Long-term in vitro culture of bovine preantral follicles: Effect of base medium and medium replacement methods

Three-dimensional culture in a bioengineered matrix and somatic cell complementation to improve growth and survival of bovine preantral follicles

Purpose

Here we explored poly(ethylene glycol) (PEG) bioengineered hydrogels for bovine preantral follicle culture with or without ovarian cell co-culture and examined the potential for differentiation of bovine embryonic stem cells (bESCs) towards gonadal somatic cells to develop a system more similar to the ovarian microenvironment.

Methods

Bovine preantral follicles were first cultured in two-dimensional (2D) control or within PEG hydrogels (3D) and then co-cultured within PEG hydrogels with bovine ovarian cells (BOCs) to determine growth and viability. Finally, we tested conditions to drive differentiation of bESCs towards the intermediate mesoderm and bipotential gonad fate.

Results

Primary follicles grew over the 10-day culture period in PEG hydrogels compared to 2D control. Early secondary follicles maintained a similar diameter within the PEG while control follicles decreased in size. Follicles lost viability after co-encapsulation with BOCs; BOCs lost stromal cell signature over the culture period within hydrogels. Induction of bESCs towards gonadal somatic fate under WNT signaling was sufficient to upregulate intermediate mesoderm ( LHX1 ) and early coelomic epithelium/bipotential gonad markers ( OSR1 , GATA4 , WT1 ). Higher BMP4 concentrations upregulated the lateral plate mesoderm marker FOXF1 . PAX3 expression was not induced, indicating absence of the paraxial mesoderm lineage.

Conclusions

Culture of primary stage preantral follicles in PEG hydrogels promoted growth compared to controls; BOCs did not maintain identity in the PEG hydrogels. Collectively, we demonstrate that PEG hydrogels can be a potential culture system for early preantral follicles pending refinements, which could include addition of ESC-derived ovarian somatic cells using the protocol described here.

CAPSULE SUMMARY

We demonstrate that three-dimensional bioengineered hydrogels could aid in the survival and growth of small bovine preantral follicles. Moreover, bovine embryonic stem cells have the potential to differentiate towards precursors of somatic gonadal cell types, presenting an alternative cell source for preantral follicle co-culture.

Method of Isolation and In Vitro Culture of Primordial Follicles in Bovine Animal Model

The mammalian ovary is a substantial source of oocytes arranged into follicles at various stages of folliculogenesis, from the primordial to the ovulatory ones. Primordial follicles constitute the most abundant source of gametes inside the mammalian ovary at any given time.The isolation of a high number of primordial follicles, together with the development of protocols for in vitro follicle growth, would provide a powerful tool to fully exploit the female reproductive potential and boost the rescue and restoration of fertility in assisted reproduction technologies in human medicine, animal breeding, and preservation of threatened species. However, the most significant limitation is the lack of efficient methods for isolating a healthy and homogeneous population of viable primordial follicles suitable for in vitro culture. Here, we provide a fast and high-yield strategy for the mechanical isolation of primordial follicles from limited portions of the ovarian cortex in the bovine animal model.

Influence of TCM 199B, α-MEM, Waymouth MB 752/1 culture media, VEGF, Estradiol-17β, GDF-9 and FGF on in vitro development of preantral follicles in sheep

A total of 2792 preantral follicles (PFs') isolated from 750 ovaries of sheep were cultured in four different experiments. The efficacy of three commercially available culture media viz., TCM 199B, α-MEM and Waymouth MB 752/1 on the growth of sheep PFs' was tested in experiment I. Among the three media TCM 199B supported better development of PFs' in culture. The remaining experiments established the best concentrations of vascular endothelial growth factor (VEGF), Estradiol-17β (E2), GDF-9, Fibroblast growth factor (FGF) and their best combinations for the in-vitro development of PFs'. Inclusion of VEGF at 10 ng/mL, Estradiol-17β at 5 ng/mL, GDF-9 at 10 ng/mL or FGF at 10 ng/mL individually in a standard medium (SM) (containing FSH, IGF-I, GH and T4) supported better nuclear maturation of the oocytes to MII stage. Different combinations of VEGF, Estradiol-17β, GDF-9 and FGF supplemented in the SM promoted similar overall follicular growth. However, (a) SM + VEGF(10 ng/mL) + E2(5 ng/mL) supported higher increase in the diameter, (b) SM without any supplements induced antrum formation in greater proportion of follicles, and (c) SM + VEGF(10 ng/mL) + GDF 9(10 ng/mL) or SM + E2 (5 ng/mL) + FGF(10 ng/mL) supported high proportion of oocytes to reach MII stage. To conclude, TCM 199B appeared to be a better medium for development of sheep PFs'. VEGF, Estradiol-17 β, GDF-9 and FGF have beneficial influence on the development of sheep PFs' when supplemented in TCM 199B.

