The never-ending search of an ideal culture system for domestic cat oocytes and embryos

Feline Wharton's jelly-derived mesenchymal stem cells as a feeder layer for oocytes maturation and embryos culture in vitro

Introduction

Due to their capacity to release growth factors and cytokines, co-culture using mesenchymal stem cells has been considered a good alternative to promoting the maturation of the oocytes and the embryo's development quality in vitro in different mammalian species. In this regard, we investigated the effect of feline Wharton's jelly MSCs as feeders layer in oocyte maturation-consequently, the development of resulting embryos in co-culture.

Methods

Oocytes with dark cytoplasm and a few layers of cumulus cells were collected and subjected to in vitro maturation and embryo culture using commercial media with and without MSCs addition. The oocytes' nuclear maturation and the degree of cumulus expansion in different groups were assessed after 24 h; the development of the embryo was evaluated every 12 h until day eight.

Results

Although MSCs increased the proportion of cumulus cells oocytes exhibiting cumulus expansion, there were no significant differences in the percentage of matured oocytes (metaphase II) among the groups (p > 0.05). However, the embryo development differs significantly, with a higher cleavage, morula, and blastocyst percentage in oocytes matured with MSC co-culture conditions than in commercial media alone (p < 0.05). Also, we observed higher morula and blastocyst rates in the embryos co-cultured with MSCs during the in vitro culture (p > 0.05).

Conclusion

Based on our results, the co-culture with MSCs during the oocyte maturation resulted in better embryo development, as well as the MSCs addition during embryo culture returned an increased number of morula and blastocysts. Further research is needed to fully understand and optimize the use of MSCs in oocyte maturation and embryo development.

A comparison of in vitro culture systems for cat embryos

The aim of this study was to compare several culture systems for cat embryos. Domestic cat oocytes were matured in vitro (IVM), fertilized (IVF), and cultured individually or in groups in drops under oil (20 μL or 50 μL) and in 16 microwell dishes (Primo Vision®). Moreover, the effects of co-culture with a) uncleaved oocytes, b) homospecific and c) heterospecific co-culture with cat and sheep companion embryos were investigated using a time-lapse system. A higher proportion of blastocysts and hatching blastocysts was observed after culture in Primo Vision® dishes compared with the classical individual (p < 0.001) and group (p < 0.05) culture systems. Culture of presumptive zygotes 16 hpi and the presence of uncleaved oocytes did not reduce blastocyst development compared with culture of embryos 24 hpi without uncleaved oocytes. Co-culture with later-stage companion cator sheep embryos accelerated development of catembryos. The highest percentage of blastocysts was obtained in the group co-cultured with sheep embryos (54%). Moreover, the blastocyst cavity formed on average 10 h faster in this group than for the control group and for embryos co-cultured with cat embryos. The proportion of hatching blastocysts was similar in the co-cultures with cat and with sheep embryos (20% vs. 22%) and significantly (p < 0.05) than in the control group (12%).

Fighting Like Cats and Dogs: Challenges in Domestic Carnivore Oocyte Development and Promises of Innovative Culture Systems

In vitro embryo production in cats and dogs still presents some challenges, and it needs to be optimized to transfer efficient protocols to related wild, endangered species. While the chemical composition of culture media has been the focus of several studies, the importance of culture substrates for oocyte and embryo culture has often been neglected. Traditional in vitro systems, i.e., two-dimensional cultures, do not resemble the physiological environments where cells develop, and they may cause morphological and functional alterations to oocytes and embryos. More modern three-dimensional and microfluidic culture system better mimic the structure and the stimuli found in in vivo conditions, and they could better support the development of oocytes and embryos in vitro, as well as the maintenance of more physiological behaviors. This review describes the different culture systems tested for domestic carnivore reproductive cells along the years, and it summarizes their effects on cultured cells with the purpose of analyzing innovative options to improve in vitro embryo production outcomes.

