Ischemic stroke is the most common cause of permanent disability and the second leading cause of death in developed countries. A new hope for treatment of this disease is brought by stem cells which represent a specific cell population which can be potentially used to regenerate damaged neurons. In this study we used the mouse strain which under the control of Thy1 gene promoter expresses yellow fluorescent protein in all parts of neurons, including nucleus, perikaryon, axon, dendrites and spines. By using a visible signal of YFP we described emergence and differentiation of neurons in cell culture and during embryonic development. These observations have been compared to processes taking place after stem cells were transplanted into the brain of experimental animal affected by stroke. MATERIAL AND METHODS In this study mice embryo and adult mice from two mice strains, B6.Cg-Tg (Thy1-YFP) 16Jrs/J (YFP-16) and C57BL/6NCrl, were used. For neural stem cell isolation pregnant females were sacrificed on the 14.5th day of pregnancy, embryos were isolated and parts of the telencephalic wall were dissociated and cells were placed in a specific proliferation medium. After formation of neurospheres and their growth, they have been dissociated and placed on special substrates containing poly-D-lysine and laminin to enable differentiation of neurons. After fixation immunocytochemical staining was performed. Pregnant females were sacrificed for embryo isolation from 9.5th to 17.5th day of pregnancy. Isolated embryos were fixed, cryosected and prepared for immnohistochemical staining. Ischemic brain injury was induced by transient left middle cerebral artery occlusion (MCAO) in adult C57Bl/6 mice. One day after MCAO, stereotaxic transplantation was performed. Brains were isolated, cryosected and prepared for immnohistochemical staining. Sections were analyzed and recorded using confocal microscope Zeiss LSM 510 META. RESULTS Results of this study showed that the number of THY1 – YFP positive cells during differentiation in vitro was constant and it was in the range from 21.3 to 22.5% of the total cell count. THY1 – YFP positive cells were positive for SOX2 and all tested neuronal markers (MAP2, β3-tubulin and NeuN). During embryonic development THY1 – YFP positive cells were observed from E12.5 and their number has been constantly increasing in the central (spinal cord, rhomencephalon and telencephalon) as well as in the peripheral (spinal nerves, cranial nerves and retina) nervous system. THY1 - YFP positive cells during embryonic development were positive for the same markers as during in vitro differentiation. In stroke affected mouse brain transplanted THY1 – YFP cells differentiated and incorporated into the injured tissue. In control animals the cells at the site of transplantation. CONCLUSION Results of this study showed that mature neurons as well as neuronal progenitor cells express THY1 – YFP construct in in vitro conditions, during embryonic development and after transplantation into the MCAO brain. This suggests that in addition to analyses of neuronal differentiation in B6.Cg-Tg(Thy1-YFP)16Jrs/J mouse, neural stem cells isolated from this strain can be successfully used in studies where tracing of cells with neuronal fate is needed. Key words: ischemic stroke, stem cells, differentiation potential, THY1, YFP