Comparison of Bovine Small Antral Follicle Development in Two- and Three-Dimensional Culture Systems

To compare the effects of two-(2D, microplate) and three-dimensional (3D, alginate) culture systems on the in vitro growth of small antral follicles in cattle, individual follicles were separately cultured in the two culture systems for 8 days. Half of the culture medium was replaced by fresh medium every 2 days; the former medium was used to assess the amount of follicular hormone secretion using ELISA. Individual follicle morphology, diameter, and survival rate were recorded every alternate day. The results showed that in 4 days, there was no significant difference between the two systems, except that the growth rate of follicles in 2D system was relatively faster. After 4 days, estradiol concentration in 3D system was higher than that in 2D system. However, progesterone concentration was lower than that in the 2D system. The survival rate and oocyte quality of follicles in 2D system were significantly lower than those in 3D system on day 8. The follicle diameter slightly increased (30-60 μm) in the entire process. Taken together, for in vitro culture of follicles within 4 days, the 2D culture system is more suitable. However, when the culture duration is >4 days, the 3D culture system is more suitable.

First pregnancy after in vitro culture of early antral follicles in goats: Positive effects of anethole on follicle development and steroidogenesis

This study aimed to evaluate the role of anethole during the in vitro culture of caprine early antral follicles. Early antral follicles were isolated from caprine ovaries and cultured for 18 days without (control) or with anethole (300 µg/ml). After culture, the cumulus-oocyte complexes were subjected to in vitro maturation, followed by parthenogenetic activation or in vitro fertilization (IVF) and embryo culture. Follicular walls were used for the quantification of messenger RNA (mRNA) of CYP19A1, CYP17, MMP-9, TIMP-2, Bax, and Bcl-2 genes, and culture medium was used for evaluation of ferric reducing antioxidant power (FRAP) and estradiol levels. After in vitro follicle culture (IVFC), anethole induced higher total antioxidant capacity, that is, it produced higher FRAP levels, reduced the Bax/Bcl-2 ratio, and increased the levels of mRNA for CYP19A1 and CYP17, which was associated with a greater estradiol production (p < .05). Also, anethole improved the ability of oocytes to resume meiosis and reach metaphase II stage, as well as yielded higher (p < .05) embryo production (e.g., morulas and blastocysts) in both parthenogenetic activation and IVF techniques. One pregnancy (Day 30) was obtained from IVFC with anethole. In conclusion, anethole promoted in vitro growth and maturation of goat early antral follicles and oocytes and enabled embryo production. Furthermore, this study reports, for the first time in goats, a pregnancy after IVF using oocytes originated from early antral follicles grown in vitro.

Supportive techniques to investigate in vitro culture and cryopreservation efficiencies of equine ovarian tissue: A review

During the reproductive lifespan of a female, only a limited quantity of oocytes are naturally ovulated; therefore, the mammalian ovary possesses a substantial population of preantral follicles available to be handled and explored in vitro. Hence, the manipulation of preantral follicles enclosed in ovarian tissue aims to recover a considerable population of oocytes of high-value animals for potential application in profitable assisted reproductive technologies (ARTs). For this purpose, the technique of preantral follicle in vitro culture (IVC) has been the most common research tool, achieving extraordinary results with offspring production in the mouse model. Although promising outcomes have been generated in livestock animals after IVC of preantral follicles, the quantity and quality of embryo production with those oocytes are still poor. In recent years, the mare has become an additional model for IVC studies due to remarkable similarities with women and livestock animals regarding in vivo and in vitro ovarian folliculogenesis. For a successful IVC system, several factors should be carefully considered to provide an optimum culture environment able to support the viability and growth of preantral follicles enclosed in ovarian tissue. The cryopreservation of the ovarian tissue is another important in vitro manipulation technique that has been used to preserve the reproductive potential in humans and, in the future, may be used in highly valuable domestic animals or endangered species. Several improvements in cryopreservation protocols are necessary to support the utilization of ovarian tissue of different species in follow-up ARTs (e.g., ovarian fragment transplantation). This review aims to provide an update on the most current advances regarding supportive in vitro techniques used in equids to evaluate and manipulate preantral follicles and ovarian tissue, as well as methodological approaches used during IVC and cryopreservation techniques.

