36 Lucrări ştiinţifice Zootehnie şi Biotehnologii, vol 40(1), (2007), Timişoara EXAMINATION OF THE GERM CELL CHIMERA FORMING POTENTIAL OF MOUSE EMBRYONIC STEM CELLS EXAMINAREA CAPACITĂŢI CELULELOR EMBRIONARE PLURIPOTENTE PENTRU FORMAREA ORGANISMELOR HIMERE GERMINALE CÂRSTEA V. B*., DOBÓ KRISZTINA*, LICHNER ZSUZSANNA*, STANCA CLAUDIA**, ILIE DANIELA***, GÓCZA ELEN* *Agricultural Biotechnology Center, Gödöllő, Hungary **Faculty of Animal Science and Biotechnology, Timişoara, România ***Research and Development Station for Bovine Raising - Arad The aim of this study was to examine the factors, which influence the chimera forming potential of mouse embryonic stem cells (ES cells). In our work, we examine the chimera producing ability of R1 and R1/E mouse ES cell lines. We found that the passage number affects chimera-forming capability of the ES cells. With the increasing of the passage number, it could be getting less chimera animal, and only the R1/E ES cell line derived cells could contribute to the germ cells. At first, we compared the marker of pluripotency using immunostaining and RT PCR, but we could not find any difference between the R1 and R1/E cell in this way. At chromosome analysis, we found, that the number of aneuploid cells, in R1 ES cell line, dramatically increased after 10 passages. We thought that the reason is that during the cell division Y chromosome could not arrange correctly between the two newly derived progeny cells. To prove our conception, we made X and Y- chromosome FISH analyses. We found, that the aneuploid R1 and R1/E ES cells contain only one X and one Y chromosome, so not the loss of Y chromosome cause the problem at the germ cell formation. At last, we made the karyotype analysis of R1 and R1/E ES cells at different passages. The karyotype analysis demonstrated that in the case of R1 ES cell line, the 41 and 42-chromosome containing cells hold trisomy. With the increasing of the passages number, the number of trisomy containing aneuploid cells increased. The aneuploid ES cells can contribute to the different tissuses of chimera animals, but cannot form viable germ cells. Key words: Es cells, chimera, FISH, karyotype Introduction The methods developed for transgenic mouse production were improved in the last few decades. With the establishment of the first mouse embryonic stem cell lines became possibly to develop new method for introduction external genetic information into the mouse genome. Although mouse ES cells have supported discovery in myriad research fields, their value was established as a tool to enable