The effect of the extracellular matrix on differentiation of bone marrow stromal cells to osteoblasts

The effect of the extracellular matrix on differentiation of bone marrow stromal cells into osteoblasts was studied in culture by morphological and biochemical analyses. Inoculation of bone marrow cells on a type I collagen gel produced calcified nodules in approximately 2 weeks, whereas inoculation on conventional plastic dishes failed to produce any calcified structures even after prolonged culturing. By transmission electron microscopy, the calcified nodules were shown to be similar in structure to the mineralized bone in animal tissue, in terms of hydroxyapatite deposits in matrix vesicles and collagen fibrils. Furthermore, the cells in the calcified nodules exhibited high levels of alkaline phosphatase activity and secreted a large amount of osteocalcin, indicating that the differentiation of bone marrow stromal cells into osteoblasts definitely occurred through the effect of the type I collagen gel. In contrast, when bone marrow cells were cultured on dishes coated with type I or IV collagen, they failed to produce any nodules, high alkaline phosphatase activity or osteocalcin. It was concluded that collagen in gel form playsan important role in the differentiation of bone marrow stromal cells to osteoblasts.