Osteogenesis in transplants of bone marrow cells.

After heterotopic (e.g. subcutaneous) transplantation of bone marrow, haemopoiesis in the graft ceases; reticular tissue develops instead, and later bone is formed (Denis, 1958). The result can be achieved by grafting either free pieces of bone marrow or those placed in diffusion chambers (Petrakova, Tolmacheva & Friedenstein, 1963; Rosin, Freiberg & Sajnek, 1963). In the case of free transplantation the bone formed is later filled with bone marrow. After transplantation in diffusion chambers haemopoiesis does not recur despite the development of a considerable mass of bone in the chambers (Friedenstein, 1965). The population of bone marrow cells is very heterogeneous, including haemopoietic cells, reticular cells and endosteum elements. According to generally accepted views this population is a mixture of individual cell lines capable of mutual transformations within certain limits (Maximov, 1927; Burwell, 1964). After transplantation some of the pathways of differentiation open to bone marrow tissue (formation of reticular and bone tissues) are stimulated, while others (haemopoiesis) are arrested. This effect could be ascribed to the death of haemopoietic stem cells and to proliferation of cells responsible for the development of reticular and bone tissue. It may, however, depend upon transformation of haemopoietic cells to reticular and osteogenic cells. An analysis of these possibilities encounters difficulties as there are no reliable data concerning the potentialities for transformation of different types of bone marrow cells, and especially of the stem cells. There is no convincing evidence for the dependence of various histogenetic pathways in bone marrow tissue (haemopoiesis, osteogenesis or the development of reticular tissue) upon individual differences of the stem cells concerned. All of them could be provided by one common line of stem cells whose differentiation is controlled by conditions within the population. Osteogenesis appearing in bone marrow grafts may serve as a model of