vitro studies of hepatic migration frequently employ 2D assays utilizing highly migratory HCC cells alongside in vivo tumor models15. These studies have provided insight into several factors influencing hepatic migration, including TGFβ-116, c-Myc17, YAP18, goosecoid19, actopaxin20, and miRNAs21,22,23. Although noteworthy progress has been made in understanding the molecular mechanisms in 3D hepatic cell migration, the distinct disparities between 2D and 3D cellular migration suggest that 2D assays possess inherent limitations. Moreover, these models often lack the inclusion of mesenchymal cell types, which are essential for both migration and growth. Although there has been advancement in the development of 3D models for liver migration that integrate the supporting mesenchyme, these models primarily emphasize co-migration rather than exploring the diverse modes of collective migration.
The ability to generate tissues from spheroids through various self-assembly and morphogenetic processes facilitates the scientific exploration of synthetic tissues. These tissues have broad applications in drug development and screening, disease modeling, therapy, and other biomedical and biotechnological applications. The methodological details of several 3D in vitro cultivation systems engineered to exhibit different modes of liver 3D collective migration are provided. These systems comprise (1) co-spheroid culture with HEP and MES spheroids in a matrix, (2) HEP spheroid matrix droplet cultured with M-CM, (3) HEP-MES mixed spheroids, and (4) HEP spheroid matrix cultured with a high density of MES cells. These systems enable robust modeling of liver collective migration in a 3D environment, thereby advancing our understanding of molecular and cellular processes underlying liver organogenesis, cancer, and therapy.