Synovial Fluid Progenitors Expressing CD90+ from Normal but Not Osteoarthritic Joints Undergo Chondrogenic Differentiation without Micro-Mass Culture Roman J. Krawetz1,2*, Yiru Elizabeth Wu1, Liam Martin3, Jerome B. Rattner2, John R. Matyas4, David A. Hart1 1 Department of Surgery, University of Calgary, Calgary, Alberta, Canada, 2 Department of Cell Biology and Anatomy, University of Calgary, Calgary, Alberta, Canada, 3 Department of Medicine, University of Calgary, Calgary, Alberta, Canada, 4 Department of Comparative Biology and Experimental Medicine, Faculty of Veterinary Medicine, University of Calgary, Calgary, Alberta, Canada Abstract Objective: Mesenchymal progenitor cells (MPCs) can differentiate into osteoblasts, adipocytes, and chondrocytes, and are in part responsible for maintaining tissue integrity. Recently, a progenitor cell population has been found within the synovial fluid that shares many similarities with bone marrow MPCs. These synovial fluid MPCs (sfMPCs) share the ability to differentiate into bone and fat, with a bias for cartilage differentiation. In this study, sfMPCs were isolated from human and canine synovial fluid collected from normal individuals and those with osteoarthritis (human: clinician-diagnosed, canine: experimental) to compare the differentiation potential of CD90+ vs. CD902 sfMPCs, and to determine if CD90 (Thy-1) is a predictive marker of synovial fluid progenitors with chondrogenic capacity in vitro. Methods: sfMPCs were derived from synovial fluid from normal and OA knee joints. These cells were induced to differentiate into chondrocytes and analyzed using quantitative PCR, immunofluorescence, and electron microscopy. Results: The CD90+ subpopulation of sfMPCs had increased chondrogenic potential compared to the CD902 population. Furthermore, sfMPCs derived from healthy joints did not require a micro-mass step for efficient chondrogenesis. Whereas sfMPCs from OA synovial fluid retain the ability to undergo chondrogenic differentiation, they require micro-mass culture conditions. Conclusions: Overall, this study has demonstrated an increased chondrogenic potential within the CD90+ fraction of human and canine sfMPCs and that this population of cells derived from healthy normal joints do not require a micro-mass step for efficient chondrogenesis, while sfMPCs obtained from OA knee joints do not differentiate efficiently into chondrocytes without the micro-mass procedure. These results reveal a fundamental shift in the chondrogenic ability of cells isolated from arthritic joint fluids, and we speculate that the mechanism behind this change of cell behavior is exposure to the altered milieu of the OA joint fluid, which will be examined in further studies. Citation: Krawetz RJ, Wu YE, Martin L, Rattner JB, Matyas JR, et al. (2012) Synovial Fluid Progenitors Expressing CD90+ from Normal but Not Osteoarthritic Joints Undergo Chondrogenic Differentiation without Micro-Mass Culture. PLoS ONE 7(8): e43616. doi:10.1371/journal.pone.0043616 Editor: Irina Kerkis, Instituto Butantan, Brazil Received February 7, 2012; Accepted July 23, 2012; Published August 29, 2012 This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Funding: This study was supported by research grants from Pfizer, Alberta Innovates-Health Solutions team grant in osteoarthritis, and Canadian Institutes for Health Research (MOP 79384). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have the following interest: This study was partly funded by Pfizer Inc. There are no patents, products in development or marketed products to declare. This does not alter the authors’ adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors. * E-mail: rkrawetz@ucalgary.ca Introduction Within the last decade, the synovium and synovial fluid have been identified as a source of mesenchymal progenitor/stem cells (sMPCs) that are functionally distinct from bone marrow MPCs [1–7]. Furthermore, these synovial membrane (smMPCs) and sfMPCs can assist in cartilage repair both in vivo and in vitro [8–11]. There is compelling evidence that these cells can be used effectively to promote cartilage repair [1–11]. sfMPCs reportedly have increased chondrogenic potential [12,13] compared to bone marrow-derived mesenchymal stem cells (bmMPCs), including increased expression of the hyaluronan receptor CD44 [13,14] and uridine diphosphoglucose dehydrogenase (UDPGD), an enzyme required for hyaluronan synthesis. Notably, bmMPCs and other MPC populations do not express UDPGD [13,14]. Evidence from in vivo studies have demonstrated that when partial- thickness defects in the articular cartilage of rabbits are formed, a continuous cell layer extending from the synovial membrane is observed to contribute to the repair of the cartilage either with or without chondrogenic inducers present [9,10]. In addition, pig and PLOS ONE | www.plosone.org 1 August 2012 | Volume 7 | Issue 8 | e43616