Multilineage-differentiating stress-enduring (Muse)-like cells exist in synovial tissue.

Introduction: Cartilage regeneration is a promising therapy for restoring joint function in patients with cartilage defects. The limited availability of autologous chondrocytes or chondrogenic progenitor cells is an obstacle to its clinical application. We investigated the existence and chondrogenic potential of synovial membrane-derived multilineage-differentiating stress-enduring (Muse)-like cells as an alternative cell source for cartilage regeneration.

Methods: Cells positive for stage-specific embryonic antigen-3 (SSEA-3), a marker of Muse cells, were isolated from the synovial membranes of 6 of 8 patients (median age, 53.5 years; range 36-72 years) by fluorescence-activated cell sorting. SSEA-3-positive cells were cultured in methylcellulose to examine their ability to form Muse clusters that are similar to the embryoid bodies formed by human embryonic stem cells. Muse clusters were expanded and chondrogenic potential of M-cluster-derived MSCs examined using a pellet culture system. Chondrogenic differentiation was evaluated by proteoglycan, safranin O, toluidine blue and type II collagen staining. To evaluate the practicality of the procedure for isolating Muse-like cells, we compared chondrogenic potential of M-cluster derived MSCs with expanded cells derived from the clusters formed by unsorted synovial cells.

Results: Synovial membranes contained SSEA-3-positive cells that after isolation exhibited Muse-like characteristics such as forming clusters that expressed NANOG, OCT3/4, and SOX2. In the pellet culture system, cell pellets created from the M-cluster-derived MSCs exhibited an increase in wet weight, which implied an increase in extracellular matrix production, displayed metachromasia with toluidine blue and safranin O staining and were aggrecan-positive and type II collagen-positive by immunostaining. Unsorted synovial cells also formed clusters in methylcellulose culture, and the expanded cell population derived from them exhibited chondrogenic potential. The histological and immunohistochemical appearance of chondrogenic pellet created from unsorted synovial cell-derived cells were comparable with that from M-cluster-derived MSCs.

Conclusions: Muse-like cells can be isolated from the human synovial membrane, even from older patients, and therefore may provide a source of multipotent cells for regenerative medicine. In addition, the cluster-forming cell population within synovial cells also has excellent chondrogenic potential. These cells may provide a more practical option for cartilage regeneration.