IGF-I Medium Supplementation Improves Singly Cultured Cat Oocyte Maturation and Embryo Development In Vitro

Embryo production is a routine procedure in several species. However, in felids, the effectiveness of this approach is far behind that in the majority of laboratory species. The development of a suitable environment starts with the proper composition of culture media. Therefore, for the improvement of assisted reproduction techniques and their outcome in cats, this is an urgent task. As the addition of insulin-like growth factors (IGF-I, IGF-II) or granulocyte-macrophage colony-stimulating factor (GM-CSF) was beneficial in other mammalian species, this study aims to check whether these components, combined with other factors (such as type of fertilisation or type of culture) can provide a benefit in the felid culture system in current use. Thus, these supplements, in different concentrations and combinations, were merged with the use of two fertilisation techniques and randomly assigned to single or group culturing. The results showed that the addition of IGF-I and/or GM-CSF produced an increase in morula and blastocyst rate in a single culture system. In particular, the supplementation with 20 ng/mL of IGF-I incremented the maturation rate by 10% and significantly increased the morula and blastocyst rates in single culturing. This result is especially remarkable for wild felids, where only a few oocytes and/or embryos are available.

Inhibition of Apoptotic Pathways Improves DNA Integrity but Not Developmental Competence of Domestic Cat Immature Vitrified Oocytes

Being a model for endangered wild felids, cryopreservation protocols for domestic cat oocytes are under continuous development. Immature vitrified oocytes (VOs) are a valuable resource for fertility preservation programs, but they often degenerate after warming and their in vitro development is poor. Since the exact mechanisms are not clear, this study assessed whether vitrification might trigger two apoptotic markers (DNA fragmentation and caspase activity, Experiment I) and the effects of a chemical inhibitor (i.e., the pan-caspase inhibitor Z-VAD-FMK) on the same markers (Experiment II) and on VOs in vitro development (Experiment III). The overarching aim was to check whether apoptosis inhibition might be a strategy to improve cat oocytes cryotolerance. In Experiment I, vitrification induced DNA fragmentation and increased caspase activity in VOs incubated for 24 h after warming (DNA fragmentation: 59.38%; caspase activity: 414.6 ± 326.8) compared to a fresh control (9.68%; 199.6 ± 178.3; p = 0.02). In Experiment II, the addition of Z-VAD-FMK to vitrification-warming and incubation media decreased DNA fragmentation and caspase activity (8.82%; 243.7 ± 106.9) compared to control (untreated) VOs (69.44%; 434.5 ± 248.3; p < 0.001). In Experiment III, Z-VAD-FMK brought maturation rates of treated VOs close to those of fresh oocytes (53.13 and 65.38%, respectively, p = 0.057), but there were no differences in VOs embryo development (cleavage rates; Z-VAD-FMK-treated VOs: 34.38%; control VOs: 31.78%; p = 0.69). In summary, vitrification increased apoptotic markers in cat VOs, and while Z-VAD-FMK was able to hinder DNA damage and caspase activity, its addition was not determinant for embryo development. To make the best use of VOs, other oocyte in vitro maturation and embryo culture strategies, such as the addition of other inhibitors or their prolonged use, should be investigated.

Granulosa cells in three-dimensional culture: A follicle-like structure for domestic cat vitrified oocytes

