Assessing the role of hematopoietic plasticity for endothelial and hepatocyte development by non-invasive lineage tracing.

Hematopoietic cells have been reported to convert into a number of non-hematopoietic cells types after transplantation/injury. Here, we have used a lineage tracing approach to determine whether hematopoietic plasticity is relevant for the normal development of hepatocytes and endothelial cells, both of which develop in close association with blood cells. Two mouse models were analyzed: vav ancestry mice, in which essentially all hematopoietic cells, including stem cells, irreversibly express yellow fluorescent protein (YFP); and lysozyme ancestry mice, in which all macrophages, as well as a small subset of all other non-myeloid hematopoietic cells, are labeled. Both lines were found to contain YFP+ hepatocytes at similar frequencies, indicating that macrophage to hepatocyte contributions occur in unperturbed mice. However, the YFP+ hepatocytes never formed clusters larger than three cells, suggesting a postnatal origin. In addition, the frequency of these cells was very low (approximately 1 in 75,000) and only increased two- to threefold after acute liver injury. Analysis of the two mouse models revealed no evidence for a hematopoietic origin of endothelial cells, showing that definitive HSCs do not function as hemangioblasts during normal development. Using endothelial cells and hepatocytes as paradigms, our study indicates that hematopoietic cells are tightly restricted in their differentiation potential during mouse embryo development and that hematopoietic plasticity plays at best a minor role in adult organ maintenance and regeneration.