The present invention relates to methods of preparing human embryonic stem cell lines using feeder cells (e.g., feeder cell layer, also known as feeder layer)-free, xeno-free culture systems and of stem cells which are capable of being maintained in an undifferentiated, pluripotent and proliferative state in culture which is free of xeno contaminants and feeder cells. 
Embryonic stem cells (ESCs), being totipotent, have the potential to develop into any type of cell and to generate any type of tissue, organ or body part, including a whole organism.  As such, it is expected that the ability to provide normal clonal human ESCs on demand and to manipulate the differentiation thereof will provide a powerful tool capable of driving radical advances in the biomedical, industrial and scientific fields.  Potential applications of ESCs are far ranging and include drug discovery and testing, generation of cells, tissues and organs for use in transplantation, production of biomolecules, testing the toxicity and/or teratogenicity of compounds and facilitating the study of developmental and other biological processes.  For example, diseases presently expected to be treatable by therapeutic transplantation of ESCs or ESC-derived cells include Parkinson's disease, cardiac infarcts, juvenile-onset diabetes mellitus, and leukemia (Gearhart J. Science 1998, 282:1061; Rossant and Nagy, Nature Biotech.  1999, 17:23). 
There are, however, significant hurdles to the practical exploitation of human ESCs. 
In order to maintain human ESC in an undifferentiated state ES cultures must be supplemented with factors which maintain cell proliferation, inhibit ES cell differentiation and preserve pluripotency. 
In addition, for cell replacement and tissue regeneration therapies human ESCs must be cultured in a complete animal-free environment and in the presence of well-defined culturing conditions which enable a complete reproduction of ES cultures. 
Currently practiced ES culturing methods are mainly based on the use of feeder cell layers which secrete factors needed for stem cell proliferation, while at the same time, inhibit their differentiation.  Feeder cell free systems have also been used in ES cell culturing, such systems utilize matrices supplemented with serum, cytokines and growth factors as a replacement for the feeder cell layer. 
Feeder-layer Based Cultures
Mouse feeder layers—The most common method for culturing ES cells is based on mouse embryonic fibroblasts (MEF) as a feeder cell layer supplemented with tissue culture medium containing serum or leukemia inhibitor factor (LIF) which supports the proliferation and the pluripotency of the ES cells [Thomson J A, Itskovitz-Eldor J, Shapiro S S, Waknitz M A, Swiergiel J J, Marshall V S, Jones J M. (1998).  Embryonic stem cell lines derived from human blastocysts.  Science 282: 1145-7; Reubinoff B E, Pera M F, Fong C, Trounson A, Bongso A. (2000).  Embryonic stem cell lines from human blastocysts: somatic differentiation in vitro.  Nat.  Biotechnol.  18: 399-404].  MEF cells are derived from day 12-13 mouse embryos in medium supplemented with fetal bovine serum.  Under these conditions mouse ES cells can be maintained in culture as pluripotent stem cells, preserving their phenotypical and functional characteristics.  However, unlike mouse ES cells, the presence of exogenously added LIF does not prevent differentiation of human ES cells (Thomson et al.,  1998, Science 282: 1145-7; Reubinoff et al.,  2000, Nat.  Biotechnol.  18: 399-404).  Furthermore, the use of feeder cells substantially increases the cost of production, and makes scale-up of human ES cell culture impractical.  Additionally, the feeder cells are metabolically inactivated to keep them from outgrowing the stem cells, hence it is necessary to have fresh feeder cells for each splitting of human ES culture.  Since at present, the separation of feeder cell components from embryonic cells prepared in bulk culture cannot be efficiently achieved, feeder cell layer-prepared ES cultures are not suitable for human therapy. 
ES cells can also be cultured on MEF under serum-free conditions using serum replacement supplemented with basic fibroblast growth factor (bFGF) [Amit M, Carpenter M K, Inokuna M S, Chiu C P, Harris C P, Waknitz M A, Itskovitz-Eldor J, Thomson J A. (2000).  Clonally derived human embryonic stem cell lines maintain pluripotency and proliferative potential for prolonged periods of culture.  Dev.  Biol.  227: 271-8].  Under these conditions the cloning efficiency of ES cells is 4 times higher than under fetal bovine serum.  In addition, following 6 months of culturing under serum replacement the ES cells still maintain their pluripotency as indicated by their ability to form teratomas which contain all three embryonic germ layers.  Although this system uses a better-defined culture conditions, the presence of mouse cells in the culture exposes the human culture to pathogens which restricts their use in cell-based therapy. 
