Oogonial stem cells: do they exist and may they have an impact on future fertility treatment?

Time to consider ovarian tissue cryopreservation for girls with Turner's syndrome: an opinion paper

Turner’s syndrome (TS) is the most common sex chromosome abnormality in women. In addition to short stature and gonadal dysgenesis, it is associated with cardiac and renal anomalies. Due to rapid follicular atresia, the majority of women with TS suffer from primary ovarian insufficiency around puberty. Thus far, donor oocyte conception has been the key fertility option for these women. With advancing technology, ovarian tissue cryopreservation (OTCP) has emerged as a clinically justifiable option especially for pre-pubertal girls with cancer. Recently published results following the use of cryopreserved ovarian tissue are reassuring. It would be prudent to consider the extension of these technological and scientific advances to other conditions, such as TS, where accelerated follicular atresia is suspected. It is possible to obtain competent oocytes from cryopreserved ovaries of girls with TS provided the ovaries were preserved before ovarian failure. However, it is a complex decision whether and when to offer OTCP as a fertility preservation (FP) option for girls with TS. The rate of decline in fertility is variable in girls with TS and can be more complex in cases with mosaicism. On the other hand, OTCP has shown some promising results in patients with cancer, which can potentially be replicated in TS and other benign indications of patients at risk of premature ovarian failure. There are proven psychological and clinical benefits of FP. Thus, an argument could be made for offering OTCP to these patients to endow these girls with the option of having biological fertility using this innovative technology. Ethical, clinical and psychological dilemmas should be considered, discussed and addressed before considering such a novel approach. We believe that the time has come to start this discussion and open this avenue of FP for girls with TS.

Artificial gametes from stem cells

The generation of artificial gametes is a real challenge for the scientific community today. In vitro development of human eggs and sperm will pave the way for the understanding of the complex process of human gametogenesis and will provide with human gametes for the study of infertility and the onset of some inherited disorders. However, the great promise of artificial gametes resides in their future application on reproductive treatments for all these people wishing to have genetically related children and for which gamete donation is now their unique option of parenthood. This is the case of infertile patients devoid of suitable gametes, same sex couples, singles and those fertile couples in a high risk of transmitting serious diseases to their progeny. In the search of the best method to obtain artificial gametes, many researchers have successfully obtained human germ cell-like cells from stem cells at different stages of differentiation. In the near future, this field will evolve to new methods providing not only viable but also functional and safe artificial germ cells. These artificial sperm and eggs should be able to recapitulate all the genetic and epigenetic processes needed for the correct gametogenesis, fertilization and embryogenesis leading to the birth of a healthy and fertile newborn.

Postnatal oogenesis in humans: a review of recent findings

In spite of generally accepted dogma that the total number of follicles and oocytes is established in human ovaries during the fetal period of life rather than forming de novo in adult ovaries, some new evidence in the field challenges this understanding. Several studies have shown that different populations of stem cells, such as germinal stem cells and small round stem cells with diameters of 2 to 4 μm, that resembled very small embryonic-like stem cells and expressed several genes related to primordial germ cells, pluripotency, and germinal lineage are present in adult human ovaries and originate in ovarian surface epithelium. These small stem cells were pushed into the germinal direction of development and formed primitive oocyte-like cells in vitro. Moreover, oocyte-like cells were also formed in vitro from embryonic stem cells and induced pluripotent stem cells. This indicates that postnatal oogenesis is not excluded. It is further supported by the occurrence of mesenchymal stem cells that can restore the function of sterilized ovaries and lead to the formation of new follicles and oocytes in animal models. Both oogenesis in vitro and transplantation of stem cell-derived "oocytes" into the ovarian niche to direct their natural maturation represent a big challenge for reproductive biomedicine in the treatment of female infertility in the future and needs to be explored and interpreted with caution, but it is still very important for clinical practice in the field of reproductive medicine.

Regenerative medicine for the treatment of reproductive system disorders: current and potential options

Regenerative medicine has opened new avenues for treating patients with severe reproductive system disorders, such as congenital abnormalities, cancer, trauma, infection, inflammation and iatrogenic injuries. Over the past two decades, scientists have advanced the field of reproductive tissue engineering to restore normal sexual function and preserve fertility in both female and male patients. In this review, we summarize recent advances in the use of cell, tissue, and organ-based regenerative medicine strategies for clinical application in reproductive system disorders.

Ovarian germline stem cells: an unlimited source of oocytes?

While there has been progress in directing the development of embryonic stem cells and induced pluripotent stem cells toward a germ cell state, their ability to serve as a source of functional oocytes in a clinically relevant model or situation has yet to be established. Recent studies suggest that the adult mammalian ovary is not endowed with a finite number of oocytes, but instead possesses stem cells that contribute to their renewal. The ability to isolate and promote the growth and development of such ovarian germline stem cells (GSCs) would provide a novel means to treat infertility in women. Although such ovarian GSCs are well characterized in nonmammalian model organisms, the findings that support the existence of adult ovarian GSCs in mammals have been met with considerable evidence that disputes their existence. This review details the lessons provided by model organisms that successfully utilize ovarian GSCs to allow for a continual and high level of female germ cell production throughout their life, with a specific focus on the cellular mechanisms involved in GSC self-renewal and oocyte development. Such an overview of the role that oogonial stem cells play in maintaining fertility in nonmammalian species serves as a backdrop for the data generated to date that supports or disputes the existence of GSCs in mammals as well as the future of this area of research in terms of its potential for any application in reproductive medicine.