The female germ line undergoes a unique sequence of differentiation processes that confers totipotency to the egg. The reconstitution of these events in vitro using pluripotent stem cells is a key achievement in reproductive biology and regenerative medicine. Here we report successful reconstitution in vitro of the entire process of oogenesis from mouse pluripotent stem cells. Fully potent mature oocytes were generated in culture from embryonic stem cells and from induced pluripotent stem cells derived from both embryonic fibroblasts and adult tail tip fibroblasts. Moreover, pluripotent stem cell lines were re-derived from the eggs that were generated in vitro, thereby reconstituting the full female germline cycle in a dish. This culture system will provide a platform for elucidating the molecular mechanisms underlying totipotency and the production of oocytes of other mammalian species in culture.

This study tested the hypothesis that murine oocytes participate in the establishment of granulosa cell phenotypic heterogeneity in preovulatory follicles. In these follicles, mural granulosa cells express LH receptors (LHR) and LHR mRNA, but expression of these molecules is low or undetectable in cumulus cells. Thus, the expression of LHR mRNA is a marker of the mural granulosa cell phenotype in preovulatory follicles. Cumulus cells expressed elevated steady-state levels of LHR mRNA when oocytes were microsurgically removed from oocyte-cumulus cell complexes, and this was prevented by paracrine factor(s) secreted by isolated oocytes. These factors also suppressed FSH-induced elevation of the level of LHR mRNA expression by mural granulosa cells isolated from small antral follicles, even when expression was augmented by culturing granulosa cells on components of basal lamina. Moreover, factor(s) secreted by oocytes suppressed the expression of LHR mRNA in mural granulosa cells isolated from preovulatory follicles already expressing elevated levels of these transcripts. The ability of oocytes to suppress the LHR mRNA expression by granulosa cells was developmentally regulated. Oocytes from preantral follicles and mature (metaphase II arrested) oocytes were less effective in suppressing expression than fully grown, germinal vesicle (GV)-stage oocytes. Furthermore, two-cell-stage embryos did not suppress LHR mRNA levels. Coculture of isolated oocytes with granulosa cells affected the synthesis of very few granulosa cell proteins detected by fluorography of two-dimensional gels after 35S-methionine labeling. Thus, oocyte suppression of FSH-induced LHR mRNA expression is specific in both the suppressing cell type and the effects on granulosa cells. It is concluded that the default pathway of granulosa cell differentiation produces the mural granulosa cell phenotype, as represented by the expression of LHR mRNA. This pathway is abrogated by oocytes. Thus, oocytes play a dominant role in establishing the fundamental heterogeneity of the granulosa cell population of preovulatory follicles.

Reconstituting gametogenesis in vitro is a key goal for reproductive biology and regenerative medicine. Successful in vitro reconstitution of primordial germ cells and spermatogenesis has recently had a significant effect in the field. However, recapitulation of oogenesis in vitro remains unachieved. Here we demonstrate the first reconstitution, to our knowledge, of the entire process of mammalian oogenesis in vitro from primordial germ cells, using an estrogenreceptor antagonist that promotes normal follicle formation, which in turn is crucial for supporting oocyte growth. The fundamental events in oogenesis (i.e., meiosis, oocyte growth, and genomic imprinting) were reproduced in the culture system. The most rigorous evidence of the recapitulation of oogenesis was the birth of fertile offspring, with a maximum of seven pups obtained from a cultured gonad. Moreover, cryopreserved gonads yielded functional oocytes and offspring in this culture system. Thus, our in vitro system will enable both innovative approaches for a deeper understanding of oogenesis and a new avenue to create and preserve female germ cells.oogenesis | primordial germ cells | follicle formation | oocytes | in vitro

The aim of this study was to establish a culture system to support the growth of bovine oocytes as enclosed in granulosa cell complexes that extend on a flat substratum. Such systems have been established for mouse oocytes but are not applicable to larger animals because it is difficult to maintain an appropriate association between the oocyte and companion somatic cells. Growing bovine oocytes with a mean diameter of 95 microm were isolated from early antral follicles: the growing stage corresponds to that of oocytes in preantral follicles of 12-day-old mice. Oocyte-granulosa cell complexes were cultured for 14 days in modified TCM199 medium supplemented with 5% fetal bovine serum, 4 mM hypoxanthine, and 0.1 microg/ml estradiol. The novel modification made for this medium was a high concentration, 4% (w/v), of polyvinylpyrrolidone (PVP; molecular weight of 360000). The flat substratum used was either an insert membrane fit in the culture plate or the bottom surface of the wells of 96-well culture plates. PVP influenced the organization of complexes, resulting in a firm association between the oocyte and the innermost layer of surrounding cells. More oocytes enclosed by a complete cell layer were recovered from the medium supplemented with 4% PVP than from the control medium. Similarly, of the oocytes initially introduced into the growth culture, a significantly larger proportion developed to the blastocyst stage from medium containing 4% PVP than from medium without PVP. When PVP medium was used, the overall yield of blastocysts was similar between the system with the insert membranes (12%) and that with the 96-well culture plates (9%). A calf was produced from one of four embryos derived from oocytes grown in 96-well culture plates, matured, and fertilized in vitro and then transferred to a recipient cow.

