During the past year, work in our group that was funded by this CIRM award has focused on three novel human stem cell systems. The first was a collection of 10 human embryonic stem cell lines that we derived from single cells, termed blastomeres, of very early stage human embryos. They were grown for three days in the lab by which time they were comprised of eight cells. Much of our work, described below, suggests that these cells have unique properties as compared to human embryonic stem cell lines that are derived by conventional means, i.e., from intact embryos that are grown for five–six days in the lab and are comprised of approximately one hundred cells. Of note, these lines were submitted to the NIH Registry in December of 2009. We were notified that they did not fit the Federal definition of a human embryonic stem cell line, which includes being derived from a five–six-day-old embryo. Therefore, a comment period was opened via the Federal Register regarding a proposed change to the definition of hESCs as coming from early embryos up to and including the 5-6 day stage. At this time the NIH is still considering the public comments. Therefore, the NIH cannot review for registration our new lines that were derived from single blastomeres and work employing this novel cell model is not eligible for Federal funding. Therefore, our experiments would not be possible without CIRM funding.
Recently, we focused on comparing the global gene expression patterns of the lines. These “molecular fingerprints” give us important clues about differences in their potential with regard to forming the various cell types that are envisioned for use in regenerative medicine therapies. For example, some of the lines express very high levels of molecules that are only made by neurons. Others seem to be acquiring the characteristics of heart muscle or liver cells. We think that this finding is important because it should be easier to make particular differentiated cell types, such as those of the pancreas, from lines that are already predisposed to differentiate down this pathway.
In additional experiments, we tested the theory that the individual molecular fingerprints of the blastomere-derived lines were a snapshot of differences that could be observed in human embryos at equivalent stages to those from which the lines were derived. Therefore, we detected molecules that were differentially expressed among the lines. In several cases, we observed expression in only a subset of the eight cells that comprise three-day-old embryos. This is evidence that, in humans, the developmental clock is running at a fast pace with differentiation evident at an earlier stage than was previously thought to occur. This finding suggests that later stage embryos from which nearly all the existing human embryonic stem cell lines have been derived may be comprised of cells that have further differentiated.
The second novel human stem cell system was committed progenitors isolated from early gestation human placentas. We were very interested in pinpointing the location of the cells, which allowed us to devise procedures for purifying them. Then we used information that our group and other investigators have generated about signals that enable their self-renewal to devise conditions that support continuous growth of these progenitors in the laboratory. The new progenitor model will allow us to study important aspects of human placental development that were previously inaccessible. This work is important because the placenta plays a large role in governing pregnancy outcome. For example, this transient organ uses an unusual tumor-like invasive process to anchor the developing baby to the uterus. Additionally, the placenta transfers nutrients and wastes to and from maternal blood, respectively. Thus, with this new model we will be able to study the formation and the function of the cells that carry out these important tasks.
The third novel human stem cell model emerged from our discovery that the lines described above, which were derived from very early-stage human embryos, could spontaneously form human placental stem cells, that is, the earliest stage precursors that ultimately give rise to committed placental progenitors. We used our knowledge of how to maintain the latter cells to devise conditions that enable growth of this population in the laboratory. Currently, we are refining methods that will enable their continuous self-renewal. We are also carrying out a detailed analysis of their developmental potential. Additionally, we want to establish banks of the cells for distribution to our colleagues who are also studying basic mechanisms of human placental development.
Finally, we completed experiments in which we devised a novel method for growing human embryonic stem cells that does not require their exposure to other cell types, a potential source of infection. We discovered a molecule that could sustain them.