Japanese Society for Animal Reproduction: award for outstanding research 2002. Cryopreservation of follicular oocytes and preimplantation embryos in cattle and horses

Study on the expression patterns and function of JUNO and CD9 in bovine oocytes during in vitro maturation

Fertilization proteins JUNO and CD9 play vital roles in sperm-egg fusion, but little is known about their expression patterns during in vitro maturation (IVM) and their function during in vitro fertilization (IVF) of bovine oocytes. In this study, qRT-PCR and immunofluorescence staining were used to detect the mRNA and protein expression levels of JUNO and CD9 genes in bovine oocytes and cumulus cells. Then, fertilization rate of MII oocytes treated with (i) JUNO antibody (1, 5 and 25 μg/ml) or (ii) CD9 antibody (1, 5 and 25 μg/ml) or (iii) CD9 antibody (5 μg/ml) + JUNO antibody (5 μg/ml) were recorded. Our results showed that the mRNA and protein expression levels of JUNO and CD9 genes significantly increased from bovine GV oocytes to MII oocytes, and similar mRNA expression patterns of JUNO and CD9 were also detected in cumulus cells. All groups of oocytes treated with CD9 antibody or JUNO antibody showed significantly decreased fertilization rates (p < .05). Particularly, the fertilization ability of oocytes treated with CD9 antibody (5 μg/ml) + JUNO antibody (5 μg/ml) sharply decreased to 3.48 ± 0.11%. In conclusion, our study revealed the expression levels of JUNO and CD9 genes in oocytes and cumulus cells increased during IVM of bovine oocytes, with JUNO protein playing a major role in the fertilization of bovine oocytes.

Generation of live offspring from vitrified mouse oocytes of C57BL/6J strain

In mammals, unfertilized oocytes are one of the most available stages for cryopreservation because the cryopreserved oocytes can be used for assisted reproductive technologies, including in vitro fertilization (IVF) and intracytoplasmic sperm injection. However, it has generally been reported that the fertility and developmental ability of the oocytes are reduced by cryopreservation. C57BL/6J mice, an inbred strain, are used extensively for the production of transgenic and knockout mice. If the oocytes from C57BL/6J mice can be successfully cryopreserved, the cryopreservation protocol used will contribute to the high-speed production of not only gene-modified mice but also hybrid mice. Very recently, we succeeded in the vitrification of mouse oocytes derived from ICR (outbred) mice. However, our protocol can be applied to the vitrification of oocytes from an inbred strain. The aim of the present study was to establish the vitrification of oocytes from C57BL/6J mice. First, the effect of cumulus cells on the ability of C57BL/6J mouse oocytes to fertilize and develop in vitro was examined. The fertility and developmental ability of oocyte-removed cumulus cells (i.e., denuded oocytes, or DOs) after IVF were reduced compared to cumulus oocyte complexes (COCs) in both fresh and cryopreserved groups. Vitrified COCs showed significantly (P<0.05) higher fertility and ability to develop into the 2-cell and blastocyst stages compared to the vitrified DOs with cumulus cells and vitrified DOs alone. The vitrified COCs developed to term at a high success rate, equivalent to the rate obtained with IVF using fresh COCs. Taken together, our results demonstrate that we succeeded for the first time in the vitrification of mouse oocytes from C57BL/6J mice. Our findings will also contribute to the improvement of oocyte vitrification not only in animals but also in clinical applications for human infertility.

Effect of dehydration prior to cryopreservation of large equine embryos

Cryopreservation of equine embryos>300microm in diameter results in low survival rates using protocols that work well for smaller equine embryos. These experiments tested the potential benefit of incorporating a dehydration step prior to standard cryopreservation procedures. Forty-six, day 7-8, grade 1, equine embryos 300-1350microm in diameter were subjected to one of the following treatments: (A) 2 min in 0.6M galactose, 10min in 1.5M glycerol, slow freeze (n=21); (B) 10min in 1.5M glycerol, slow freeze (n=15); (C) 2min in 0.6M galactose, 10min in 1.5M glycerol, followed by exposure to thaw solutions, then culture medium (n=5); (D) transferred directly to culture medium (n=5). Frozen embryos were thawed and subjected to a three-step cryoprotectant removal. Five embryos from each treatment were evaluated morphologically after 24 and 48h culture (1=excellent, 5=degenerate/dead). All treatments had at least 4/5 embryos with a quality score >or=3 at these time points except treatment B (2/5 at 24h, 1/5 at 48h). Subsequent embryos from treatment A (n=16) or B (n=10) were matched in sets of two for size and treatment, thawed, and immediately transferred in pairs to 13 recipients. Only two recipient mares were pregnant; one received two 400microm embryos from treatment A, and the other one 400 and one 415microm embryo from treatment B. There was no advantage of incorporating a 2min dehydration step into the cryopreservation protocol for large equine embryos.

Ultrastructure of human mature oocytes after slow cooling cryopreservation using different sucrose concentrations†

BACKGROUND

We studied the ultrastructural characteristics of human mature oocytes frozen/thawed (F/T) using different concentrations of sucrose. Fresh human mature oocytes were used as controls.

METHODS

The oocytes (n = 48) were fixed in 1.5% glutaraldehyde at sampling (n = 16) or after freeze/thawing performed using a slow cooling method with propane-1,2-diol 1.5 mol/l and sucrose at either 0.1 mol/l (n = 16) or 0.3 mol/l (n = 16) in the freezing solution. The oocytes were then processed for electron microscopy observations.

RESULTS

Fresh and F/T oocytes belonging to both study groups were regularly rounded in sections, with a homogeneous cytoplasm and an intact zona pellucida (ZP). Organelles (mainly mitochondria-smooth endoplasmic reticulum aggregates and mitochondria-vesicle complexes) were abundant and uniformly dispersed in the ooplasm. The amount and density of cortical granules appeared to be abnormally reduced in some F/T samples, independently of the sucrose concentration in the freezing solution: this feature was frequently associated with an increased density of the inner ZP, possibly related to the occurrence of zona 'hardening'. Furthermore, slight to moderate microvacuolization was revealed in the ooplasm of some F/T oocytes, particularly in those treated with sucrose 0.3 mol/l.

CONCLUSIONS

Freeze/thawing procedures are associated with ultrastructural alterations in specific oocyte microdomains, presumably linked to the reduced developmental potential of mature cryopreserved oocytes. Further work is needed to determine whether or not a high concentration of sucrose plays a role, at least in part, in producing the above alterations.