Progress toward species-tailored prematuration approaches in carnivores

In the past four decades, the bovine model has been highly informative and inspiring to assisted reproductive technologies (ART) in other species. Most of the recent advances in ART have come from studies in cattle, particularly those unveiling the importance of several processes that must be recapitulated in vitro to ensure the proper development of the oocyte. The maintenance of structural and functional communications between the cumulus cells and the oocyte and a well-orchestrated chromatin remodeling with the gradual silencing of transcriptional activity represent essential processes for the progressive acquisition of oocyte developmental competence. These markers are now considered the milestones of physiological approaches to increase the efficiency of reproductive technologies. Different in vitro approaches have been proposed. In particular, the so-called "pre-IVM" or "prematuration" is a culture step performed before in vitro maturation (IVM) to support the completion of the oocyte differentiation process. Although these attempts only partially improved the embryo quality and yield, they currently represent a proof of principle that oocytes retrieved from an ovary or an ovarian batch shouldn't be treated as a whole and that tailored approaches can be developed for culturing competent oocytes in several species, including humans. An advancement in ART's efficiency would be desirable in carnivores, where the success is still limited. Since the progress in reproductive medicine has often come from comparative studies, this review highlights aspects that have been critical in other species and how they may be extended to carnivores.

Reliability of Hoechst 33342 staining under wide-field microscopy for evaluation of the nuclear status of living dog oocytes

Due to the marked cytoplasmic opacity of canine oocytes, the diagnosis of their nuclear status is difficult. The objective of the present study was to evaluate the accuracy of Hoechst staining observed under epifluorescence wide-field microscopy [living oocyte observation (LivOO)] by comparison to a reference technique [DNA staining with ethidium homodimer-2 under confocal microscopy; fixed oocyte observation (FixOO)]. Four Hoechst 33342 concentrations (200 ng, 500 ng, 1 μg, 2 μg/mL) were tested and 1 μg/mL was the lowest one with the lowest proportion of oocytes in which DNA was missed. At this concentration, LivOO procedure did not affect the degeneration rate. On 379 oocytes observed individually with the two techniques successively, diagnosis of meiosis resumption by LivOO was exact in 87.3% of the cases, but the meiosis resumption rate was underestimated (23.5% versus 34.3% with FixOO; p < 0.001). Diagnosis for metaphase II was exact in 80% of the cases, but LivOO detected only 72.7% of the metaphase II oocytes present. Metaphase rates did not differ between LivOO and FixOO. This study contributes to a better interpretation of in vitro maturation results. The developmental potential of metaphase II canine oocytes sorted after Hoechst staining is to be evaluated.

Effects of hormonal supplementation on nuclear maturation and cortical granules distribution of canine oocytes during various reproductive stages

The aim of this study was to evaluate the effects of hCG, progesterone and oestradiol supplementation on nuclear and cytoplasmic maturation of canine oocytes cultured for 24, 48, 72 and 96 h. Oocytes obtained from 18 healthy bitches were divided into three groups according to their reproductive status (follicular, luteal and anoestrus stages) and cultured in TCM 199 + 25 UI/ml of hCG + 1 μg/ml of progesterone + 1 μg/ml of 17-β oestradiol or without hormonal supplementation (control) for different periods. Then, they were stained with FITC-LCA-Hoescht for chromatin configuration and cortical granules distribution and evaluated under an epifluorescence microscope. Culture time and the influence of different stages of the oestrous cycle were also evaluated. The present study demonstrated that there was no significant difference among the reproductive stages. With regards to culture medium, only oocytes from the supplemented medium were able to complete meiosis; however, significant difference was only noticed in the percentage of MI stage oocytes (p < 0.05) in the follicular and luteal group at 72 h of culture. Most oocytes in germinal vesicle, germinal vesicle breakdown and metaphase I stage had cortical granules distributed throughout the cytoplasm (immature pattern), irrespective of the culture period (p < 0.05). Cortical granules distributed immediately beneath the plasma membrane (mature) was only observed in metaphase II stage oocytes, but not all of them presented matured cytoplasm. Our results reveal that cortical granules distribution in canine oocytes matured in vitro did not progressed in correspondence with nuclear stage changes and are in accordance with those from other species.

The canine oocyte: uncommon features of in vivo and in vitro maturation

The biology of the canine oocyte is unusual compared with that of other mammalian females. The present paper reviews both in vivo and in vitro specificities of canine oocytes. Final follicular growth in the bitch is characterised by an early appearance of LH binding sites in the granulosa, a high proportion of polyovular follicles and a preovulatory luteinisation, starting at the time of the LH surge. Through follicular fluid, preovulatory oocytes are thus exposed to high levels of progesterone, as high as 1000-fold plasma concentrations. The composition of the follicular fluid is affected by the size of the female. The more specific aspect of oocyte biology in the bitch is ovulation: oocytes are expelled immature, at the Prophase I stage. Ovulatory follicles are 6–8 mm in diameter, releasing oocytes from 110 µm, with dark cytoplasm. Resumption of meiosis occurs from 48 h postovulation, MII stages appearing 48–54 h after ovulation. The mechanisms controlling such a late meiotic resumption are still unknown. Granulosa cells seem to play a central role as in other mammalian species, but not with cAMP as the principal mediator. The importance of a transient reactivation of oocyte transcription a few hours before meiotic resumption is to be explored. These specific features may contribute to the low efficiency of IVM. Only 10–20% oocytes reach the metaphase stage and suffer from a poor cytoplasmic maturation. Moreover, in vitro culture of canine oocytes is associated with a high proportion of degeneration. To date, IVM of the oocytes is the main limiting factor for the development of assisted reproductive techniques in the canine. A better knowledge of the basic physiology of folliculogenesis and the molecular mechanisms controlling oocyte meiosis resumption in this species may allow us to overcome this obstacle.