Ultrastructure of oocyte maturation, fertilization, and early embryo development in vitro in the Siberian tiger (Panthera tigris altaica)

Ultrastructural changes in feline oocytes during ovary storage for 24- and 48-hours

Despite established microscopic markers of feline oocyte quality, little is known about their ultrastructural traits. To the best of our knowledge, there is no published report analysing the effect of 24 and 48 h ovarian storage time on the domestic cat oocytes characteristics at the ultrastructural level. Oocytes (n = 30) were classified using the light microscopy as good or bad quality and then proceeded for TEM observations. The location, shape, size and distribution of each organelle was noted in each examined oocyte. In in good quality oocytes the cytoplasmic organelles were generally easier to identify, and furthermore its distribution pattern was more obvious to spot than in bad quality ones. Whereas bad quality oocytes were typically characterised by the lower visibility of the cellular structures and cytoplasmic architecture was less apparent and often arranged without a predictable pattern. In good quality oocytes obtained from fresh ovaries cytoplasmic vacuoles (CVs) occupied a significantly larger area (0,72 vs. 0.18 CVs/μm², respectively) than in bad quality ones, whereas in bad quality and stored oocytes more cytoplasm was occupied by lipid droplets (LDs) than in fresh good oocytes (0,22 ± 0,09 vs. 0,09 ± 0,05 respectively). It can be concluded that ultrastructure changes in feline oocytes during 24 and 48 h ovarian storage cannot be assessed in light microscopy. The ultrastructure of oocytes was seriously disturbed after 48 h of ovary storage, despite being classified as good quality. However, further investigations utilizing more cells are necessary to confirm reported traits of ultrastructure changes in stored and non-stored oocytes of good and bad quality.

Brilliant cresyl blue staining allows the selection for developmentally competent immature feline oocytes

The outcome of in-vitro-maturation and in-vitro-fertilization of feline oocytes depends on the selection of high quality oocytes, and is often restricted to morphological criteria. The aim of this study was to test whether the Brilliant cresyl blue (BCB) staining is suitable for pre-selection of feline oocytes before in-vitro-maturation. Cumulus-oocytes-complexes (COC) were released from domestic cat ovaries obtained after ovariectomy and were subjected to BCB staining. BCB+ stained oocytes were characterized by a violet/pale blue staining of the ooplasma, BCB- oocytes remained unstained. Transmission electron microscopy indicated for a slightly advanced stage of BCB- oocytes within the maturation process. After 24 h in-vitro-maturation, almost 75% of BCB+ and 21.5% of BCB- oocytes were able to reach metaphase II. Also, after in-vitro-fertilization, significantly more oocytes developed to morulae (19.2%) if oocytes were preselected for BCB staining, although 8% of unstained COC still reached advanced embryo stages. Prolonged storage of ovaries before COC retrieval for 16-20 h at 4 °C was accompanied by reduced number of BCB+ oocytes (96 of 210, 45.7%) in comparison to freshly isolated COC (151 of 225, 67.1%), and impaired cleavage rate (19.8%) and morula rate (9.4%) of BCB+ oocytes but the rate of embryos which developed to advanced stages remained unchanged (∼50%). To conclude, BCB staining is a very useful tool to preselect immature COC of feline species ensuring higher developmental rates.

Ultrastructure of canine oocytes during in vivo maturation

The aim of the present study was to describe the canine oocyte ultrastructural modifications during in vivo maturation, with precise reference to the timing of the LH surge and of ovulation. Twenty-five bitches were ovariectomized at specific stages between the onset of proestrus and the fifth day post-ovulation: 65 oocytes were observed by transmission electron microscopy (TEM), either before the LH surge (n = 10), between the LH surge and ovulation (n = 12) or after ovulation (n = 43). Prior to the LH surge, the oocyte nucleus had already begun its displacement to the vicinity of the oolemma and reticulated nucleoli were infrequent. The cytoplasm showed signs of immaturity (few organelles preferentially located in the cortical zone, "mitochondrial cloud", scarce cortical granules). The LH surge was immediately followed by cumulus expansion but the ovulation occurred 2 days later. Retraction of the transzonal projections and the meiotic resumption occurred after another 3 days (5 days after the LH peak). The ovulation was then followed by gradual cytoplasmic modifications. Nucleoli re-assumed a reticulated aspect around 24 hr post-ovulation. From 48 hr post-ovulation mitochondria and SER were very numerous and evenly distributed. In conclusion canine oocyte maturation began prior to the LH surge and no cytoplasmic or nuclear modifications followed immediately the LH surge and ovulation. This study suggests that two distinct signals are needed for the final in vivo maturation: one prior to the LH surge (to induce maturation) and another one, around 3 days post-ovulation (to induce meiotic resumption).

In vitro production and transfer of cat embryos in the 21st century

Appreciable progress has been made in the development of assisted reproductive technology (ART) for creating in vitro embryos in cats. Moreover, the extent of advancement in the last decade has been similar, albeit of more modest magnitude, to that seen in some other domestic and laboratory species, particularly when the disparities in financial, and, hence, scientific, resources are considered. The recent progress in domestic felid ART has made it possible to envisage their potential role in supporting the conservation of endangered felid species, which, in reality, is a multifarious process requiring wide-ranging, yet coordinated approaches. The prospect of incorporating ART into that intricate domain, with limited exceptions, remains a long-term, but highly motivating objective. Meanwhile, the straightforward accessibility and abundant supply of domestic cat gametes from local veterinary clinics provides a valuable and practical source of material for further research on the basic aspects of in vitro oocyte maturation, fertilization and early embryo development. Furthermore, extrapolating the domestic biotechniques to non-domestic felids has produced encouraging results in some species.

