Here, I provide an important update.
What’s going on now?
Here I provide answers to the key questions.
How many papers are there on iPS cells?
An interesting and unexpected trend for this still early stage in the evolution of the iPS cell field is (as you can see from the figure at left) that the total number of iPS cell papers is leveling off dramatically. Depending on how things go it is even conceivable that for the first time in the field’s existence that the number of papers in one year (2011) could be lower than in the previous year (2010). Although the verdict is out on that, nonetheless it is clear that the tremendous velocity in rate of increase in the number of iPS cell papers is now history and is likely to stay that way.
Why is the number of papers leveling off so strikingly? There are a number of possible explanations. First, the field is “maturing” and some of the initial excitement may have worn off. Another notion is that a near maximal number of labs around the world are working and publishing on iPS cells given the resources such as space and funding and total number of people available. It would not surprise me to see the total number of iPS cell papers actually fall in 2012, but in this exciting field things are still very unpredictable.
What is going on with citations?
Another trend that is somewhat distinct from the leveling off of total papers is the ever upward trend in the number of citations mentioning iPS cells in way one or another (see figure to the right). I should note that for my searches for this post I used ISI and used the title search parameter to include “iPS cell(s)”, “induced pluripotent stem cell(s)”, and “induced pluripotency”. It is possible I missed some papers using these parameters, but it should have done a pretty good job of being inclusive.
The fact that the number of citations continues to grow even as the number of papers is leveling off is not surprising given that all the papers from 2006-2011 published on iPS cells are available to be cited by new papers. Still the growth in the number of iPS cell-related citations is slowing down relatively speaking and I expect that while it will continue to increase, looking forward the increases will be less dramatic.
What institutions are publishing the most iPS cell papers?
Here there are some expected places but also some surprises amongst the top 20 (see below). The top institution was Kyoto University with 77 followed closely behind by Harvard with 74 (32 of which mentioned specifically its stem cell institute). Then we have a dramatic dropoff to include Univ. of Tokyo at 36, Chinese Academy of Sciences (CAS) at 34, and Stanford at 32.
I am very impressed by the number from CAS and China more generally.
In terms of California institutions, Stanford was #6 out of the top 20, the Salk was #10, UCSD was #15, Gladstone Institute (UCSF) was #16, and UCLA was #18. Overall I would say that is pretty good.
What are all these iPS cell papers focused on in terms of research areas?
There are still a very large number of iPS cell papers that are almost entirely methodological. These are of course crucial, but some might argue incremental in terms of their contributions. Still, very often they are published in very high impact journals. Many iPS cell papers are nowadays on disease modeling. Relatively speaking, as a percentage of all iPS cell papers, there seem to be far fewer papers on the basic science of induced pluripotency than there were in the past, making one wonder if the field has gotten ahead of itself.
Who are the scientists authoring these iPS cell papers?
The leader of the field is Shinya Yamanaka, who was the discoverer of iPS cells. Another top iPS cell scientist in terms of productivity is George Daley. However, the number of authors has grown and broadened dramatically in the last few years, which I think reflects the maturing of the field and the increased interest.
What journals are publishing iPS cell papers?
In the past some scientists raised concerns that we blogged (here) about that the research from the iPS cell field was predominantly being published in too few journals, which had a dramatic effect on the course of the research and could promote “group think” if an unusually high fraction of papers were being written, reviewed, and supervised by a relatively very small group of people.
Since that time the number and diversity of journals publishing papers that are primarily about iPS cells has increased substantially. The top three journals in order of iPS cell publications are Blood (45), Stem Cells (42), and Cell Stem Cell (37).
For me, a big surprise is journal #4, which is PLoS One with 34 papers.
What this tells me is that researchers working on iPS cells are fed up with being told that their papers are not sexy enough for certain journals, and hence turn to PLoS One which judges papers based on a peer review process focused on the quality of the science, not some subjective notion about the novelty or impact of a paper.
I also find it quite interesting that the journal Regenerative Medicine comes in at 5th with 31 papers since it was lower down the list in the past. The fact that the journal Circulation comes in at 6th with 25 papers is intriguing, because together with Blood at #1, it suggests to me that the fate of iPS cells and their potential clinical use is gaining a lot more interest.
Overall, what does this all mean?
What we are seeing is the evolution of a new field from the very beginning as reflected in its publishing trends.
The iPS cell field is maturing. It is less about basic science and relatively speaking less about methods than it used to be. It is now more focused on disease modeling and on potential clinical applications. The “holy grail” of an entirely chemical, non-genetic, efficient method is still out there as a goal toward which many are still focused. I see the iPS cell field as healthy, dynamic, and on course to change regenerative medicine. It is just that many people were very unrealistic at the beginning and expected wondrous things in just a couple years. Perhaps the progress of the iPS cell field should be measured in 5-year increments. We have had about 5 years so far and the field has advanced enormously. I think the next 5 years will be equally exciting and important.