Morphologic characterization of isolated bovine early preantral follicles during short-term individual in vitro culture

A spotlight on factors influencing the in vitro folliculogenesis of isolated preantral follicles

Female fertility preservation via complete in vitro folliculogenesis is still chimerical. Due to many factors affecting the efficiency of isolation and culture of preantral follicles, the improvement of techniques geared to fertility preservation in higher mammals seems to be at an impasse. We need an objective view of the current stand to understand how to progress further. As such, a survey was conducted to analyze the relative distribution of studies performed in ten mammalian species on preantral follicle culture available on PubMed. Using the bovine as a reference model, we explore some factors influencing data variation that contribute to the difficulty in reproducing studies. While years of research have enabled the recapitulation of folliculogenesis from as modest as the early antral follicle stage ex vivo, in vitro preantral folliculogenesis remains elusive. Herein, we revisit the classical evidence that laid the foundations for understanding preantral folliculogenesis and review the length, breadth, and depth of information that the era of big data has currently levied. Moving forward, we recognize the urgency of synthesizing the multi-disciplinary approaches to mimic folliculogenesis in vitro to achieve a translational landscape of infertility at individual and large-scale conservation levels.

Preservation of connexin 43 and transzonal projections in isolated bovine pre-antral follicles before and following vitrification

PURPOSE

Gap junctions and transzonal projections play a crucial role in intercellular communication between different follicular components and are necessary for follicle development. We aimed to demonstrate gap junction protein connexin 43 (Cx43) and transzonal projections (TZPs) in viable, category 1, isolated bovine pre-antral follicles (PAFs) during short-term culture and after vitrification and warming.

METHODS

This study involved four experimental groups: fresh control, 2-day culture, 4-day culture, and vitrified secondary PAFs. Isolated PAFs were vitrified using a simple and efficient cryopreservation method by means of mini cell strainers.

RESULTS

Cx43 and TZPs were detected in pre-antral follicles of all stages, as well as in every experimental group. The group fresh follicles showed a higher percentage of follicles that were positive for Cx43 (91.7%) than the follicles that were vitrified (77.4%). All follicles that were cultured for 2 days were Cx43-positive (100%). Follicles cultured for 4 days (65.8%) (P = 0.002) showed the lowest percentage of follicles that were Cx43-positive. The percentages of the presence or (partial) absence of the TZP network were shown to be very heterogeneous between follicles in different treatment groups.

CONCLUSIONS

These results suggest the maintenance of communication between the oocyte and the somatic companion cells after vitrification and warming. The varying percentages of the expression of the TZP network within groups suggests that it will be of interest to investigate whether this is truly due to variability in TZP integrity and follicle quality or due to methodological limitations.

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.

Characterization of isolated bovine preantral follicles based on morphology, diameter and cell number

Preantral follicles are a potential reservoir of oocytes to be used in assisted reproductive technologies. With the increasing interest in developing techniques to grow preantral follicles in vitro, and as the bovine emerges as an appropriate model species to understand human folliculogenesis, the establishment of an accurate classification of developmental stages is needed. Classification of bovine preantral follicles has been mostly based on histological analysis and estimation models, which may not translate well to correctly characterize preantral follicles isolated from the ovary. In this study, we classified bovine preantral follicles by morphology upon isolation, determined diameter and number of granulosa cells by direct counting, and compared our results with previous studies reporting bovine preantral follicle classification. Follicles were isolated via homogenization of ovary tissue and classified into primary, early secondary and secondary stage based on morphology and number of layers of granulosa cells. Diameter was individually measured and Hoechst 33342 was used as a nuclear stain to count granulosa cells. We found that follicles classified by morphology into primary, early secondary, and secondary had different mean diameter and cell number (P < 0.01); cell number and diameter were positively correlated, as were cell density and cell number in each developmental stage (P < 0.01). Results obtained here were mostly in agreement with previous classifications based on histological sections and on isolated follicles, with some discrepancies. The present data add accuracy to classification of bovine preantral follicles that is critical to optimize culture conditions to produce developmentally competent oocytes.

Heat stress impairs in vitro development of preantral follicles of cattle

Although detrimental effects of heat stress on antral follicle development have been well studied, long-term effects - affecting the preantral follicle pool - are still largely unknown. The goal of this study was to evaluate effects of heat stress on growth, viability, gene expression and ATP production of preantral follicles of cattle. Follicles at the primary, early secondary and secondary stages were isolated from cattle ovaries and individually cultured while imposing physiological (CON; 38.5 °C) or intermittent heat stress (HS; 38.5 °C for 16 h and 41 °C for 8 h daily) conditions for 7 days. Individual follicles were subjected to real-time qPCR for determination of relative abundance of BAX, HSPA1A and SOD1 mRNA transcripts and evaluated for ATP production. Treatment for 7 days with intermittent HS decreased viability (P =  0.01) and diameter (P =  0.03) of preantral follicles. Relative abundances of BAX and HSPA1A mRNA transcripts were greater in follicles of the CON and HS groups that became non-viable during culture (P <  0.05); relative abundance of SOD1 mRNA transcript, however, was only greater in non-viable follicles of the HS group (P <  0.05), but not non-viable follicles of the CON group (P =  0.3). The ATP production was not different between viable follicles of the CON and HS group (P = 0.86). In conclusion, all stages of growing preantral follicles of cattle were susceptible to negative effects of heat stress. Follicles at the secondary stage of development were most sensitive, followed by early secondary and primary follicles.

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.

High diluted and dynamised follicle stimulating hormone modulates steroid production in isolated porcine preantral follicles cultured in vitro

Objective: This study investigated the effect of two different follicle stimulating hormone (FSH) preparations (diluted/dynamised and diluted) on the in vitro development and steroid production (estradiol, progesterone and testosterone) of isolated porcine preantral follicle after in vitro culture.

Methods: Secondary follicles were cultured in Alpha Minimum Essential Medium (α-MEM+) supplemented with grain ethanol (AL – 0.2%, v/v), diluted/dynamised FSH (rFSH 6cH – 0.05 fg/mL) or diluted-only FSH (1.5 ng/mL) for 4 days. Follicle development was evaluated on the basis of follicular growth, morphology and hormone production.

Results: The percentage of follicular integrity and extrusion were not affected by the treatments after culture. For all treatments, follicular diameter increased significantly from Day 0 to Day 4. On Day 2 of culture, the estradiol production was significantly higher in AL and diluted-only FSH treatments compared with diluted/dynamised FSH. However, diluted/dynamised FSH showed a significantly higher progesterone production on Day 2. Only on Day 4, the testosterone production was higher in the AL than diluted-only FSH treatments, but similar to diluted/dynamised FSH treatment. Except for diluted/dynamised FSH treatment, progesterone production increased (P < 0.05) from Day 2 to Day 4; only for AL treatment, a significant increase of testosterone production was observed during culture.

Conclusion: Compared to control the diluted/dynamised FSH addition increased progesterone production but decreased the estradiol production after in vitro culture of isolated porcine preantral follicles. Taken together the results suggest that at least for progesterone production the mechanism of action of diluted/dynamised FSH differs from its vehicle.