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When freshly isolated rabbit marrow cells were cultured either in vitro or in diffusion chambers in vivo, the hemopoietic cells disappeared and there was a proliferation of the stromal cell population. The colonies formed in vitro were mainly fibroblastic, and this cell type predominated in confluent cultures. Staining for alkaline phosphatase activity and for the Von Kossa reaction was negative in in vitro cultures. However, marrow cell suspensions or fibroblasts harvested from in vitro culture of marrow cells, gave rise to a mixture of bone, cartilage and fibrous tissue in diffusion chambers implanted into the peritoneal cavity. In contrast, only a soft fibrous tissue developed from spleen fibroblasts in diffusion chambers. Differentiation of osteogenic tissue within diffusion chambers fell into two categories: (1) Formation of bone in a fibrous layer surrounding cartilage; (2) intramembranous bone formed directly within fibrous tissue unassociated with cartilage. In both cases alkaline phosphatase activity appeared before the onset of mineralization, and decreased as the first signs of mineral became apparent. The present results suggest that postnatal marrow contains osteogenic precursors with the potential to differentiate via either of the two major paths followed during skeletal development in the embryo. Clonal analysis of the marrow stromal cell population will be required to clarify whether osteo-, chondro-, and fibrogenic cells are the products of one stromal cell line modulated by the microenvironment, or whether there are distinct cell lines for each type.
Ashton et al. (Mon,) studied this question.