On 2014 May 15, G L Francis commented:

In setting out to produce a simplified chemically defined culture medium for human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) the authors demonstrated removal of bovine serum albumin (BSA) and other constituents was possible. By judicious screening the resultant optimized medium (E8) was greatly simplified compared to the starting medium (TeSR) and provided comparable outcomes for long term undifferentiated proliferation of both human ESCs and iPSCs.

The authors appear to show quite convincingly that in the original TeSR medium BSA serves the purpose of neutralizing the potential toxicity of β-mercaptoethanol (BME) (see Fig 1 : Title. ‘’Albumin is not required for hESC culture’’). The only question I raise is about the veracity of the overall conclusion is that albumin presence in the absence of BME does not enhance the proliferation (I assume equals combined survival of plated ESCs and subsequent proliferation) measured at 120h after plating in TeSR-based medium, as depicted in Fig.1 panel (f).The first bar in Fig.1 panel (f) representing the activity for ‘’complete TeSR medium (with BSA & BME)’’, is stimulated a further two- fold in the ‘’without BME’’ medium (presumably with BSA!) represented in the last bar of that panel (f). If that is the case, does that not indicate that addition of albumin significantly improves the performance of the medium in the long term proliferation of hESCs – apparently at odds with the conclusions argued in the Discussion of the paper?

Moreover, in Figure 2 titled “Essential medium components for human ESC survival and proliferation” in panel (c) the improvement in proliferation at 129 h for plated hESCs over the starting TeSR medium by a number of additions to a base E8 medium was only 40% (compare this to the improvement of TeSR by TeSR –BME+BSA above in Fig.1). Admittedly the further addition of other components, and finally Nodal, led to incremental improvement over the base components of E8.

I understand the importance of eliminating an animal or human sera sourced component (also what about the transferrin?) This still prompts the question as what would have been the result if albumin was added to the final E8 medium in terms of long term proliferation of hESCs in this study. For the experimental evaluation of this question the best performing batch of BSA would have been most appropriate, however, to address the issues raised about the use of this xenobiotic material, a human recombinant source of albumin could have been evaluated. Human recombinant albumin is now produced at scale under GMP conditions and I believe the price has become more acceptable - I agree the final acceptance of that would rely on a cost benefit analysis at the process level.

On 2014 May 15, G L Francis commented:

In setting out to produce a simplified chemically defined culture medium for human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) the authors demonstrated removal of bovine serum albumin (BSA) and other constituents was possible. By judicious screening the resultant optimized medium (E8) was greatly simplified compared to the starting medium (TeSR) and provided comparable outcomes for long term undifferentiated proliferation of both human ESCs and iPSCs.

The authors appear to show quite convincingly that in the original TeSR medium BSA serves the purpose of neutralizing the potential toxicity of β-mercaptoethanol (BME) (see Fig 1 : Title. ‘’Albumin is not required for hESC culture’’). The only question I raise is about the veracity of the overall conclusion is that albumin presence in the absence of BME does not enhance the proliferation (I assume equals combined survival of plated ESCs and subsequent proliferation) measured at 120h after plating in TeSR-based medium, as depicted in Fig.1 panel (f).The first bar in Fig.1 panel (f) representing the activity for ‘’complete TeSR medium (with BSA & BME)’’, is stimulated a further two- fold in the ‘’without BME’’ medium (presumably with BSA!) represented in the last bar of that panel (f). If that is the case, does that not indicate that addition of albumin significantly improves the performance of the medium in the long term proliferation of hESCs – apparently at odds with the conclusions argued in the Discussion of the paper?

Moreover, in Figure 2 titled “Essential medium components for human ESC survival and proliferation” in panel (c) the improvement in proliferation at 129 h for plated hESCs over the starting TeSR medium by a number of additions to a base E8 medium was only 40% (compare this to the improvement of TeSR by TeSR –BME+BSA above in Fig.1). Admittedly the further addition of other components, and finally Nodal, led to incremental improvement over the base components of E8.

I understand the importance of eliminating an animal or human sera sourced component (also what about the transferrin?) This still prompts the question as what would have been the result if albumin was added to the final E8 medium in terms of long term proliferation of hESCs in this study. For the experimental evaluation of this question the best performing batch of BSA would have been most appropriate, however, to address the issues raised about the use of this xenobiotic material, a human recombinant source of albumin could have been evaluated. Human recombinant albumin is now produced at scale under GMP conditions and I believe the price has become more acceptable - I agree the final acceptance of that would rely on a cost benefit analysis at the process level.