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In 24 hours adherent marrow cell cultures (AMCC) were represented by single stretched fibroblasts. In non-feeder-supplemented AMCC most of the CFU-f remained single fibroblasts or passed through 1-3 cell doublings [correction of dudlings]. The colony stimulating activity of irradiated marrow cells was found to be diffuse across the Millipore filter, which seems to indicate that haemopoietic marrow cells produce a colony stimulating factor which is required for triggering the CFU-f from the Go-period of the cell cycle into cell proliferation.

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Colony-forming fibroblast precursors were detected in circulating blood of adult guinea pigs by CFUf in vitro colony assay. SFU amount to 0.9 +/- 0.2 (M +/- m) per 10(5) explanted leucocytes ranging from 0.04 X 10(-5) to 3.0 X 10(-5) for individual donors. The presence of collagen type I and lack of factor VIII antigen and of FC-receptors proved that CFU-derived colonies in the blood cultures were composed of fibroblasts.

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Bone formation in adult human bone marrow organ cultures is described. When culturing marrow fragments, thick bone lamina is formed. It has well-mineralized trabecular bone matrix with bone cells incorporated and is lined with osteoblast-like cells. In cultures of marrow deaggregated cell suspensions thin layers of the bone are only formed. Osteoclast-like cells develop in the cultures.

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Na-beta-glycerophosphate was added to the organ culture medium of mice marrow fragments. New bone ground substance is formed, its mineralization degree and morphology being highly dependent upon glycerophosphate addition and removal periods. Complete or partial ground substance mineralization occurs, in last case calcium insoluble salts may be found in young apical or in old basal parts only. Ground substance deposition and mineralization dynamics in vitro is discussed, paying attention to possibility of demineralization process.

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Adult mouse bone marrow cell suspensions prepared by trypsinization were cultivated in gelatin sponges on millipore filters. When HAWP filters were used, multilayer bone structure was formed. It contained mineralized ground substance, incorporated bone cells and osteoblast layer. With the use of AUFS filters, bone tissue developed not only on the top surface, but also inside the filter.

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Bone tissue composed of typical bone trabeculae containing ground substance with incorporated osteogenic cells and osteoblast layer was formed in organ cultures of bone marrow obtained from adult mice. Electron microscopic properties of the bone formed in vitro were identical to those of the bone tissue in vivo. The mineralization of the bone took place only in the presence of Na-beta-glycerophosphate in the culture medium.

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The cells of syngeneic bone marrow were added in the primary monolayer cultures of bone marrow, thymus and peritoneal fluid of the mouse. Discrete plaques with several dozens to several thousands of myeloid cells developed on the surface of fibroblast colonies within 2--7 days. It was shown that both the size and rate of formation of myeloid plaques were markedly higher in the culture of thymus fibroblasts than in those of bone marrow and peritoneal fluid fibroblasts. Hence, the stromal fibroblasts of different origin differ from each other by their ability to interact in vitro with hemopoietic precursor cells forming myeloid plaques.

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Stromal fibroblasts of bone marrow, spleen and thymus origin as well as fibroblasts of peritoneal fluid origin are shown to have no difference in the ability to bind morphologically distinguishable granuloid cells in monolayer cultures. On the contrary, thymus fibroblast colonies encourage the development of myeloid cell plaques (which are formed by bone marrow hemopoietic precursor cells) a great deal stronger than do bone marrow and peritoneal fluid fibroblast colonies. It means that their different origin may be revealed when they interact with hemopoietic precursor cells but not with differentiated myeloid cells.

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A culture system favouring myeloid cell formation directly on the surface of mouse stromal fibroblast colonies has been developed. It has been shown that the plaques formed are both much more numerous and essentially larger on stromal fibroblast colonies of thymus than of bone marrow origin. Consequently, hemopoietic precursor cells that form the myeloid plaques in the monolayer cultures interact dissimilarly with stromal mechanocytes of different origin.

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Explantation of suspensions of the cells isolated from the guinea-pig thymus by mechanical treatment leads to the formation in monolayer cultures of discrete fibroblast colonies, meanwhile explantation of the cells obtained by trypsinization results in fibroblast colonies apart from foci of the epithelium. These two types of the cells are discriminated on the basis of their fixation with antifibronectin and antifibroblast sera as well as by adhesiveness of epithelial cells and fibroblasts.

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In vitro adhesion of guinea pig bone marrow and spleen cells to cultured fibroblasts of bone marrow, spleen, thymus and peritoneal fluid origin was studied. Much better binding of myeloid cells to fibroblasts than to macrophages was observed, but no difference in adhesion to stromal fibroblasts of different origin was focund. The number of adhereing cells per one stromal mechanocyte depends on the number of adhesion sites on the surface of target cells. According to our data, adhesion sites for myeloid cells are much more numerous, than adhesion sites for lymphoid cells.

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