after initial establishment and derivation of human embryonic stem cells ( hesc ; thomson et al . , 1998 ; reubinoff et al . , 2000 ) first the infection free status of the donors has to be addressed , in europe couples are tested before any fertility treatment is offered , but the cells themselves have to be tested , too ( hovatta , 2011 ) . second , optimized good manufacturing practice ( gmp ) compliant systems must be implemented for derivation , scaling - up , banking of cells , and their corresponding quality assurance controls ( unger et al . , 2008 ; ausubel et al . , 2011 ) . the culture systems currently encounter the problem of suboptimal quality of xeno - free culture constituents . thus strategies are needed to overcome this difficulty ( sidhu et al . , 2008 ) . steps have been taken ; initially , hesc were grown on irradiated mouse feeders , later human fore - skin fibroblast were used ( hovatta et al . , 2003 ) , now we can use gmp compliant coating substrates specially designed for hesc growth ( rodin et al . , steps were also taken for the generation defined xeno - free gmp compliant medium for derivation and for expansion ( ludwig et al . , 2006 ; the potential of somatic cell reprogramming via expression of specific transcription factors and thus the generation of hesc induced pluripotent stem cells ( hipsc ; takahashi and yamanaka , 2006 ; takahashi et al . , 2007 ) has the advantage that they could be generated from the recipient patients own cells . there is no deep understanding of the effects that the reprogramming events have ; for instance on extracellular signaling ( okita et al . , 2011 ) , and the way that this could lead to immune reaction . hence fast reactivity is already present in healthy individuals for controlling any rapidly amplifying cells ( dhodapkar et al . , 2010 ) . un - silenced expression of the reprogramming factor oct-4 might then cause undesired immunoreactivity on the transplanted cells . immunoreactivity toward graft - derived hipsc of the same genetic background was also shown in animal models ( zhao et al . , 2011 ) . for successful reprogramming of somatic cells , many epigenetic changes must occur in an adequate manner . dna methylation changes during reprogramming must occur in important developmental and oncogenic regions , which increases the oncogenic risk of the reprogrammed cells ( doi et al . there is an additional risk for abnormalities and high tumorigenic potential , especially if c - myc is used as one of the transcription factors ( okita et al . , 2007 ) . also , genetic and epigenetic stability and large - scale genomic rearrangements after reprogramming and subsequent culture ( kim et al . , 2010 ; gore et al . , 2011 ; hussein et al . , 2011 ; lister et al . , 2011 ) it is also important to address the safety of long - term culture , as shown recently ; the occurrence of chromosomal rearrangements in long - term culture of 125 hesc and 11 hipsc ( amps et al . , 2011 ) . there is a consensus that undifferentiated pluripotent stem cells ( psc ) will not be used directly in any clinical transplantations procedure , but instead their psc derived differentiated cells recently , results using hesc derived dopaminergic neurons have shown correct phenotype differentiation and grafting potential given by no tumor formation , maintenance of the grafted cells , and functional recovery in parkinsonian animal models in mice , rats , and monkeys ( kriks et al . , 2011 ) . the protocols designed for this cell replacement assay were optimal regarding the phenotype , quantity of the cells , functionality , and immunological properties . integration of transplanted cells was achieved when single cells were transplanted , the use of proper biodegradable scaffolds must also be considered . in addition to this initial report regarding the neural lineage , differentiation protocols for other cell types are needed . even if transplantation in animal models is successful , it is important to generate safety strategies before clinical trials to appropriately remove undifferentiated psc from their psc derived therapeutic cells . strategies such as inserting suicide gene ( drobyski et al . , 2003 ; uchibori et al . , 2009 ) might have controversial outcomes under clinical trials given their safety ( yi et al . alternatively , strategies such as removal of undifferentiated cells using antibodies might be safer ( tang et al . , 2011 ) . as discussed earlier , an optimal engraftment and cell replacement strategy should account for a minimal immune reaction in the recipient . this immune reaction occurs because the immune system of the recipient recognizes the grafted cells as foreign material or mismatched cellular components and thus generates a cascade of events that ultimately results in destruction and rejection of the grafted cells . this destruction can also compromise the recipient s immune status ( petersen et al . , 1975 ) . immunoreactivity toward the graft is mainly caused by t cell response toward unmatched major histocompatibility