The Osteogenic Potential of Culture-Expanded Rat Marrow Mesenchymal Cells Assayed In Vivo in Calcium Phosphate Ceramic Blocks

When whole marrow is introduced into porous calcium phosphate ceramic, bone forms on the walls of the pores. As an extension of earlier studies, bone marrow cells derived from the femora of inbred rats were introduced into tissue culture, and the adherent cells were cultivated, mitotically expanded, subcultured, harvested, placed in small cubes of porous calcium phosphate ceramic, and grafted into subcutaneous sites of syngeneic rats. Primary marrow-derived, cultured mesenchymal cells introduced into ceramic showed strong osteogenic potential, with bone forming in the pore regions of ceramic as early as two weeks after in vivo implantation; cartilage was observed infrequently in pores that appeared to be avascular. Osteogenesis could be observed after the 18th subculture (over 36 population doublings) when the cells were tested in ceramic at subcutaneous sites, whereas chondrogenesis was observed with only the first and second subcultured cells in the ceramic delivery vehicle. With increasing numbers of subcultures, the initiation of osteogenesis and the apparent rate of bone formation declined, and the course of osteogenesis was delayed. Cultured, marrow-derived mesenchymal cells, even after the 21st subculture (over 40 population doublings), exhibited a positive histochemical reaction for alkaline phosphatase. However, the in vivo osteogenic potential of these cells was not correlated with their alkaline phosphatase activity. The implantation of cell pellets or the injection of cell suspensions of fresh or cultured, adherent marrow cells never produced bone or cartilage in heterotopic sites. These data indicate that porous ceramic provides an excellent delivery vehicle for cells that are capable of osteogenic expression and suggest that the composite graft of marrow-derived mesenchymal cells and porous ceramic may be useful for repair of massive bone defects. It may be possible to culture marrow mesenchymal cells as a source for reparative cells for implantation back into autogeneic sites.