Distribution and viability of spermatozoa in the canine female genital tract during post-ovulatory oocyte maturation

BACKGROUND

Unlike other domestic mammals, in which metaphase-II oocytes are ovulated, canine ovulation is characterized by the release of primary oocytes, which may take 12 to up to 36 hours. Further 60 hours are needed for maturation to secondary oocytes which then remain fertile for about 48 hours. Oestrus takes 7 to 10 days on average and may start as early as a week before ovulation. This together with the prolonged process of post-ovulatory oocyte maturation requires an according longevity of spermatozoa in the female genital tract in order to provide a population of fertile sperm when oocytes have matured to fertilizability. Therefore the distribution and viability of spermatozoa in the bitch genital tract was examined during post-ovulatory oocyte maturation.

METHODS

Thirteen beagle bitches were inseminated on the day of sonographically verified ovulation with pooled semen of two beagle dogs containing one billion progressively motile spermatozoa. Ovariohysterectomy was performed two days later (group 1, n = 6) and four days later (group 2, n = 7). The oviduct and uterine horn of one side were flushed separately and the flushing's were checked for the presence of gametes. The oviducts including the utero-tubal junction and the uterine horns, both the flushed and unflushed, were histologically examined for sperm distribution.

RESULTS

The total number of spermatozoa recovered by flushing was low and evaluation of viability was limited. Prophase-I oocytes were collected from oviduct flushing in group 1, whereas unfertilized metaphase-II oocytes were detected in group 2. From day 2 to day 4 after ovulation a significant decrease in the percentage of glands containing sperm (P<0.05) and a marked reduction of the mean sperm number in uterine horn glands were observed. A concomitant diminution of spermatozoa was indicated in the utero-tubal junction accompanied by a slight increase in sperm numbers in the mid oviduct.

CONCLUSIONS

Oocyte maturation to metaphase-II stage is accompanied by a continuous sperm detachment and elimination in the uterine horns. Entrance of spermatozoa into the caudal oviduct seems to be steadily controlled by the utero-tubal junction thus providing a selected sperm population to be shifted towards the site of fertilization when oocyte maturation is completed.

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.

The influence of 9-cis-retinoic acid on nuclear and cytoplasmic maturation and gene expression in canine oocytes during in vitro maturation

Retinoids have important roles in regulation of oocyte nuclear and cytoplasmic maturation. The present study investigated the effects of a retinoid metabolite on nuclear maturation, cytoplasmic maturation, and gene expression in canine oocytes during in vitro maturation (IVM). Cumulus-oocyte complexes (COCs) were harvested from ovaries by slicing. Only oocytes that were >120 μm in diameter, with a homogeneous dark cytoplasm and three or more layers of compact cumulus cells were used. Varying concentrations of 9-cis retinoic acid (9-cis-RA; 0, 5, 50, and 500 nm) were included in the maturation medium, and the following were measured: (i) oocyte nuclear maturation after culture for 48 h; (ii) cytoplasmic granular migration by labeling of oocytes with fluorescein isothiocyanate labeled lectins; and (iii) relative expression of genes related to apoptosis (BAX and BclII) in cumulus cells detached from oocytes, by semiquantitative reverse transcriptase-polymerase chain reaction. After 48 h culture with IVM, the highest percentage of oocytes that had developed to the metaphase II (MII) stage were in the 5 nm 9-cis-RA treatment group (18.3 ± 2.5%; P < 0.05). Complete granular migration was observed in oocytes matured with 5 nm 9-cis-RA, consistent with a commensurate gain in developmental competence. Treatment with 5 nm 9-cis-RA had no effect on BclII gene expression, but downregulated BAX expression. In conclusion, since 5 nm 9-cis-RA was beneficial to nuclear and cytoplasmic maturation of canine oocytes, we inferred an important role for 9-cis-RA during IVM.