Human mesenchymal stem cells (MSCs) have been extensively characterized with respect to their in vitro expansion and differentiation potential, especially with respect to osteogenesis. Dexamethasone (Dex) is a well-known inducer of osteogenic differentiation of MSCs, but little is known about the effect of Dex treatment on apoptosis in MSCs. In this study, apoptosis in MSCs was examined with respect to cell density and Dex supplementation, using DAPI staining and DNA fragmentation ELISA Assay. In MSC cultures initiated at 1.0, 3.0, and 9.0 Â 10 3 cells per cm 2 , it was found that higher MSC density correlated with increased apoptosis and that this apoptotic effect was diminished in cultures containing 100 nM Dex. MSCs and fibroblasts were co-cultured, along with empty insert controls, and assayed for apoptosis by ELISA and DAPI counts to determine if soluble factors accounted for the cell density-related apoptosis. No difference was seen between MSCs cultured with inserts containing either MSCs, fibroblasts, or empty control. To determine cell contact effects, BrdU-labeled MSCs were cultured alone or with unlabeled chondrocytes at 2Â and 8Â the number of MSCs, with and without Dex, and apoptosis levels quantified. The results showed increased apoptosis at greater cell densities, and that the amount of apoptosis was greatly diminished in cultures containing Dex. These results show that apoptosis in MSCs is cell density-related, requires direct cell contact, and that Dex treatment reduces or eliminates this density-related apoptosis. These results may impact how MSCs should be cultured for clinical applications. Keywords: dexamethasone; mesenchymal stem cells; apoptosis Human bone marrow-derived mesenchymal stem cells (MSCs) are a rare population of undifferentiated cells that have the capacity to self-renew and can differentiate into mesodermal phenotypes including osteoblasts, chondrocytes, myocytes, and adipocytes. 1-3 Induction of osteoprogenitor differentiation of MSCs by dexamethasone (Dex) is well known, 4,5 and culture supplementation with Dex at 10 À8 M has been reported to promote osteoprogenitor cell differentiation in marrow stromal cells. 6,7 In addition, Dex is also a component of medium used for differentiation into chondroblasts, myocytes, and adipocytes in vitro. [8][9][10] With respect to Dex effects on apoptosis, the results have often been contradictory, depending upon the cell type being studied. For example, Dex treatment has been shown to induce apoptosis in thymocytes, 11,12 lymphocytes, 13-15 some tumor cells, 16,17 and respiratory epithelium, 18,19 whereas primary cultured hepatocytes, 20 neutrophils, 21,22 and some solid tumors 23,24 show reduced apoptosis after Dex treatment.Interestingly, while Dex has been used extensively with MSCs as an inductive factor for multiple end-stage phenotypes, few studies have assessed the effects of Dex on proliferation and apoptosis. To date, only one other study addressed the issue of Dex effects on MSC apoptosis wherein it was shown that hu...