Preantral follicles containing oocytes of 70-89.5 microns in diameter were isolated from pig ovaries and cultured in collagen gel for up to 16 days, in the presence of serum, FSH and oestradiol. Formation of follicular antra occurred as the culture proceeded. The oocytes had been enclosed by granulosa cells and contacts between the oocytes and processes of the enclosing cumulus cells were maintained over the culture period. After 16 days of culture, 30-40% of the oocytes were of normal appearance, and the diameters of about half of these oocytes were larger than 100 microns. When the oocytes grown in vitro were liberated from the follicles and cultured for a further 48 h in modified Krebs-Ringer bicarbonate solution, 6, 30 and 60% of the oocytes larger than 90, 100 and 110 microns underwent germinal vesicle breakdown, respectively. Progression to metaphase II was observed in 40% of oocytes that were over 110 microns in diameter, whereas no oocyte less than 90 microns in diameter resumed meiosis. The relationship between the size and meiotic competence of oocytes was similar for oocytes grown in vitro or in vivo. Oocytes grown and matured in vitro were penetrated by spermatozoa and formed a female pronucleus, but decondensation of the sperm head was incomplete. The results demonstrate for the first time that pig oocytes from preantral follicles can grow up to their final size, acquire meiotic competence, and be penetrated by spermatozoa in vitro.

The molecules involved in determining meiotic competence were determined in porcine oocytes isolated from preantral and antral follicles of different sizes. Oocytes isolated from preantral follicles had a mean diameter of 78 u.m, contained diffuse filamentous chromatin in the germinal vesicle and were incapable of progressing from the G 2 to the M phase of the cycle even after 72 h in culture. Oocytes from early antral follicles had a mean diameter of 105 u,m, showed a filamentous chromatin configuration and about half resumed meiosis but arrested at metaphase I (MI) when cultured. Oocytes from mid-antral (3-4 mm) and large antral follicles (5-6 mm) had mean oocyte diameters of 115 and 119 u,m respectively, contained condensed chromatin around the nucleolus and progressed to metaphase II (Mil) in 48% and 93% of instances respectively. Analysis of p34 cdc2 , the catalytic subunit of maturation promoting factor (MPF), by immunoblotting indicates that the inability of small (78 u.m) oocytes to resume meiosis is due, at least in part, to inadequate levels of the catalytic subunit of MPF. On the other hand, the inability of intermediate-sized (105 |xm) oocytes from antral follicles to complete the first meiotic division by progressing beyond MI appears not to be limited by levels of p34 cdc2 , which are maximal by this stage. We postulate that an inadequacy of molecules other than p34 cdc2 limits progression of MI to Mil; the acquisition of these molecules during the final stages of growth may be correlated with the formation of the perinucleolar chromatin rim in the germinal vesicle.

Mammalian oogenesis occurs concomitantly with folliculogenesis in a coordinated manner in the ovaries. In vitro growth (IVG) culture systems of the oocytes have been developed as a new technology for utilizing incompetent oocytes in the ovary as a source of mature oocytes as well as for studying oogenesis, folliculogenesis, and oocyte-somatic cell interactions. The results of IVG experiments have suggested that direct association of oocytes and surrounding granulosa cells supports oocyte viability and growth through the gap junctions, which are efficient conduits for low molecular weight substances. It has been revealed that granulosa cells metabolize some molecules which are in turn transported into the oocytes. IVG systems have also provided evidence that FSH promotes the development of follicles at secondary or later stages by its stimulation of proliferation and differentiation of granulosa cells, and perhaps by its anti-apoptotic effects. In addition, interactions between granulosa cell-derived KIT ligands and oocyte KIT receptors have been suggested as initiating oocyte growth and follicular development. Furthermore, recent findings suggest there are growth factors derived from oocytes such as GDF-9 and BMP-15. With such factors, oocytes participate in follicular development by regulating the differentiation of surrounding somatic cells. These bidirectional communications between oocytes and somatic cells are important for oocyte growth and follicular development. IVG systems should provide further information regarding oogenesis and folliculogenesis in the ovary.

This protocol is an extension to: Nat. Protoc. 8, 1513-1524 (2013); doi: 10.1038/nprot.2013.090; published online 11 July 2013Generation of functional oocytes in culture from pluripotent stem cells should provide a useful model system for improving our understanding of the basic mechanisms underlying oogenesis. In addition, it has potential applications as an alternative source of oocytes for reproduction. Using the most advanced mouse model in regard to reproductive engineering and stem cell biology, we previously developed a culture method that produces functional primorial germ cells starting from pluripotent cells in culture and described it in a previous protocol. This Protocol Extension describes an adaptation of this existing Protocol in which oogenesis also occurs in vitro, thus substantially modifying the technique. Oocytes generated from embryonic stem cells (ESCs) or induced pluripotent stem cells give rise to healthy pups. Here, we describe the protocol for oocyte generation in culture. The protocol is mainly composed of three different culture stages: in vitro differentiation (IVDi), in vitro growth (IVG), and in vitro maturation (IVM), which in total take ∼5 weeks. In each culture period, there are several checkpoints that enable the number of oocytes being produced in the culture to be monitored. The basic structure of the culture system should provide a useful tool for clarifying the complicated sequence of oogenesis in mammals.