Neither Notch1 Expression nor Cellular Size Correlate with Mesenchymal Stem Cell Properties of Adult Articular Chondrocytes

<i>Background:</i> Tissue repair is thought to be regulated by progenitor cells, which in other tissues are characterized by their Notch1 expression or small cellular size. Here we studied if these traits affect the chondrogenic potential and are markers for multipotent progenitor cell populations in adult articular cartilage. <i>Methods:</i> Directly isolated articular chondrocytes were sorted with regard to their Notch1 expression or cellular size. Their colony forming efficiency (CFE) and their potential to differentiate towards adipogenic, osteogenic and chondrogenic lineages were investigated. The different sorted populations were also expanded in monolayer and analyzed in the same manner as the directly isolated cells. <i>Results:</i> No differences in CFE or adipogenic, osteogenic and chondrogenic potentials were detected among the sorted populations. Expanded cells displayed a higher osteochondral potential than directly isolated cells. <i>Conclusion:</i> Cellular size or Notch1 expression is not per se a specific marker for mesenchymal progenitor cells in adult articular cartilage. Monolayer-expanded adult chondrocytes contain a larger mesenchymal progenitor cell-like population than directly isolated cells, highly likely as a result of dedifferentiation. If there are resident Notch1-positive cells or cells of a specific size in adult articular cartilage with functional features of progenitor cells, the population consists of only a very small number of cells.