Advances in in vitro folliculogenesis in domestic ruminants

The in vitro follicle culture (IVFC) represents an outstanding tool to enhance our understanding of the control of folliculogenesis and to allow the future use of a large number of immature oocytes enclosed in preantral follicles (PFs) in assisted reproductive techniques in humans as well as in others mammalian species including the ruminants. So far, the best results of IVFC were reported from mice with the production of live offspring from primordial follicles cultured in vitro. Live birth has been obtained after the in vitro culture of bovine early antral follicles. However, in other ruminant species, these results have been limited to the production of a variable number of mature oocytes and low percentages of embryos after in vitro culture of goat, buffalo and sheep isolated secondary preantral follicles. The present review presents and discusses the main findings, limitations, and prospects of in vitro folliculogenesis in ruminants focusing on bovine, caprine, and ovine species.

In vitro ovarian follicle growth: a comprehensive analysis of key protocol variables†

Folliculogenesis is a complex process that requires integration of autocrine, paracrine, and endocrine factors together with tightly regulated interactions between granulosa cells and oocytes for the growth and survival of healthy follicles. Culture of ovarian follicles is a powerful approach for investigating folliculogenesis and oogenesis in a tightly controlled environment. This method has not only enabled unprecedented insight into the fundamental biology of follicle development but also has far-reaching translational applications, including in fertility preservation for women whose ovarian follicles may be damaged by disease or its treatment or in wildlife conservation. Two- and three-dimensional follicle culture systems have been developed and are rapidly evolving. It is clear from a review of the literature on isolated follicle culture methods published over the past two decades (1980-2018) that protocols vary with respect to species examined, follicle isolation methods, culture techniques, culture media and nutrient and hormone supplementation, and experimental endpoints. Here we review the heterogeneity among these major variables of follicle culture protocols.

In vitro growth of immature bovine follicles and oocytes

The limitation in the supply of mature, fertilisable oocytes constitutes a major impediment to increasing the success of assisted reproduction, stem cell derivation and cloning in domestic species. Techniques are being developed to grow immature oocytes invitro that have the potential to increase the supply of oocytes. Mouse oocytes can be cultured from initial stages of development to maturity, and live young have been produced, but for domestic species, such as cows, with long growth periods, invitro systems that allow complete growth of oocytes contained within primordial follicles to maturity is technically challenging and has not yet been achieved. For cows, several culture systems have been developed that support specific developmental stages, but a multistep culture system will be required for complete growth invitro. This review highlights the steps that will be required to achieve the goal of growing oocytes invitro.

Is the pre-antral ovarian follicle the 'holy grail'for female fertility preservation?

Fertility preservation is not only a concern for humans with compromised fertility after cancer treatment. The preservation of genetic material from endangered animal species or animals with important genetic traits will also greatly benefit from the development of alternative fertility preservation strategies. In humans, embryo cryopreservation and mature-oocyte cryopreservation are currently the only approved methods for fertility preservation. Ovarian tissue cryopreservation is specifically indicated for prepubertal girls and women whose cancer treatment cannot be postponed. The cryopreservation of pre-antral follicles (PAFs) is a safer alternative for cancer patients who are at risk of the reintroduction of malignant cells. As PAFs account for the vast majority of follicles in the ovarian cortex, they represent an untapped potential, which could be cultivated for reproduction, preservation, or research purposes. Vitrification is being used more and more as it seems to yield better results compared to slow freezing, although protocols still need to be optimized for each specific cell type and species. Several methods can be used to assess follicle quality, ranging from simple viability stains to more complex xenografting procedures. In vitro development of PAFs to the pre-ovulatory stage has not yet been achieved in humans and larger animals. However, in vitro culture systems for PAFs are under development and are expected to become available in the near future. This review will focus on recent developments in (human) fertility preservation strategies, which are often accomplished by the use of in vitro animal models due to ethical considerations and the scarcity of human research material.