complex ( mhc ) ; in humans called human leukocyte antigen ( hla ) . if the profile is unmatched , it will result in rejection ( lechler et al . , 2005 ) . this rejection can occur via direct allorecognition of the donor antigen presenting cells ( apc ) or via indirect recognition of apoptotic cells ingested by the recipients apc , in both cases apcs presenting unmatched mhcs ( walsh et al . , 2004 ) . several groups have studied mhc profiles of hesc and their differentiated cells ( swijnenburg et al . , 2008 ; pearl et al . , 2011 ) . findings are that undifferentiated cells express mhc i antigens , though at low levels compared with somatic cells ; but they do not express mhc ii molecules ( drukker et al . , 2002 , 2006 ; li et al . , 2004 ) . during in vitro differentiation toward germ lineages , embryoid body ( eb ) formation , or teratoma formation mhc i expression increases dramatically ( drukker and benvenisty , 2004 ) . also culture methods of hesc can change antigen expression levels ( rajala et al . , 2010 ) . careful selection of culture conditions , both for the undifferentiated hesc and for their differentiated derivatives is needed . human embryonic stem cells adopts the expression of non - human cell surface markers if exposed to such substances during culture ( martin et al . , 2005 ; hisamatsu - sakamoto et al . , 2008 ) hence , optimal culture conditions must be xeno - free from the initial derivation and onward . these culture conditions must be carefully analyzed and scientific consensus must be achieved in order to raise current methodologies . challenges with the immunoreactivity of the transplantable cells could be addressed by rigorous immunosuppressive treatments . unfortunately , this is not desired , since there is a clear correlation between the length and intensity of exposure to immunosuppressive therapy and post - transplant risk of malignancy and tumor aggressiveness ( gutierrez - dalmau and campistol , 2007 ) . an interesting solution is costimulatory blockage of t cell response ( grinnemo et al . , 2006 , 2008 ; swijnenburg et al . , 2008 ; pearl et al . , 2011 . this immunosuppression strategy will generate tolerance to the grafted cells and thus increase graft survival ; initial pharmaceutical agents have been developed and pending clinical applications to the fda are to give in the near future more information . in this mini - review we highlighted the most important areas to be considered under a cell replacement therapy . the possibility of using hipsc derived therapeutic cells in cell replacement therapies requires still long - term studies in non - human animal models addressing the questions of immunogenicity , epigenetic and genetic stability of these cells , and the optimized differentiation of the cells . the importance of profiling immunogenic markers as part of the stem - ness characterization and profiling of cells allocated in stem cell banks must be consider . such information has to be well protected so that it will not be lost in any given situation . adequate culture conditions , supporting correct immunogenicity of the cells under a transplantation assay is also required . next , the management of immunosuppression schemes must aim to a minimal time influencing the immunological status of the recipient . from all the information obtained , these profiles can then be used in combination with methodologies focused at monitoring the status of the transplanted cells . in a given scenario that undesired cells persist in the transplant , adequate counteracting actions have immediately to be taken . such possibilities have to be tested and the removal of undesired effects confirmed before starting cell transplantations . the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
human embryonic stem cells ( hesc ) and induced pluripotent stem cells ( hipsc ) are an attractive cell source for regenerative medicine . 
 these cells can be expanded to vast numbers and can be differentiated to many desired pluripotent stem cells ( psc ) derived therapeutic cells . 
 cell replacement bears promises , but also challenges 
 . the introduction of exogenous cells in a recipient must address several different topics ; its safety , the exclusion of tumor formation , the immunological response and possible rejection , the cells cleanliness and their biological quality , and quantity representing the functionality of the psc derived therapeutic cells . 
 tumor formation requires the removal of any psc remaining after differentiation . 
 immunological rejection can be addressed with immunomodulation of the cells and the recipient . 
 cleanliness can be optimized using good manufacturing practice quality systems . at last 
 , the functionality of the cells must be tested in in vitro and in animal models . after addressing these challenges , 
 precise strategies are developed to monitor the status of the cells at different times and in case of undesired results , corresponding counteracting strategies must exist before any clinical attempt .