ISSN 1990-519X, Cell and Tissue Biology, 2010, Vol. 4, No. 5, pp. 419–423. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.M. Mikhailov, E.V. Kaminskaya, B.V. Popov, S.N. Kuzovatov, N.S. Skripkina, G.P. Kosyakova, A.M. Zaichik, T.M. Grinchuk, N.N. Nikolsky, 2010, published in Tsitologiya, Vol. 52, No. 10, 2010, pp. 853–858. 419 Mesenchymal or stromal stem cells (MSCs) from the bone marrow of adult mammals are the most examined stem cells. The interest in MSCs as an instrument for cell therapy and tissue engineering accounts for their multilinage potential, as well as by their accessibility and relatively easy identification, cultivation, and fractionation. MSCs differentiate spontaneously onto bone tissue with bone marrow (Friedenstein and Lalikina, 1973; Friedenstein, 1976). MSCs can be induced to differentiate in vitro into bone, cartilaginous, adipose, cardiomyocyte, and neuronal tissues (Pittenger et al. 1999; Woodbury et al., 2000; Vladimirskaya et al., 2005; Tseng et al., 2007; Liu et al., 2009). There are data on spontaneous genome destabiliza- tion and the tumor transformation of human and mouse MSCs during long-term cultivation (Rubio et al., 2004, 2008; Miura et al., 2006; Li et al., 2007; Grigoryan and Kruglikov, 2009; Røsland et al., 2009). However, other workers believe that human MSCs do not undergo spontaneous oncogenic transformation after long-term in vitro culture (Choumerianou et al., 2008; Bernardo et al., 2009). Transformed cells are known to retain their capac- ity for differentiation during long-term cultivation in vitro. It is assumed that the spontaneous malignization of cultivated cells in vitro returns cells to living condi- tions in vivo and, therefore, discloses differential potential of transformed cultivated cells (Vakhtin, 1974). The assessment of the differential potentiation of tumor stem cells allows one to evaluate epigenetic changes during successive events of MSC differentia- tion. Previously, we have found that MSCs from trans- genic C57BL/6 mice that express GFP (MSC-GFP) transplanted into muscles of mdx mice generate tumors in 2 months (Popov et al., 2008, 2009). Cells exhibited tumorigenic properties at the 43rd–45th, but not 15th, passages (Popov et al., 2008, 2009). Tumors had areas of differentiated tissues. MSC-GFP cells grafted to C57BL/6 did not produce tumors. Characteristics of Tumors Developed in mdx Mice after Transplantation of GFP-Positive Mesenchymal Stem Cells Isolated from Bone Marrow of Transgenic C57BL/6 Mice V. M. Mikhailov a , E. V. Kaminskaya a , B. V. Popov a , S. N. Kuzovatov a , N. S. Skripkina b , G. P. Kosyakova c , A. M. Zaichik d , T. M. Grinchuk a , and N. N. Nikolsky a a Institute of Cytology, Russian Academy of Sciences, St. Petersburg, Russia b St. Petersburg Pathological Anatomy Bureau, St. Petersburg, Russia c Research Institute of Agricultural Animals Genetics, St. Petersburg, Russia d Postgraduate Medical Academy, St. Petersburg, Russia e-mail: vmikhailov@mail.cytspb.rssi.ru Received March 15, 2010 Abstract—The purpose of the work was morphological and histochemical examination of tissue differentia- tion in tumors developed in mdx mice after the intramuscular transplantation of GFP-positive mesenchymal bone-marrow stem cells (MSC-GFP) derived from C576BL/6 transgenic mice and cultivated for 43–45 pas- sages. These cells did not generate tumors in syngeneic adult C57BL/6 mice. The tumors were classified as mesenhymomas, fibrosarcomas, and sarcoma. Adipocyte and chondrocyte clusters, as well as bone areas with erythroid, myeloid, and thrombocyte hematopoiesis and neural tissue with glia cells were observed inside of tumors. Types of tissue tumor differentiation were similar to those described in the literature for MSCs induced to differente in vitro by specific treatment. However, the differentiation spectrum in MSC-GFP- produced tumors was broader than the differentiation of tumors derived from adult mouse MSCs spontane- ously transformed or transfected in vitro. The results presented here, along with our previous data, demon- strate that the transfection of stem cells, including totipotent stem cells, with genetic constructs is accompa- nied by the destabilization of the cell genome, even if the activity of inserted gene (GFP) does not affect gen- eral cell functioning. Key words: GFP-positive mesenchymal stem cell, transplantation, mdx mice, tumors, differentiation. DOI: 10.1134/S1990519X10050020