Clonal mesenchymal progenitors from human bone marrow differentiate in vitro according to a hierarchical model

Bone marrow stromal cells can give rise to several mesenchymal lineages. Th e existence of a common stem/progenitor cell, the mesenchymal stem cell, ha s been proposed, but which developmental stages follow this mesenchymal mul tipotent progenitor is not known. Based on experimental evidence, a model o f mesenchymal stem cell differentiation has been proposed in which individu al Lineages branch directly from the same progenitor. We have verified this model by using clonal cultures of bone marrow derived stromal fibroblasts, We have analyzed the ability of 185 non-immortalized human bone marrow str omal cell clones to differentiate into the three main lineages: osteo-, cho ndro- and adipogenic, All clones but one differentiated into the osteogenic lineage. About one third of the clones differentiated into all three linea ges analyzed. Most clones (60-80%) displayed an osteo-chondrogenic potentia l. We have never observed clones with a differentiation potential limited t o the osteo-adipo- or to the chondro-adipogenic phenotype, nor pure chondro genic and adipogenic clones. How long the differentiation potential of a nu mber of clones was maintained was assessed throughout their life span. Clon es progressively lost their adipogenic and chondrogenic differentiation pot ential at increasing cell doublings, Our data suggest a possible model of predetermined bone marrow stromal cell s differentiation where the tripotent cells can be considered as early mese nchymal progenitors that display a sequential loss of lineage potentials, g enerating osteochondrogenic progenitors which, in turn, give rise to osteog enic precursors.