Since their isolation in 1998, human embryonic stem (hES) cells have been shown to be capable of adopting various cell fates in vitro. Here, we present in vitro data demonstrating the directed commitment of human embryonic stem cells to the osteogenic lineage. Human ES cells are shown to respond to factors that promote osteogenesis, leading to activation of the osteogenic markers osteocalcin, parathyroid hormone receptor, bone sialoprotein, osteopontin, cbfa1, and collagen 1. Moreover, the mineralized nodules obtained are composed of hydroxyapatite, further establishing the similarity of osteoblasts in culture to bone. These results show that osteoblasts can be derived from human ES cultures in vitro and provide the basis for comparison of adult and embryonic-derived osteogenesis, and for an investigation of potential applications for hES cells in orthopaedic tissue repair.

Human embryonic stem cells (hESCs) have the potential to generate multiple cell types and hold promise for future therapeutic applications. Although undifferentiated hESCs can proliferate indefinitely, hESC derivatives significantly downregulate telomerase and have limited replication potential. In this study we examine whether the replicative lifespan of hESC derivatives can be extended by ectopic expression of human telomerase reverse transcriptase (hTERT), the catalytic component of the telomerase complex. To this end, we have derived HEF1 cells, a fibroblast-like cell type, differentiated from hESCs. Infection of HEF1 cells with a retrovirus expressing hTERT extends their replicative capacity, resulting in immortal human HEF1-hTERT cells. HEF1-hTERT cells can be used to produce conditioned medium (CM) capable of supporting hESC growth under feeder-free conditions. Cultures maintained in HEF1-CM show characteristics similar to mouse embryonic fibroblast CM control cultures, including morphology, surface marker and transcription factor expression, telomerase activity, differentiation, and karyotypic stability. In addition, HEF1-hTERT cells have the capacity to differentiate into cells of the osteogenic lineage. These results suggest that immortalized cell lines can be generated from hESCs and that cells derived from hESCs can be used to support their own growth, creating a genotypically homogeneous system for the culture of hESCs. Stem Cells 2004;22: 972-980 STEM CELLS 2004;22:972-980 www.StemCells.com

Thiazolidinediones are insulin-sensitizing agents and in clinical use for the treatment of type II diabetes. Under specific experimental conditions, these molecules induce adipogenic differentiation of mesenchymal precursor cells at the expense of osteoblasts in vitro, suggesting possible negative effects on the skeleton. We measured effects of the thiazolidinedione BRL49653 on bone tissue of intact and estrogen-deprived skeletally mature adult female Wistar rats (6-9 months old). Weight gain and decreased plasma triglyceride levels confirmed the effectiveness of the treatment. However, no change in bone mass or fat marrow volume was observed in intact rats treated for 8 weeks with 5, 10, or 20 mg/kg of BRL49653. Study of marrow cultures established at necropsy revealed a higher responsiveness to adipogenic differentiation protocols of cultures established from the 10-mg/kg group compared to vehicle control. In a second study, the effects of thiazolidinedione treatment on the skeleton of estrogen-deprived rats were investigated. Application of 10 mg/kg of BRL49653 for 12 weeks resulted in enhanced bone loss (+31%; pQCT) and increased fat marrow volume (+117%; histomorphometry) compared to vehicle-treated OVX control. Interestingly, osteoblast number was comparable in both cases. Bone resorption parameters were significantly increased in the treatment group (+27% osteoclast number, +30% eroded surface). Enhanced bone loss due to treatment was consistently observed in the tibia, femur, and the lumbar spine. Our data indicate that thiazolidinediones may enhance bone loss induced by estrogen deprivation.

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