Developmental and Ultrastructual Characteristics of Mouse Oocytes Grown in Vitro from Primordial Germ Cells

Complete in vitro oogenesis: retrospects and prospects

Precise control of mammalian oogenesis has been a traditional focus of reproductive and developmental biology research. Recently, new reports have introduced the possibility of obtaining functional gametes derived in vitro from stem cells. The potential to produce functional gametes from stem cells has exciting applications for regenerative medicine though still remains challenging. In mammalian females ovulation and fertilization is a privilege reserved for a small number of oocytes. In reality the vast majority of oocytes formed from primordial germ cells (PGCs) will undergo apoptosis, or other forms of cell death. Removal occurs during germ cell cyst breakdown and the establishment of the primordial follicle (PF) pool, during the long dormancy at the PF stage, or through follicular atresia prior to reaching the ovulatory stage. A way to solve this limitation could be to produce large numbers of oocytes, in vitro, from stem cells. However, to recapitulate mammalian oogenesis and produce fertilizable oocytes in vitro is a complex process involving several different cell types, precise follicular cell-oocyte reciprocal interactions, a variety of nutrients and combinations of cytokines, and precise growth factors and hormones depending on the developmental stage. In 2016, two papers published by Morohaku et al. and Hikabe et al. reported in vitro procedures that appear to reproduce efficiently these conditions allowing for the production, completely in a dish, of a relatively large number of oocytes that are fertilizable and capable of giving rise to viable offspring in the mouse. The present article offers a critical overview of these results as well as other previous work performed mainly in mouse attempting to reproduce oogenesis completely in vitro and considers some perspectives for the potential to adapt the methods to produce functional human oocytes.

Effects of in vitro culture of mouse fetal gonads on subsequent ovarian development in vivo and oocyte maturation in vitro

Under organ culture, female fetal gonads in mice cannot develop beyond the preantral follicle stage unless the follicles are individually isolated and cultured again. In this study, we investigated the effect of in vitro culture of female fetal gonads before transplantation on subsequent in vivo development. The gonads derived from female fetuses 12.5 days postcoitum were organ-cultured for 0, 7 and 14 days, and then were grafted underneath the kidney capsules of severe combined immunodeficient mice and recovered at 21, 14 and 7 days post-transplantation, respectively. The histological analysis of the grafts showed that the in vitro culture of the fetal gonads restricted follicular development to the antral follicle stage post-transplantation. In the grafts cultured for 14 days, particularly, no antral follicle was observed. However, the oocytes in these follicles had grown to around 65 microm in diameter and had competence to resume meiosis in vitro. When the fetal gonads were grafted after culture for 7 and 14 days, 13.0% and 6.8% of the oocytes progressed to the metaphase II stage, respectively. These data showed significant differences (P < 0.05) in comparison with the control group (25.3%). Our results indicate that the in vitro culture of female fetal gonads before transplantation affects the subsequent in vivo development of both follicular cells and oocytes, and in vitro oocyte maturation. However, this effect seems to be more severe in terms of follicular development when compared with oocyte growth and maturation.

Tissue culture before transplantation of frozen-thawed human fetal ovarian tissue into immunodeficient mice

OBJECTIVE

To evaluate the effects of tissue culture on the viability and development of follicles in frozen-thawed human fetal ovarian tissue before transplantation into severe combined immunodeficient (SCID) mice and to determine the optimal duration of pretransplant tissue culture.

DESIGN

Experimental prospective study.

SETTING

Animal center and reproductive laboratories in university hospitals.

INTERVENTION(S)

Frozen-thawed human fetal ovarian tissue samples from 20-week-old abortuses were randomly divided into four groups and cultured in vitro for 0, 3, 6, or 9 days before being xenografted into kidney capsules of bilaterally oophorectomized severe combined immunodeficient (SCID) mice. Grafts were removed 16 weeks after transplantation. Histological analysis and assessment of proliferative cell nuclear antigen (PCNA) expression levels were used to evaluate the survival and development of follicles.

RESULT(S)

The proportion of growing follicles was significantly increased in groups cultured before transplantation as compared with the noncultured group. Sixteen weeks after transplantation, the number of follicles in the cultured grafts was higher than that in the noncultured grafts. Grafts cultured for 6 or 9 days showed higher proportions of preantral and antral follicles than grafts cultured for 0 or 3 days. PCNA immunohistochemical analysis indicated that follicle cells were in a proliferative state after culture and transplantation.

CONCLUSION(S)

The viability and development of human fetal follicles may be improved by pretransplant tissue culture. The optimal culture duration before transplantation of fetal ovarian tissue is 6 days.