Recent updates on the biological basis of heterogeneity in bone marrow stromal cells/skeletal stem cells

Deepika Arora, Pamela Gehron Robey

Figure 2. Gold standard assays by which to assess differentiation capacity. Many of the currently used “standard” assays of differentiation are prone to artifact or misinterpretation. However, there are assays that can faithfully report differentiation capacity: 1) the in vitro cartilage pellet assay, whereby one can see chondrocytes lying in lacunae surrounded by extracellular matrix that stains purple with toluidine blue, 2) the in vivo transplantation assay whereby donor cells are able to make bone matrix, osteocytes, osteoblasts, and in some cases, support haematopoiesis and formation of marrow adipocytes (the latter two properties are not shared by all forms of skeletal stem cells), and 3) the in vitro myogenic assay, whereby myotubes are formed in the absence of exogenous myoblasts (which will spontaneously fuse with any fibroblastic population). Adapted in part from Sacchetti et al.30 BM: bone marrow; DAPI: diamidino-2-phenylindole; H&E: hematoxylin and eosin; hp: haematopoiesis; MSC: mesenchymal stem/stromal cell; MU: muscle; MyHC: myosin heavy chain.