29 clonal strains of the second and third passages obtained from primary cultures of rabbit bone marrow were placed in diffusion chambers and implanted intraperitoneally to homologous animals. Studies have shown that 45% of bone marrow monoclonal strains have osteogenic potencies. Exceptionally reticular tissue contained 9 chambers, but together with bone and cartilage tissue, it was present in 13 more chambers, which accounted for 76% of all strains. In chambers of type O, where differentiation of both bone and cartilage tissue was possible, 16 strains were studied. In four chambers (25%), both bone and cartilage tissue was formed. It should be noted once again that in the studies of R. Chaylakhyan and coauthors (2001), individual progenitor cells underwent from 31 to 34 doublings in the composition of the cell strain, and their progeny amounted to 0.9-2.0 x 109 cells. The number of mitoses to which the precursor cells of the polyclonal strains were exposed did not practically differ from that of the cells of monoclonal strains. Moreover, the rate of development of polyclonal strains, especially in the first phase of their formation, substantially depended on the number of colonies used to initiate the strains. Diploid strains of human embryonic fibroblasts (WI-38) when reclining at 12-15th doubling levels also formed colonies that differ in cell diameter and cell content. Large colonies containing more than 103 cells comprised only 5-10%. With an increase in the number of divisions, the percentage of large colonies decreased. Mono- and polyclonal strains of stromal bone marrow fibroblasts maintained a diploid set of chromosomes after 20 or more doublings, and their development trend was comparable with the dynamics of development of diploid strains of embryonic fibroblasts. Analysis of the differentiation potential of individual bone marrow stromal progenitor cells, carried out by transplantation of monoclonal strains into diffusion chambers, showed that half of them are osteogenic. Large colonies accounted for 10% of their total number. Consequently, the number of osteogenic colony-forming cells corresponded to approximately 5% of their total population. In the total mass of osteogenic progenitor cells identified by the authors, there were cells capable of simultaneously forming bone and cartilage tissue. At the same time, it was first established that for these two types of tissue in the adult body there is a common progenitor cell: 25% of the tested clones were created by similar cells, and their number among the total population of progenitor cells was at least 2.5%. Thus, heterotopic transplantation of individual clones of bone marrow fibroblasts has opened up new aspects of the structural organization of the population of mesenchymal progenitor cells. Stromal progenitor cells capable of transferring a specific microenvironment at once for all hematopoiesis sprouts were found, the number of which among the large clones studied in different models is from 5 to 15% (0.5-1.5% of the total number of detected progenitor cells). Along with clones transferring a complete bone marrow microenvironment, there are precursor cells determined only for osteogenesis, which form bone tissue during transfer in the open system that does not support the development of hematopoiesis. Their number from the total number of progenitor cells is 1.5-3%. Some of these cells are able to form bone tissue with a limited period of self-maintenance. Therefore, the population of stromal progenitor cells is heterogeneous in its differentiation potential. Among them there is a category of cells claiming to be the role of stem stromal cells capable of differentiating in all three directions characteristic of bone marrow stromal tissue, forming bone, cartilage and reticular tissue. The data given allow us to hope that withof various cell markers, it will be possible to determine the contribution of each type of stromal cells to the organization of a specific microenvironment and support of hematopoiesis in dexter cultures.