Multiple mesenchymal progenitor cell subtypes with distinct functional potential are present within the intimal layer of the hip synovium.

Background: The synovial membrane adjacent to the articular cartilage is home to synovial mesenchymal progenitor cell (sMPC) populations that have the ability to undergo chondrogenesis. While it has been hypothesized that multiple subtypes of stem and progenitor cells exist in vivo, there is little evidence supporting this hypothesis in human tissues. Furthermore, in most of the published literature on this topic, the cells are cultured before derivation of clonal populations. This gap in the literature makes it difficult to determine if there are distinct MPC subtypes in human synovial tissues, and if so, if these sMPCs express any markers in vivo/in situ that provide information in regards to the function of specific MPC subtypes (e.g. cells with increased chondrogenic capacity)? Therefore, the current study was undertaken to determine if any of the classical MPC cell surface markers provide insight into the differentiation capacity of sMPCs.

Methods: Clonal populations of sMPCs were derived from a cohort of patients with hip osteoarthritis (OA) and patients at high risk to develop OA using indexed cell sorting. Tri-differentiation potential and cell surface receptor expression of the resultant clones was determined.

Results: A number of clones with distinct differentiation potential were derived from this cohort, yet the most common cell surface marker profile on MPCs (in situ) that demonstrated chondrogenic potential was determined to be CD90/CD44/CD73. A validation cohort was employed to isolate cells with only this cell surface profile. Isolating cells directly from human synovial tissue with these three markers alone, did not enrich for cells with chondrogenic capacity.

Conclusions: Therefore, additional markers are required to further discriminate the heterogeneous subtypes of MPCs and identify sMPCs with functional properties that are believed to be advantageous for clinical application.