hfMSCs Differentiate toward Growth Plate Cartilage Figure 6. Protein interactions of differentially expressed genes between undifferentiated hfMSC and prepubertal growth plate cartilage. Analysis of protein interactions of genes that are differentially expressed in undifferentiated hfMSC (week 0) compared to average expression profiles of growth plate cartilage of 3 prepubertal donors using STRING 9.0. Large clusters of interacting proteins can be identified (A–F). Many other genes are identified that have no known protein interactions with other genes in the list. doi:10.1371/journal.pone.0044561.g006 necessary specific supplements, such as T3, dexamethasone and Wnt3a, for MSC differentiation into bone tissue, suggesting that when these factors are added to our cultures during the later stages of differentiation, one may observe induction of late hypertrophic markers and mineralization [37–39]. The model we describe offers insight into the route that MSCs take during the chondrogenic differentiated into growth plate cartilage as depicted in figure 6. This enables us to perform different types of research in the field of bone tissue engineering and early growth plate differentiation To our knowledge this is the first time human fetal mesenchymal stromal cells are followed on their route toward the chondrogenic differentiation. We followed the differentiation during 5 weeks with microarray data every week and compared the obtained gene profiles to the gene expression profile GP chondrocytes from prepubertal and fetal growth plate. Using analysis of protein-protein interaction networks, we observed that the typical markers for chondrogenesis and early stages of hypertrophic differentiation are present, but other protein clusters, such as genes important for late hypertrophy and endochondral ossification are not induced. Using traditional PLOS ONE | www.plosone.org 8 November 2012 | Volume 7 | Issue 11 | e44561