Ovarian control for assisted reproduction in the domestic cat and wild felids

Of the 37 felid species, all but the domestic cat are classified as threatened with extinction in all or part of their native range. Additionally, the domestic cat is a valuable model for human biomedical research. Propagating some wild felids as well as domestic cat populations serving as human models is a major challenge primarily due to difficulties in transporting animals between facilities to ensure the pairing of genetically matched individuals, behavioral incompatibility between pairs and low fertility. Artificial insemination (AI) and in vitro fertilization/embryo transfer (IVF/ET) are powerful tools for helping manage rare populations. Developing successful assisted reproductive techniques requires knowledge of the female reproductive cycle and precise control of ovarian activity. Successful ovarian stimulation for AI and IVF/ET has been achieved in at least one-third of all cat species. However, sensitivity to a given gonadotropin treatment appears highly species-specific, and poor responses are common, particularly in felid species that exhibit spontaneous ovulations. Furthermore, current gonadotropin regimens have been demonstrated to perturb female reproductive function often leading to reduced fertility. Overall, ovarian response to exogenous hormonal stimulation has been highly variable, and pregnancy success after AI or IVF/ET remains low (<20%) in most species. Therefore, there is an immediate need to develop improved regimens that would allow more predictable ovarian responses in felids. We contend that recent research involving the use of progestins to control the ovary prior to gonadotropin stimulation shows promise for providing consistent ovarian stimulation in felids.

Follicle-stimulating hormone receptor in felids: intra- and interspecies variation

Assisted reproductive technologies are increasingly applied to support breeding efforts for many endangered felids. To explain the highly variable responses among felids to exogenous gonadotropins (FSH, eCG), we analyzed a 567bp fragment spanning a hyper-variable region of the FSH receptor in the domestic cat (catFSHR) and nine wild felid species/subspecies (felFSHR). Phylogenetic analysis indicated that the newly sequenced felFSHRs, together with the bear FSHR, belong to the carnivore group closely related to the ungulate clade. Within Felidae, genetic distances were 0.0089 +/- 0.0018 for nucleotide and 0.0183 +/- 0.0044 for amino acid (aa) sequences. In pairwise comparisons among catFSHR and all new felFSHRs, similarity ranged from 98.6 to 99.5% for nucleotides and from 97.4 to 98.9% for aa. Besides interspecies variability, intraspecies variation was also detected on both the cDNA and the protein level. There were no indications for an expression of tissue-specific isoforms of FSHR in testis and ovary.

The potential for gamete recovery from non-domestic canids and felids

Species are becoming extinct at a rate 100 times the natural background rates. Considering all mammalian orders, 24% of all Carnivora species are threatened. The goal of carnivore conservation is to reverse the decline in populations and to secure remaining populations in ways that will assure enduring public support. In this context, biotechnology is a tool with tremendous potential for assisting the conservation of endangered canid and felid species. As the first step for biotechnology development is the gamete obtainment, this review will discuss the potential of gamete recovery from non-domestic canids and felids, based on learning how to apply these procedures in the domestic carnivores. Thus, electroejaculation and obtaining both epidydimal spermatozoon and spermatogonial germ cells are indicated as techniques for male gametes recovery. In the female gametes retrieval, different methods for oocyte recovery from both antral and preantral follicles, and the possibility for ovarian tissue transplantation are discussed. Furthermore, the study discusses the responsibilities involved in the use of assisted reproduction in endangered species conservation.

Efficacy of porcine gonadotropins for repeated stimulation of ovarian activity for oocyte retrieval and in vitro embryo production and cryopreservation in Siberian tigers (Panthera tigris altaica)

A comparison of the amino acid sequences demonstrated that Siberian tiger gonadotropins are more homologous with those of porcine than any other commercially available preparation. The present study measured the efficacy of repeated ovarian stimulation with purified porcine gonadotropins on the follicular, hormonal, and immunogenic responses in Siberian tigers as well as on the ability of oocytes retrieved by laparoscopic follicular aspiration to fertilize and cleave in vitro. Controlled rate and vitrification cryopreservation methods were also compared for their ability to support ongoing cleavage following thawing of presumptive 2- to 4-cell tiger embryos generated in vitro. Vitrification supported continued embryonic cleavage in vitro while controlled rate freezing did not. Stereological microscopy indicated an excellent ovarian response with the recovery of quality cumulus-oocyte complexes that apparently fertilized and cleaved in vitro. However, ultrastructural and physiological examination revealed abnormal and unnatural responses such as the failure of some cumulus-oocyte complexes to reach maturity and progestagen levels to approach normalcy. At the same time, analyses of blood for antibodies failed to detect an immune reaction to these foreign gonadotropins in an assay that tested positive for the chorionic gonadotropin-stimulated domestic cat. Together, these observations suggest that porcine gonadotropins may be effective for the ovarian stimulation of tigers but that some modifications to administration protocols are needed to produce a more natural response.