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Multipotent Cells from Human Fetal Liver

Dan YY et al, Proc Natl Acad Sci USA 2006;103:9912-17
Summarized by Reema Patel, Advanced Stem Cell Graduate Course, Fall 2006

Figure 1.  The human Fetal Liver Multipotent Progenitor Cells isolated were found to express stem cell, epithelial, and mesenchymal markers. The cells could be induced to form both endodermal and mesodermal cells.

The potential application of stem cells for regenerative medicine is expected to involve complex processes. It is vital to understand the underlying processes in tissue development and response to injury. In an effort to study liver development and regeneration, Dan et al sought for a liver stem cell in the human fetal liver. This study reports on the isolation and characterization of human fetal liver multipotent progenitor cells (hFLMPC) from legally aborted fetuses. As shown in Figure 1, the cells have markers (surface proteins) which include stem cell markers, epithelial markers, and some mesenchymal markers. The hFLMPC were able to differentiate into cells of the liver and bile duct. In addition, hFLMPCs was shown to differentiate into fat, bone, cartilage, and endothelial cells.

Liver cells derived from hFLMPCs were shown to be functional, as determined in an in vivo model of tissue injury. The hFLMPCs were able to integrate into the liver. Although the results are preliminary, this suggests that hFLMPCs could have potential in liver regeneration.

Interestingly, the hFLMPCs formed cells similar to those that can be developed from mesenchymal stem cells. However, hFLMPCs differ from mesenchymal stem cells based on the expressions of membrane markers, morphology, and evidence of spontaneous generation of liver cells. The experimental evidence suggests that hFLMPCs are in mesenchymal-epithelial transition, a needed step to generate liver cells. The hFLMPCs appears to be bipotential and can generate mesodermal and endodermal origin. Additional studies and experiments are required to understand how this novel stem cell fits into those that have been studied, such as mesenchymal stem cells.

The potential application of stem cells for regenerative medicine is expected to involve complex processes. It is vital to understand the underlying processes in tissue development and response to injury. In an effort to study liver development and regeneration, Dan et al sought for a liver stem cell in the human fetal liver. Liver stem cells had previously been cultured from the rodent fetal liver, but not from the human fetal liver. And, while it has been known for some time that hematopoietic stem cells (HSC) are present in the fetal liver, research on nonhematopoietic stem cells in the fetal liver has been limited. This study reports on the isolation and characterization of human fetal liver multipotent progenitor cells (hFLMPC). The cells have markers (surface proteins) which include stem cell markers, epithelial markers, and some mesenchymal markers. As shown in Figure 1, the hFLMPC were able to differentiate into hepatocytes and bile duct cells, as well as fat, bone, cartilage, and endothelial cells.
 
The hFLMPCs come from legally aborted first- and second-trimester fetuses. The cells from the fetal liver are cultured for 3 months before progenitor cells are isolated and passaged. Then serial dilution is used to clone and maintain cells for up to 6 months, which includes over 100 population doublings and 20 passages. The cells self-renewed and did not show telomere shortening, and are therefore considered to be stem cells.

The immunophenotype of hFLMPCs includes stem cell markers, epithelial markers, and some mesenchymal markers, but does not include liver-specific markers. Figure 1 shows the detailed immunophenotype. Co-expression of markers from two main lineages (epithelial and mesenchymal) was found to occur within the same cells. As expected, when the hFLMPCs are differentiated into hepatocytes, liver-specific marker expression increases and mesenchymal marker expression decreases.

The authors found that when subjected to different microenvironments, the hFLMPCs can differentiate into diverse types of cells. When put on collagen plates, the cells differentiate into hepatocytes, and when put on 3D collagen gels, they also differentiate into bile duct cells. Additionally, hepatocytes derived from hFLMPCs were found to be functional in an in vivo mouse injury model, where the cells were found integrated into liver parenchyma. Although the results are preliminary, this also suggests that hFLMPCs could have potential in liver regeneration. An important aspect that was not investigated is the potential tumorigenicity of implanted hFLMPCs. While more immature stem cells can differentiate into more cell types, they are also more likely to cause tumors upon implantation.

Interestingly, the hFLMPCs formed cells similar to those that can be developed from mesenchymal stem cells. However, hFLMPCs differ from mesenchymal stem cells based on the expressions of membrane markers, morphology, and evidence of spontaneous generation of liver cells. The experimental evidence suggests that hFLMPCs are in mesenchymal-epithelial transition, a needed step to generate liver cells. The hFLMPCs appears to be bipotential and can generate mesodermal and endodermal origin. Additional studies and experiments are required to understand how this novel stem cell fits into those that have been studied, such as mesenchymal stem cells.

COMMENTS

Dan et al reports on the integration of differentiated hFLMPCs in the liver parenchyma in a mouse injury model, indicating their therapeutic potential. An important feature to investigate is the tumorigenicity of these cells. In addition to integrating into the liver, the cells may be forming tumors in the animal model as embryonic stem cells do.

Additionally, although not mentioned, the gestation of the fetus would be expected to alter the characteristics of the hFLMPCs and there may even be optimal gestational periods, depending on the application. While culturing the cells, another important consideration to take is the possibility of fusion with feeder layer cells and murine protein contamination.