Follicle-like structures are three-dimensional matrices joint with living cells that allow the in vitro development of female gametes in more physiological conditions. They have been shown to be beneficial to fresh oocytes in different species, and in this study, domestic cat (Felis silvestris catus) granulosa cells were used to create a functional follicle-like structure aimed at supporting the in vitro maturation of conspecific vitrified oocytes, key players of fertility preservation programmes that usually struggle to acquire their full developmental competence after warming. Cat granulosa cells were cultured for up to 6 days in three-dimensional barium alginate microcapsules (i.e. follicle-like structures) or in two-dimensional monolayers, and their steroidogenic ability (estradiol and progesterone secretion) was assessed to confirm their functionality. The same systems were used (on day 2 or 6 of granulosa cells culture) for the in vitro maturation (IVM) of Cryotop^® vitrified immature cat oocytes and compared with microdrops of IVM medium without cells (control). Granulosa cells were able to maintain their functionality in vitro in both the conditions, even if with a different extent of hormonal secretion along culture (p = .02). Vitrified oocytes resumed meiosis at higher rates when cultured with 2 days old granulosa cells (p = .03), but full maturation rates slightly raised when granulosa cells were cultured longer, albeit without differences with the control group. This study paved the road for the creation of enriched culture systems in the domestic cat, but innovations are strongly needed for vitrified oocytes that deserve better chances to develop in vitro.

Developmental Competence of Domestic Cat Vitrified Oocytes in 3D Enriched Culture Conditions

Cryoinjuries severely affect the competence of vitrified oocytes (VOs) to develop into embryos after warming. The use of culture conditions that provide physical and chemical support and resemble the in vivo microenvironment in which oocytes develop, such as 3D scaffolds and coculture systems, might be useful to improve VOs outcomes. In this study, an enriched culture system of 3D barium alginate microcapsules was employed for the in vitro embryo production of domestic cat VOs. Cryotop vitrified-warmed oocytes were in vitro matured for 24 h in the 3D system with or without fresh cumulus-oocyte complexes (COCs) in coculture, whereas a control group of VOs was cultured in traditional 2D microdrops of medium. After in vitro fertilization, presumptive embryos were cultured in 3D or 2D systems according to the maturation conditions. Vitrified oocytes were able to mature and develop into embryos in 3D microcapsules (17.42 ± 11.83%) as well as in 2D microdrops (14.96 ± 8.80%), but the coculture with companion COCs in 3D resulted in similar proportions of VOs embryo development (18.39 ± 16.67%; p = 1.00), although COCs presence allowed for blastocyst formation (0.95 ± 2.52%). In conclusion, embryos until late developmental stages were obtained from cat VOs, and 3D microcapsules were comparable to 2D microdrops, but improvements in post-warming conditions are still needed.

The use of human and bovine commercial media for oocyte maturation and embryo development in the domestic cat (Felis catus)

The aim of this study was to examine the suitability of commercial media designed for humans and cattle for oocyte maturation and embryo culture in the domestic cat. In Exp. I, feline oocytes collected ex vivo were subjected to in vitro maturation in a laboratory-made culture medium (based on M199) or a commercial medium designed for cattle cells (BO-IVM^® ). In Exp. II, ICSI-derived feline embryos were cultured for 7 days in a commercial human (Continuous Single Culture^® ) or bovine (BO-EC^® ) cell medium. The rates of cleavage, morula and blastocyst formation were evaluated at 24 hr, 6 days and 7 days after ICSI, respectively, and compared between experimental groups. At the end of culture, embryos were assessed for viability and apoptotic changes. In Exp. I, no statistically significant difference in oocyte maturation outcome between laboratory-made (52.7%) and commercial media (58.9%) was observed. However, the use of a commercial medium prepared for use with bovine cells resulted in a significantly lower variance of the maturation rate. In Exp. II, no statistically significant differences between two commercial media were observed for cleavage (67.5% and 64.5%), morula (39.3% and 47.1%) and blastocyst rates (25.0% and 19.6%), as well as for the percentage of late apoptotic blastomeres. Morulae cultured in medium marketed for humans exhibited significantly more early apoptotic (43.2 ± 31.2% vs. 23.4 ± 23.2%) and necrotic (60.6 ± 47.6% vs. 29.4 ± 22.6%) blastomeres. In conclusion, both commercial media tested are suitable for in vitro oocyte maturation and embryo culture procedures in cats. It is remarkable that a culture medium designed for use in cattle for in vitro maturation of cat oocytes provides more reproducible results.