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What are the challenges in utilizing a serum-free, feeder-free, defined cell culture system for human embryonic stem cells?
With the field of stem cell biology rapidly evolving, researchers today face new challenges in translating bench science research towards practical clinical applications. The potential utility of clinically relevant stem cell types, such as pluripotent human embryonic stem cells (hESCs), in therapeutic applications for regenerative medicine is still technically limited. This is due to their reliance on established cell culture systems that utilize mouse embryonic fibroblasts (MEFs) as a feeder cell layer and the inclusion of Fetal Bovine Serum (FBS). Use of these traditional cell culture systems is extremely problematic for researchers hoping to avoid variability caused by undefined biological substances and factors derived from FBS and MEFs. Variability causes inconsistent results, which makes manufacturing scale-up challenging due to lack of reproducibility. It is also necessary to eliminate animal-derived components that may have the potential for transmission of pathogens in stem cell therapy applications.
Recent advancements have produced several alternatives designed to overcome the challenges associated with traditional cell culture methods and create a more ‘defined’ cell culture system. Today several companies have responded to the shift in market need for hESC by developing commercially available serum replacement. The replacement is packaged as part of a ‘defined’ cell culture system, a basement membrane mixture extract, and other extracellular matrices substrates. In addition, others have introduced the use of alternative feeder cells, such as human foreskin fibroblasts, to replace MEFs. Thes alternative feeder cells can be utilized as a source of conditioned medium to maintain hESCs. However, these new advancements have drawbacks and similar disadvantages to the traditional cell culture method. For example, several commercially available serum replacement media contains animal-derived components, including basement membrane mixture derived from mouse sarcoma and comprised of undefined components. Similar to serum and MEFs, many of these alternative solutions are not practical for commercial scale-up, have batch to batch variability, and require additional cell culturing of another cell type.
Many scientists have discovered that by switching to these new cell culture systems they must have a set criteria and validation assays to ensure that the characteristics of hESCs are maintained. Although it is unclear the long-term effects of these new culture conditions, researchers understand key indicators of a healthy cell culture (i.e. morphology, pluripotency, and differentiation capability). It is critical that these systems are able to maintain the inherent, unique properties of hESCs through specific validation endpoints such as the presence of pluripotency markers, stable karyotype, and hESCs in an undifferentiated state over long-term passaging, in vitro propagation, and teratoma formation in immunocompromised mice. Many hurdles still remain in the development of a serum-free, feeder-free, defined cell culture system. Much promise in progress towards creating such a system may be in the use of recombinant human proteins and elucidating the necessary components required for the regulation of hESCs self-renewal and differentiation capacity.