Differentiation of rat osteoblast-like cells in monolayer and micromass cultures

During intramembranous bone formation, preosteoblasts condense, differentiate into osteoblasts and deposit bone matrix. We compared the differentiation process of rat calvarial osteoblast-like cells inoculated as micromasses, which mimic the in vivo condensation process, with cells inoculated as monolayers. The cells were analysed morphologically at 1,2 and 3 weeks by light microscopy (alkaline phosphatase activity, mineralization), by transmission electron microscopy, and biochemically (collagen typing, alkaline phosphatase activity, protein and DNA content). The cells inoculated as monolayers formed alkaline phosphatase positive and mineralized nodules during the culture period. The cells inoculated as a micromass formed a large mineralized area consisting of smaller nodules. The ultrastructure of the cells in both culture systems showed the typical features of osteoblasts and osteocytes. The main difference between monolayer and micromass cultures was found after 1 week in culture. The cells inoculated as a micromass formed a multilayer of cells. The cytoplasm contained rER, mitochondria, vesicles and ribosomes. There were abundant collagen fibrils in membrane folds and in the extracellular matrix. This was in contrast to the cells in monolayer culture which showed hardly any collagen fibrils in the extracellular matrix. The promotion of the differentiation was also confirmed by biochemical data showing that the DNA content was lower in the micromass than in the monolayer cultures during the culture period. These results show that micromass, as compared to monolayer, culture promotes the differentiation of rat osteoblast-like cells in vitro.