Methods and Strategies for Lineage Tracing of Mesenchymal Progenitor Cells.

Mesenchymal progenitors (MP) are found to varying extents in most tissues and organs. Their relationship to bone marrow-derived mesenchymal stem cells (MSCs) remains unclear, however, both populations appear to share a number of properties as defined by functional assays, clonogenic activity, and genetic and cell surface markers. MSCs were originally defined by their in vitro colony forming unit-fibroblast (CFU-F) activity and their ability to contribute to various mesenchymal lineages (i.e. cartilage, bone, and fat). MSCs also appear to exhibit some unique properties, in that expanded clones in the absence of bone-inducing factors generate bone spicules/organs in vivo. Subsequent analysis of these elements has demonstrated that the transplanted cells directly contribute to multiple mesenchymal lineages. Our ability to study MP and/or MSC behavior and lineage potential in vivo has been hampered by a lack of suitable Cre lines in which to effectively genetically mark and follow the fate and activity of these cells in development, growth, homeostasis and following injury or in disease. The emergence of several new genetic lines is enabling us to now address critical questions regarding MP/MSC location, behavior, function, and fate. The use of these lines and others in conjunction with suitable reporter lines will be described for MP/MSC cell fate analysis.