Bone morphogenetic proteins (BMPs) are members of the TGF-β superfamily and play a critical role in skeletal development, bone formation and stem cell differentiation. Disruptions in BMP signaling result in a variety of skeletal and extraskeletal anomalies. BMP9 is a poorly characterized member of the BMP family and is among the most osteogenic BMPs, promoting osteoblastic differentiation of mesenchymal stem cells (MSCs) both in vitro and in vivo. Recent findings from various in vivo and molecular studies strongly suggest that the mechanisms governing BMP9-mediated osteoinduction differ from other osteogenic BMPs. Many signaling pathways with diverse functions have been found to play a role in BMP9-mediated osteogenesis. Several of these pathways are also critical in the differentiation of other cell lineages, including adipocytes and chondrocytes. While BMP9 is known to be a potent osteogenic factor, it also influences several other pathways including cancer development, angiogenesis and myogenesis. Although BMP9 has been demonstrated as one of the most osteogenic BMPs, relatively little is known about the specific mechanisms responsible for these effects. BMP9 has demonstrated efficacy in promoting spinal fusion and bony non-union repair in animal models, demonstrating great translational promise. This review aims to summarize our current knowledge of BMP9-mediated osteogenesis by presenting recently completed work which may help us to further elucidate these pathways.

Adipose tissue is an alternative source of mesenchymal stem cells and human adipose-derived stem cells (ASCs) display an attractive and substantial therapeutic potential when transplanted in animal models. To this end, an understanding of ASC biology is necessary and the knowledge of mechanisms that maintain ASCs in an undifferentiated state with no loss of differentiation potential during ex vivo expansion represents a crucial step. However, these mechanisms remain to be identified because appropriate human cellular models are scant. In this review we will describe a cellular model isolated from human adipose tissue displaying all the features of stem cells. Then, we will focus on the identification of intrinsic and extrinsic factors regulating the balance between human ASC proliferation and differentiation. We will point out the role of factors secreted by undifferentiated ASCs, such a FGF2, activin A, BMP4, Hedgehog molecules and secreted by adipose tissue macrophages. Finally, we will outline the role of miRNAs in these processes.

It is a great pleasure to ring in the year 2012 with the launch of the American Journal of Stem Cells (www.ajsc.us). This new peer reviewed open access online journal aims to provide to the biomedical community cutting edge insight into the latest developments in the fast paced stem cell field. The format of the journal will facilitate rapid dissemination of research findings to the broadest audience. The topics are open to all adult and embryonic stem cells and everything in between. Due to the multidisciplinary nature of the stem cell field, it is expected that some manuscripts will bring for example computational or biophysical expertise into the fold. Stem cell research has blossomed in the past several decades and shows no signs of slowing down. The research has moved from early observations of stem cells being defined as functional entities to now including prospective isolation and purification of stem cells based on their immunophenotypic, molecular, metabolic, and biochemical make up. Additional new insights into stem cell heterogeneity and advances in study of single stem cells, as opposed to populations, promise to further advance purification and directed differentiation based on finding the right “recipe” of transcription factors and target genes. Such major advances will be critical for improving our understanding of the biological and pathological development of tissues and organs, as well as the mechanisms underlying self-renewal and tissue-specific regeneration, which will ultimately facilitate downstream therapeutic applications. The great potential for adult stem cells has already been realized in the clinic using stem cell enriched populations and new applications and optimizations are in progress. The initial excitement regarding embryonic stem cells and induced pluripotent stem cells is tempered only by the realization that many obstacles need to be carefully passed in order to develop a safe and effective product for clinical testing. The pace of discovery in this area has been remarkable and equally so the pace for overcoming technical limitations and improving safety suggests that these cells will eventually be ready for prime time. There is clearly no better time to publish in the stem cell field. All of the Editors-in-Chief and Editorial Board are enthusiastic about the ability of this journal to provide ongoing contributions to the field. The Editors are committed to advancing the discipline of stem cells and regenerative medicine and welcome your manuscripts covering all aspects of stem cell biology. The American Journal of Stem Cells is your resource and we are looking forward to receiving your best reviews and original articles.