Role of germinal vesicle on protein synthesis in rat oocyte during in vitro maturation

Factors and molecules that could impact cell differentiation in the embryo generated by nuclear transfer

Somatic cell nuclear transfer is a technique to create an embryo using an enucleated oocyte and a donor nucleus. Nucleus of somatic cells must be reprogrammed in order to participate in normal development within an enucleated egg. Reprogramming refers to the erasing and remodeling of cellular epigenetic marks to a lower differentiation state. Somatic nuclei must be reprogrammed by factors in the oocyte cytoplasm to a rather totipotent state since the reconstructed embryo must initiate embryo development from the one cell stage to term. In embryos reconstructed by nuclear transfer, the donor genetic material must respond to the cytoplasmic environment of the cytoplast and recapitulate this normal developmental process. Enucleation is critically important for cloning efficiency because may affect the ultrastructure of the remaining cytoplast, thus resulting in a decline or destruction of its cellular compartments. Nonetheless, the effects of in vitro culturing are yet to be fully understood. In vitro oocyte maturation can affect the abundance of specific transcripts and are likely to deplete the developmental competence. The epigenetic modifications established during cellular differentiation are a major factor determining this low efficiency as they act as epigenetic barriers restricting reprogramming of somatic nuclei. In this review we discuss some factors that could impact cell differentiation in embryo generated by nuclear transfer.

Transfer of germinal vesicle to ooplasm of young mice could not rescue ageing-associated chromosome misalignment in meiosis of oocytes from aged mice

BACKGROUND

[corrected] Transferring a germinal vesicle (GV) from an aged woman's oocyte into ooplasm from a younger woman has been proposed as a possible way to overcome the problem of age-related decline in female fertility. Here we assessed this possibility by determining whether ooplasts derived from young mice could rescue ageing-associated chromosome misalignment in meiosis of oocytes from aged mice.

METHODS

Three groups of reconstructed oocytes, young GV-young cytoplast (group YY), aged GV-young cytoplast (group AY), and young GV-aged cytoplast (group YA), were created by micromanipulation and electrofusion.

RESULTS

Nuclear transplantation was successful in 89.8-94.4% of GV-ooplast complexes, and maturation rate of the reconstructed oocytes was 93.5-97.9%. Confocal microscopy analysis showed a significantly higher rate (49.2%) of chromosome misalignment in ageing mice than in young mice (16.9%), and 57.1% of oocytes in group AY exhibited chromosome misalignment, while the abnormality rate in groups YY and YA was 16.3 and 16.7% respectively. Calcium imaging showed that the three groups of reconstructed oocytes exhibited a similar pattern of calcium oscillations upon stimulation with bovine sperm extracts. Fertilization rate and developmental capacity to 2-cell embryos were also similar among the three groups of oocytes.

CONCLUSIONS

Our findings suggest that: (i) the ooplasm from young mice could not rescue ageing-associated chromosome misalignment in meiosis of GV from aged mice; and (ii) behaviour of chromosome alignment over metaphase spindle is predominantly determined by GV material.

Germinal vesicle material is essential for nucleus remodeling after nuclear transfer

Successful cloning by nuclear transfer has been reported with somatic or embryonic stem (ES) cell nucleus injection into enucleated mouse metaphase II oocytes. In this study, we enucleated mouse oocytes at the germinal vesicle (GV) or pro-metaphase I (pro-MI) stage and cultured the cytoplasm to the MII stage. Nuclei from cells of the R1 ES cell line were injected into both types of cytoplasm to evaluate developmental potential of resulting embryos compared to MII cytoplasmic injection. Immunocytochemical staining revealed that a spindle started to organize 30 min after nucleus injection into all three types of cytoplasm. A well-organized bipolar spindle resembling an MII spindle was present in both pro-MI and MII cytoplasm 1 h after injection with ES cells. However, in the mature GV cytoplasm, chromosomes were distributed throughout the cytoplasm and a much bigger spindle was formed. Pseudopronucleus formation was observed in pro-MI and MII cytoplasm after activation treatment. Although no pronucleus formation was found in GV cytoplasm, chromosomes segregated into two groups in response to activation. Only 8.1% of reconstructed embryos with pro-MI cytoplasm developed to the morula stage after culture in CZB medium. In contrast, 53.5% of embryos reconstructed with MII cytoplasm developed to the morula/blastocyst stage, and 5.3% of transferred embryos developed to term. These results indicate that GV material is essential for nucleus remodeling after nuclear transfer.

Viable rabbits derived from reconstructed oocytes by germinal vesicle transfer after intracytoplasmic sperm injection (ICSI)

Abnormal oocyte spindle due to the improper function of ooplasm is associated with female infertility of advanced maternal age. A possible way to overcome this problem is to transfer an oocyte germinal vesicle (GV) which contains genetic materials of a patient with a history of poor embryo development to the cytoplast from a donor oocyte. Here we demonstrate that GV transfer is feasible using a rabbit model. When the GVs were transferred to auto- or hetero-cytoplasts of GV stage oocytes, around 80% of the reconstructed oocytes could mature in vitro and 7.1-9.4% of the oocytes developed to blastocyst stage after intracytoplasmic sperm injection (ICSI). Transfer of 93 fertilized eggs reconstructed via GV transfer into six recipients resulted in two live offspring. Results of this experiment indicate that GV transfer can potentially become a new approach in treatment of infertility because of advanced maternal age.

In vitro maturation of human preovulatory oocytes reconstructed by germinal vesicle transfer

OBJECTIVE

To describe a micromanipulation-electrofusion procedure for transferring germinal vesicles (GVs) between immature human oocytes.

DESIGN

Pilot study to assess oocyte maturation after an invasive micromanipulation procedure.

SETTING

Research laboratory at a university medical center.

PATIENT(S)

Immature oocytes were discarded from intracytoplasmic sperm injection (ICSI)-IVF cycles of patients 23-48 years of age.

INTERVENTION(S)

Initially, GV removal and transfer were performed on the same oocyte; these "self-reconstructed" oocytes were then cultured in vitro for up to 50 hours and examined periodically for maturation as judged by the extrusion of the first polar body. In a second study, GVs from oocytes of "old" patients (>38 years old) were successfully transferred into enucleated immature oocytes of "young" patients (<31 years old).

MAIN OUTCOME MEASURE(S)

Extrusion of the first polar body was monitored in "reconstructed" and control oocytes; karyotypes also were analyzed at meiosis II.

RESULT(S)

From 48 oocytes from old patients, 12 GVs were successfully removed, transferred, and fused into previously enucleated oocytes from young patients. After in vitro culture, 7 of these "reconstructed" oocytes matured to meiosis II, a maturation rate not significantly different from that observed in nonmanipulated controls. A normal, second meiotic metaphase chromosome complement was observed in 4 of 5 reconstructed oocytes.

CONCLUSION(S)

Normal meiosis can occur after the transfer of a GV into an enucleated host oocyte. Germinal vesicle transfer may be a valuable research procedure that generates cell models to characterize the cytoplasmic-nuclear interplay for cell cycle regulation, maturation, and fertilization in the human oocyte; it also may be a potentially attractive alternative to oocyte donation.