Effects of vitrification on the viability of alginate encapsulated isolated bovine pre-antral follicles

PURPOSE

Individual follicle cryopreservation techniques, without hydrogel support, are labor-intensive and a substantial proportion of isolated follicles are lost during handling and after warming. Therefore, the viability and morphology of isolated bovine (as a model for human) pre-antral follicles after vitrification and warming, when encapsulated in alginate beads, were investigated.

METHODS

Bovine pre-antral follicles were mechanically isolated and divided into four different groups: (1) culture in 2% alginate beads (3D system) and vitrification in beads using mesh cups (3DVIT), (2) culture in 2% alginate beads (3DCUL), (3) culture in 96-well plates (2D system) and vitrification using High Security Vitrification straws® (2DVIT), (4) culture in a 2D system (2DCUL). The same vitrification and warming protocols were used for embedded (3DVIT) and non-embedded follicles (2DVIT).

RESULTS

No differences were observed in follicle viability between group 2DCUL and 3DCUL. Group 3DVIT showed the lowest viability (45.9%) according to calcein and neutral red staining among all groups. Group 2DVIT displayed the highest viability (87.5%) and largest percentage of follicles with a well-preserved morphology.

CONCLUSIONS

Our results show that, using a vitification protocol optimized for non-embedded follicles, 2D culture is more effective in vitrifying isolated follicles. However, embedding in alginate allow to handle follicles more efficiently, i.e., without excessive manipulation and thus less labor-intensive in combination with a reduced loss of follicles during the procedure. Based on the increased work efficiency, but lower viability and higher proportion of follicles showing impaired morphology, we consider it advantageous to optimize the protocol for the vitrification of embedded follicles to increase survival and maintain morphology after vitrification.

Effect of bone morphogenetic proteins 2 and 4 on survival and development of bovine secondary follicles cultured in vitro

This study evaluated the effect of bone morphogenetic proteins 2 (BMP2) and 4 (BMP2) on follicle development and mRNA expression for GDF9, Cyclin B1, BMPR1A, BMPR1B, BMPRII, FSHR and SMAD1 in bovine secondary follicles cultured in vitro. Isolated secondary follicles were cultured for 18 days in TCM199⁺ medium alone or supplemented with BMP2 (10 ng/mL), BMP4 (100 ng/mL) or combination of both BMP2 and 4. Real-time PCR was used to analyze mRNA levels in fresh and cultured follicles. After 18 days of culture, follicles cultured with BMP2 alone or with BMP4 alone had larger diameters when compared to control (P < .05). In addition, all treatments promoted antrum formation and maintained a high viability rate through the growing period. The presence of BMP2, BMP4 or both together did not influence mRNA expression for the tested genes. However, the in vitro culture induces down-regulation for mRNA expression of BMPR1A. In conclusion, the addition of BMP2 or BMP4 alone in cultured medium promotes follicular growth and antrum formation in bovine follicles after 18 days of in vitro culture.

Effects of FSH addition to an enriched medium containing insulin and EGF after long-term culture on functionality of equine ovarian biopsy tissue

The effect of FSH supplementation on an enriched cultured medium containing insulin (10 ng/mL) and EGF (50 ng/mL) was investigated on in vitro culture of equine ovarian biopsy tissue. Ovarian tissue fragments were collected from mares (n = 10) and distributed in the following treatments: noncultured control, cultured control, and cultured + FSH. Both treated groups were cultured for 7 or 15 days. The end points evaluated were: follicular morphology, estradiol levels in the culture medium, fluorescence intensity for TUNEL, EGFR and Ki-67 detection, and gene expression of GDF-9, BMP-15, and Cyclin-D2 in the ovarian tissue. After seven days of culture, medium supplemented with FSH had a similar (P > 0.05) percentage of morphologically normal follicles compared to the noncultured control group. Estradiol levels increased (P < 0.05) from Day 7 to Day 15 of culture for both treated groups. No difference (P > 0.05) was observed for TUNEL and EGFR intensity between the noncultured control group and the treated groups after 15 days of culture. Ki-67 intensity did not differ (P > 0.05) between treated groups after 15 days of culture, but decreased (P < 0.05) when compared with the noncultured control group. Similar (P > 0.05) mRNA expression for GDF-9, BMP-15, and Cyclin-D2 was observed among all treatments after 15 days of culture. In conclusion, an enriched medium supplemented or not with FSH was able to maintain the functionality of equine ovarian biopsy tissue after a long-term in vitro culture.