Chondrogenic Potential of CD90+ Progenitors Figure 4. Aggregation and Chondrogenesis of sfMPCs. Normal and OA CD90+ sfMPCs were induced to differentiate without prior micro-mass aggregation. By day 14 normal sfMPCs expressed Sox9, Collagen2 and Aggrecan mRNA (A,D) while OA sfMPCs expressed severely reduced levels (A,D). Furthermore, normal sfMPCs generated a tissue-like structure that stained positive for Alcian Blue (B,G), whereas OA sfMPCs did not (C,F) and remained as a monolayer. Scale bars represent 200 mM. doi:10.1371/journal.pone.0043616.g004 were otherwise derived and cultured under identical conditions. Moreover, it would appear that this aggregation behaviour (which was consistent across the 5 human and 2 canine normal individuals tested) is lost within the OA phenotype (again consistent across the 5 OA patients and 2 canines tested). We speculate that MPCs in the OA joint might be missing, or have masked, a cell surface receptor necessary for spontaneous aggregation. In continuing studies, we will examine if OA sfMPCs demonstrate any changes in gene expression compared to normal sfMPCs, specifically in genes related to pre-condensation (CCN-1, CNN-2) and/or within condensation (N-CAM, Tenascin C) [31]. To complement this line of investigation, micro-array studies comparing the global gene expression patterns of normal vs. OA sfMPCs have been completed and the data analysis is underway. Previously, studies have demonstrated that chondrocyte aggrega- tion is mediated by b1-Integrin [32], while Cadherins seem to play a dominant role in the aggregation of MSCs during chondrogen- esis [33]. Therefore, the mechanism behind the failure of OA sfMPCs aggregation should be studied further to determine if genes expressed during the normal chondrogenic developmental process are aberrantly expressed. PLOS ONE | www.plosone.org 6 August 2012 | Volume 7 | Issue 8 | e43616