Human embryonic fibroblasts or adult fallopian epithelial cells as feeder cell layers—Human ES cells can be grown and maintained using human embryonic fibroblasts or adult fallopian epithelial cells.  When grown on these human feeder cells the human ES cells exhibit normal karyotypes, present alkaline phosphatase activity, express Oct-4 and other embryonic cell surface markers including SSEA-3, SSEA-4, TRA-1-60, and GCTM-2, form teratomas in vivo, and retain all key morphological characteristics [Richards M, Fong C Y, Chan W K, Wong P C, Bongso A. (2002).  Human feeders support prolonged undifferentiated growth of human inner cell masses and embryonic stem cells.  Nat.  Biotechnol.  20: 933-6].  However, the major disadvantage of using human embryonic fibroblasts or adult fallopian tube epithelial cells as feeder cells is that both of these cell lines have a limited passage capacity of only 8-10 times, thereby limiting the ability of a prolonged ES growth period.  For a prolonged culturing period, the ES cells must be grown on human feeder cells originated from several subjects which results in an increased variability in culture conditions. 
Foreskin feeder layers—Human ES cells can be cultured on human foreskin feeder layer as disclosed in U.S. patent application Ser.  No.  10/368,045.  Foreskin derived feeder cell layers consist of a complete animal-free environment suitable for culturing human ES cells.  In addition, foreskin cells can be maintained in culture for as long as 42 passages since their derivation, providing the ES cells with a relatively constant environment.  Under these conditions the human ES cells were found to be functionally indistinct from cells grown with alternate protocols (e.g., MEF).  Following differentiation, ES cells expressed genes associated with all three embryonal germ layers, in vitro, and formed teratomas in vivo, consisting of tissue arising from all three germ layers.  In addition, unlike human fallopian epithelial cells or human embryonic fibroblasts, human ES cells cultured on foreskin feeder layers were maintained in culture in a pluripotent and undifferentiated state for at least 87 passages.  However, although foreskin cells can be maintained in culture for long periods (i.e., 42 passages), the foreskin culture system is not well-defined due to differences between separate batches.  In addition, human feeder layer-based culture systems would still require the simultaneous growth of both feeder layers and hES cells.  Therefore, feeder cells-free culturing systems have been developed. 
Feeder Cells-free Cultures
Stem cells can be grown on a solid surface such as an extracellular matrix (e.g., Matrigel™ or laminin) in the presence of a culture medium.  Unlike feeder-based cultures which require the simultaneous growth of feeder cells and stem cells and which may result in mixed cell populations, stem cells grown on feeder cells-free systems are easily separated from the surface.  The culture medium used for growing the stem cells contains factors that effectively inhibit differentiation and promote their growth such as MEF-conditioned medium and bFGF.  However, commonly used feeder cells-free culturing systems utilize an animal-based matrix (e.g., Matrigel™) supplemented with mouse or bovine serum, or with MEF conditioned medium [Xu C, et al. ( 2001).  Feeder cells-free growth of undifferentiated human embryonic stem cells.  Nat Biotechnol.  19: 971-4] which present the risk of animal pathogen cross-transfer to the human ES cells, thus compromising future clinical applications. 
As is further disclosed in U.S. patent application Ser.  No.  10/368,045, stem cells can be cultured on a matrix surface supplemented with foreskin-derived conditioned medium.  However, this medium, although present an animal-free system is yet not fully-defined in terms of culture composition. 
Recent attempts to culture human embryonic stem cells on a more defined culture composition utilized Matrigel or laminin surfaces and a mixture of growth factors.  However, as disclosed in U.S. Pat Appl.  No.  20030017589 under these conditions only 50-70% of the cells exhibited undifferentiated cell morphology.  In addition, the stem cells further exhibited a relatively short doubling time of 19 hours, which suggests that the stem cells became tumorigenic (see Amit et al, 2000, Dev.  Biol.  227: 271-8). 
There is thus a widely recognized need for, and it would be highly advantageous to have, a feeder cells-free, xeno-free culturing system, capable of maintaining human ES cells in a proliferative, pluripotent and undifferentiated state devoid of the above limitations.