Submitted by FertilityLab Tue 07/03/2012
The short answer to the title question is, that Yes!, we are getting considerably closer to achieving therapeutic applications of stem cell technology to replenish ovarian reserve. Dr. Jonathan Tilly and his colleague Dr. Dori Woods recently summarized the dramatic advances in ovarian stem cell technology in their paper “The next (re) generation of ovarian biology and fertility in women: is current science tomorrow’s practice?” What are stem cells? Stem cells are rare cells that have the capacity to produce more cells identical to themselves. Some of these clones become dedicated to a particular cell type and others remain as stem cells to continually replenish this “starter” population of cells. In a previous blog post, Rejuvenating gonads with stem cells, I discussed Jonathon Tilly’s landmark studies which showed that mouse ovarian stem cells exist well into adulthood. His work was met, as he himself put it, with “considerable skepticism” at first because it upset the accepted dogma that all eggs a female will ever have are present at birth. His work has now been validated by numerous other investigators. In fact, in the mouse model, stem cells recovered from adult mice have been grown in culture for months as cell lines and under in vitro conditions have produced follicles and eggs in culture. Furthermore, the transplantation of these stem cells to recipient mice also results in the in vivo production of oocytes in mice. Of course, that’s all well and good for mice, but what about humans? Do adult women have these persistant stem cells in their ovaries and do they still have the capacity to produce eggs well into adulthood? Dr. Tilly and his colleagues recently reported that reproductive age women in their 20′s and early 30′s still retain this rare population of stem cells within their ovaries. Stem cells isolated from human ovarian tissue can be grown in culture and produce oocytes in vitro. Furthermore, human ovarian stem cells can be injected into human ovarian substrate tissue and follicles begin to grow in this tissue and produce human eggs when the tissue is transplanted into mice hosts. These studies proved two very important points: first, that reproductive age women have persistent ovarian stem cells and second, these stem cells retain the capacity in adulthood to undergo the process of follicle formation (folliculogenesis) and produce eggs both in vitro and also when transplanted into a host model. It is important to note that the stem cells used in these studies are ovarian stem cells, not embryonic stem cells. In theory, embryonic stem cells could also be used to produce oocytes but in practice, it has been much more difficult and to date impossible to create a fertilizable oocyte starting with an embryonic stem cell. The developmental road from embryonic stem cell to ovarian stem cell and then to follicle containing an oocyte is even less understood and requires, obviously, more steps. This makes the identification of ovarian stem cells so important and since these ovarian stem cells are recovered from adults, not embryos, the ethical issues associated with embryo use are bypassed. The next step to advance the ultimate goal of reproductive therapies is to develop reliable in vitro culture systems that take the ovarian stem cell all the way from stem cell to oocyte, without the use of an animal host. Interestingly, there have been promising results with in vitro culture of primordial (early stage) follicles recovered from ovarian tissue which when cultured for a period of months can produce mature follicles containing oocytes. Since isolated ovarian stem cells can produce a primordial follicle, the starting point for the culture system, Tilly and his group would propose combining these two methods (isolation of ovarian stem cells and effective primordial follicle culture) to create the whole culture path. Tilly envisions several possibilities for future clinical use of ovarian stem cells including:
