The goal of our work is to understand how the initial stages of human development occur. Although the scientific community has learned a great deal about the factors that control the first developmental decisions in model organisms such as mice, very little is known about the parallel processes in humans. Much of what we have learned suggests that there are fundamental similarities and fundamental differences, but their extent has remained largely unexplored. We went to map this uncharted territory.
How will we go about approaching this scientific question? We are studying the first few days of human development by growing embryos and studying their physical and molecular properties. Briefly, frozen human embryos, approximately the size of the head of a small pin, were donated to the UCSF Gamete and Embryo Bank by couples at the conclusion of fertility treatments. After written informed consent was obtained, we used these early embryos in our studies. When they are grown under the proper conditions, they continue to develop, expanding from two cells into aggregates that contain four, eight, sixteen, and eventually up to 100 cells without an increase in size. This process mimics what we think is happening during the first few days of an actual pregnancy. Thus, by studying the changes that occur during the first few days of human embryonic development in the laboratory, we think that we will gain a much better understanding of this process.
During the current grant period, we made a great deal of progress toward accomplishing our overall goals. With regard to studying the physical properties of embryos, we showed that between the 8-cell and the 16-cell stage of development the component cells begin to segregate based on their ability to attach to one another. The cells on the outer surface of the embryo adhere tightly to one another. They also have a distinct orientation with morphological and molecular markers asymmetrically and systematically distributed at one end or the other. Once cells develop these specializations, we think that they are fated to form the placenta, which attaches the offspring to the uterus and supports its development for the rest of pregnancy. In contrast, cells on the inside of the embryo, which fail to develop these specializations, go on to form the so-called “inner cell mass” that develops into the offspring.
In the coming year, we want to put these new findings into a molecular context. Specifically, we want to determine whether orientation of the cells that form the outer surface of the embryo happens before or after the cells start to express markers that suggest they have assumed a placental fate. These experiments will help us understand whether orientation or molecular signatures are the primary drivers of this initial developmental decision. We are also interested in the interplay of these two forces as related to the continued differentiation of the cells that go on to form components of the inner cell mass and, subsequently, all the cells of the offspring.
What are the practical implications of this work? We think that stem cell researchers will use our findings to optimize protocols for differentiating human embryonic stem cells along the major lineages. Currently, we know that the most robust protocols for generating many types of cells (e.g., cardiomyocytes/muscle, neurons, pancreatic beta cells) involve triggering stepwise differentiation processes that recapitulate what happens during normal development. The roadmap that is generally used has been drawn using animal models. We think it will be very important to get equivalent information about the early stages of human embryonic development, which can be used to customize these protocols for the generation of differentiated human cell-based therapies.
Our work also has relevance to treating human infertility. For example, it is very difficult to discern how embryonic development might be going awry when we know almost nothing about the analogous normal processes. Thus, we envision that the results of our work can be used as a backdrop for developing molecular correlates of embryo quality that can be used to identify the subset with the best potential for further development. We think that the ability to recognize and transfer these embryos will help improve pregnancy rates in couples who seek assisted reproductive technologies. Finally, establishing landmarks during the first few days of human development will help us optimize growth conditions for human embryos, an important consideration since the goal is to replicate as closely as possible the environment in which these initial developmental steps normally take place.
In summary, we are making a great deal of progress in understanding the initial stages of human development. Thus far, our data suggest that the rules that have been learned studying animal models have been bent or reinvented in our species. Our continued work will further illuminate this principle.