In vitro osteogenesis assays: influence of the primary cell source on alkaline phosphatase activity and...

In trabecular bone fracture repair in vivo, osteogenesis occurs through endochondral ossification under hypoxic conditions, or through woven bone deposition in the vicinity of blood vessels. In vitro osteogenesis assays are routinely used to test osteoblastic responses to drugs, hormones, and biomaterials for bone and cartilage repair applications. These cell culture models recapitulate events that occur in woven bone synthesis, and are carried out using primary osteoblasts, osteoblast precursors such as bone marrow-derived mesenchymal stromal cells (BMSCs), or various osteoblast cell lines. With time in culture, cell differentiation is typically assessed by examining levels of alkaline phosphatase activity (an early osteoblast marker) and by evaluating the assembly of a collagen (type I)-containing fibrillar extracellular matrix that mineralizes. In this review, we have made a comparative analysis of published osteogenic assays using calvarial cells, calvaria-derived cell lines, and bone marrow stromal cells. In all of these cell types, alkaline phosphatase activity shows similar progression over time using a variety of osteogenic and mineralizing media conditions; however, levels of alkaline phosphatase activity are not proportional to observed mineralization levels.