Haemopoietic long‐term bone marrow cultures from adult mice show osteogenic capacity in vitro on 3–dimensional collagen sponges

Abstract. Adult murine bone marrow cells, cultured under conditions for long‐term haemopoietic marrow cultures, produce bone matrix proteins and mineralized tissue in vitro, but only after the adherent stromal cells were loaded on a 3‐dimensional collagen sponge. Provided more than 8 × 10⁶ cells are loaded, mineralization as measured by ⁸⁵Sr uptake from the culture medium, occurred in this 3‐dimensional configuration (3‐D) within 6 days. In contrast if undisrupted marrow fragments (containing more than 10⁷ cells) are placed directly on a collagen sponge, then it requires more than 10 days before significant mineralization can similarly be detected. The 2‐dimensional (2‐D) long‐term marrow culture system allows prior expansion of the stromal cells and some differentiation in an osteogenic direction within the adherent stromal layer. This is suggested by the presence of type I collagen and alkaline phos‐phatase positive cells. However, synthesis of osteonectin and a bone specific protein, osteocalcin, as well as calcification are only observed in 3‐D cultures. Electron microscopy demonstrated hydroxyapatite mineral on collagen fibres, osteoblast‐like cells, fibroblasts, cells which accumulated lipids, and macrophages which were retained on the collagen matrices. Irradiation of confluent long‐term bone marrow cultures, prior to their loading on the collagen sponge showed that haemopoietic stem cells are not necessary for the mineralization.