The capacity of bone grafts to repair critical size defects can be greatly enhanced by the delivery of mesenchymal stem cells (MSCs). Adipose tissue is considered the most effective source of MSCs (ADSCs); however, the efficiency of bone regeneration using undifferentiated ADSCs is low. Therefore, this study proposes scaffolds based on polycaprolactone (PCL), which is widely considered a suitable MSC delivery system, were used as a three-dimensional (3D) culture environment promoting osteogenic differentiation of ADSCs. PCL scaffolds enriched with 5% tricalcium phosphate (TCP) were used. Human ADSCs were cultured in osteogenic medium both on the scaffolds and in 2D culture. Cell viability and osteogenic differentiation were tested at various time points for 42 days. The expression of RUNX2, collagen I, alkaline phosphatase, osteonectin and osteocalcin, measured by real-time polymerase chain reaction was significantly upregulated in 3D culture. Production of osteocalcin, a specific marker of terminally differentiated osteoblasts, was significantly higher in 3D cultures than in 2D cultures, as confirmed by western blot and immunostaining, and accompanied by earlier and enhanced mineralization. Subcutaneous implantation into immunodeficient mice was used for in vivo observations. Immunohistological and micro-computed tomography analysis revealed ADSC survival and activity toward extracellular production after 4 and 12 weeks, although heterotopic osteogenesis was not confirmed -probably resulting from insufficient availability of Ca/P ions. Additionally, TCP did not contribute to the upregulation of differentiation on the scaffolds in culture, and we postulate that the 3D architecture is a critical factor and provides a useful environment for prior-to-implantation osteogenic differentiation of ADSCs.

Epithelial cells with osteoinductive potential (KB and WISH cell lines, transitional epithelium of several species) are rich in alkaline phosphatase activity. In contrast, cells devoid of osteoinductive ability are low in this enzyme activity. However, there were no differences between the two classes of cells with respect to acid phosphatase activity.

Osteoblastic and chondroblastic (i.e., osteogenic) cells belong to the stromal cell system, which is associated with bone marrow, and bone and is separate from the hematopoietic stem-cell system. Stromal stem cells are capable of producing reticular, fibroblastic, osteogenic, and adipose stromal lines. Marrow-derived osteogenic cells are a component of marrow stroma, which in vitro form fibroblastic-type colonies. These colonies are a heterogeneous population with varying enzymatic expressions and potencies that differentiate into fibroblastic, reticular, adipocytic, and osteogenic populations. It is postulated that these colonies are a component of the stem-and progenitor cell populations. Progenitors of osteogenic cells are widely distributed in the extraskeletal organs. On contact with an adequate inductor, they differentiate into chondro-and/or osteoblasts, thus producing ectopic (i.e., induced) cartilage and/or bone. Such osteoprogenitor cells were termed inducible osteoprogenitor cells, in contrast to the determined osteoprogenitor cells, which are present in the bone marrow stroma and produce bone spontaneously. To the class of determined osteoprogenitors also belong endosteal cells, periosteal cells, and osteoblastic established cell lines. There is no evidence of the presence of osteogenic cells in the blood and peritoneal fluid. The concept of mesenchymal cells as an osteoblastic precursor in adult organisms is open to question.

Background: Differences in duration of bone healing in various parts of the human skeleton are common experience for orthopaedic surgeons. The reason for these differences is not obvious and not clear.

2006. Metaplasia of chondrocytes into osteoblasts. Folia biol. (Kraków) 54: 75-80.Hypertrophic chondrocytes, commonly considered as terminal cells responsible for apoptotic elimination in endochondral osteogenesis, have the potential to switch their metabolic role and enter osteoblastic differentiation, based on histochemical, immunohistochemical, biochemical and cytological analysis.During endochondral osteogenesis, some osteocytes are derived from hypertrophic chondrocytes. Also non-hypertrophic chondrocytes are able to transform into osteogenic cells, and the bone thus formed is termed "transchondroid bone". In this review a summary and discussion of reports on chondrocyte transdifferentiation is given.