Embryonic stem cells can provide an unlimited supply of pluripotent cells for tissue engineering applications. Bone tissue engineering by directly differentiating ES cells (ESCs) into osteoblasts has been unsuccessful so far. Therefore, we investigated an alternative approach, based on the process of endochondral ossification. A cartilage matrix was formed in vitro by mouse ESCs seeded on a scaffold. When these cartilage tissue-engineered constructs (CTECs) were implanted s.c., the cartilage matured, became hypertrophic, calcified, and was ultimately replaced by bone tissue in the course of 21 days. Bone aligning hypertrophic cartilage was observed frequently. Using various chondrogenic differentiation periods in vitro, we demonstrated that a cartilage matrix is required for bone formation by ESCs. Chondrogenic differentiation of mesenchymal stem cells and articular chondrocytes showed that a cartilage matrix alone was not sufficient to drive endochondral bone formation. Moreover, when CTECs were implanted orthotopically into critical-size cranial defects in rats, efficient bone formation was observed. We report previously undescribed ESCbased bone tissue engineering under controlled reproducible conditions. Furthermore, our data indicate that ESCs can also be used as a model system to study endochondral bone formation.osteoblast ͉ cartilage ͉ endochondral ossification ͉ scaffold ͉ in vivo

Tissue engineering of large bone defects is approached through implantation of autologous osteogenic cells, generally referred to as multipotent stromal cells or mesenchymal stem cells (MSCs). Animal-derived MSCs successfully bridge large bone defects, but models for ectopic bone formation as well as recent clinical trials demonstrate that bone formation by human MSCs (hMSCs) is inadequate. The expansion phase presents an attractive window to direct hMSCs by pharmacological manipulation, even though no profound effect on bone formation in vivo has been described so far using this approach. We report that activation of protein kinase A elicits an immediate response through induction of genes such as ID2 and FosB, followed by sustained secretion of bone-related cytokines such as BMP-2, IGF-1, and IL-11. As a consequence, PKA activation results in robust in vivo bone formation by hMSCs derived from orthopedic patients.bone tissue engineering ͉ osteogenesis ͉ PKA signaling

Although a large number of mutations causing malfunction of complex I (NADH:ubiquinone oxidoreductase) of the OXPHOS system is now known, their cell biological consequences remain obscure. We previously showed that the bradykinin (Bk)-induced increase in mitochondrial [ATP] ([ATP](M)) is significantly reduced in primary skin fibroblasts from a patient with an isolated complex I deficiency. The present work addresses the mechanism(s) underlying this impaired response. Luminometry of fibroblasts from 6 healthy subjects and 14 genetically characterized patients expressing mitochondria targeted luciferase revealed that the Bk-induced increase in [ATP](M) was significantly, but to a variable degree, decreased in 10 patients. The same variation was observed for the increases in mitochondrial [Ca(2+)] ([Ca(2+)](M)), measured with mitochondria targeted aequorin, and cytosolic [Ca(2+)] ([Ca(2+)](C)), measured with fura-2, and for the Ca(2+) content of the endoplasmic reticulum (ER), calculated from the increase in [Ca(2+)](C) evoked by thapsigargin, an inhibitor of the ER Ca(2+) ATPase. Regression analysis revealed that the increase in [ATP](M) was directly proportional to the increases in [Ca(2+)](C) and [Ca(2+)](M) and to the ER Ca(2+) content. Our findings provide evidence that a pathological reduction in ER Ca(2+) content is the direct cause of the impaired Bk-induced increase in [ATP](M) in human complex I deficiency.

Introduction Large inter-donor differences exist in human mesenchymal stem cell (hMSC) yield and the response of these cells to osteogenic stimuli. The source of these differences may be clinical differences in stem cell characteristics between individuals or the aspiration procedure itself.Methods From a total of 23 donors, we aimed to take 2 consecutive 10-mL aspirates from the same site in 17 donors and in 6 donors we aimed to take a 5-mL and a 20-mL aspirate. The aspiration was stopped either when the syringe was full or when no more bone marrow came through. Mononuclear cell yield (MNC), MSC yield, and differentiation capacity were analyzed for intra-donor and inter-donor variation. We analyzed the effect of the dilution with peripheral blood by drawing 20 mL at once.Results There was a high correlation between the first and second aspiration volumes, and aspirates with a volume of less than 8 mL showed a large variation in cellular yield. The second 10-mL aspirate, and also 20-mL aspirates, contained a lower concentration of nucleated cells and yielded lower numbers of mesenchymal stem cells. No effect of the aspiration procedure on the biological characteristics of the mesenchymal stem cells was seen.Conclusion We recommend collection volumes of bone marrow aspirates of at least 8 mL to reduce the risk of obtaining aspirates with low cell numbers. From the same site, a second aspiration or an aspirate of > 10 mL can be drawn without any loss of biological quality due to dilution with peripheral blood.

Collagen is the main protein component of the extracellular matrix of bone, and it has structural and instructive properties. Collagen undergoes many post-translational modifications, including extensive crosslinking. Although defective crosslinking has been implicated in human syndromes (e.g., osteogenesis imperfecta or Ehlers-Danlos syndrome), it is not clear to what extent crosslinking is necessary for collagen's instructive properties during bone formation. Here we report that inhibition of collagen crosslinking in the mouse pre-osteoblast cell line MC3T3-E1 impairs the osteogenic program. Genome-wide expression profiling of beta-aminopropionitrile-treated and control cells revealed that matrix deposition by MC3T3-E1 cells provides a feed back signal, driving cells through the differentiation process, that is strongly impaired when crosslinking is inhibited. Inhibition of crosslinking did not affect osteogenic differentiation of human mesenchymal stem cells (hMSCs), shown by the expression of alkaline phosphatase and genome-wide gene expression analysis, although it enhances matrix mineralization. In conclusion, collagen crosslinking harbors instructive properties in MC3T3-E1 differentiation but plays a more-passive role in differentiation of bone marrow-derived hMSCs.