Chromosome aberrations in mouse embryos and fetuses produced by assisted reproductive technology

Successful ICSI in Mice Using Caput Epididymal Spermatozoa

Spermatozoa undergo their last phase of spermiogenesis, known as maturation, as they pass through the epididymis. A recent report indicates that mouse immature spermatozoa retrieved from the caput epididymis for intracytoplasmic sperm injection (ICSI) give rise to embryos with multiple developmental defects. Further, these embryos were unable to develop to term after their transfer to surrogate mothers. Herein, we examined the potential of mouse caput spermatozoa to produce normal embryos by comparing the use of caput vs. cauda epididymal spermatozoa for in vitro fertilization (IVF) or ICSI. Two methods for the separation of sperm heads prior to ICSI were also compared: freezing/thawing or drawing through a syringe. We found that in contrast to caudal spermatozoa, caput spermatozoa failed to produce embryos via IVF, confirming their immature state. However, regardless of the method employed for the separation of sperm heads, similar efficiencies of blastocyst production in vitro and development to term after transfer to surrogate mothers were observed following ICSI using both caput and cauda epididymal spermatozoa. It therefore seems that mice spermatozoa recovered from the caput epididymis are as valid as caudal spermatozoa for the production of embryos and offspring by ICSI.

Production of mouse fetuses using spermatozoa exposed temporarily to high temperature or continuously to room temperature after freeze-drying in Na+-free/K+-rich EGTA buffer

Present study aimed to determine to what extent freeze-dried spermatozoa were able to withstand high-temperature conditions: transient increase in storage temperature and long-term exposure to room temperature. Mouse spermatozoa were freeze-dried in EGTA/Tris-HCl buffered solution alkalinized using KOH (K-ETBS, pH 7.7), and then stored for up to 7 months at 4 °C or 25 °C. After 2 months' storage, some of the 4°C-stored spermatozoa were exposed to 40 °C for 1 week or 1 month, then again stored at 4 °C for the remaining storage period. Following storage, rehydrated spermatozoa were injected into mouse oocytes. The resulting zygotes were assessed for chromosome damage, in vitro development up to the blastocyst stage, and post-implantation development to normal fetuses on day 18 of gestation. In storage at 4 °C, one-week exposure to 40 °C had no adverse effect on the chromosome integrity and developmental competence compared to non-exposure to 40 °C (continuous storage at 4 °C). In contrast, one-month exposure to 40 °C caused an increasing level of chromosome damage (36%, P < 0.05) and reduced frequencies of blastocysts (54%, P < 0.05) and normal fetuses (36%, P < 0.05) compared to the frequencies obtained by continuous storage at 4 °C (15%, 82% and 52%, respectively). Storage at 25 °C resulted in accumulation of chromosome damage (27%, P < 0.05), leading to decreased blastocyst formation (63%, P < 0.05). But, the frequency of normal fetus (44%) was not significantly different from that obtained by continuous storage at 4 °C. Consequently, mouse spermatozoa freeze-dried in K-ETBS withstood temporary exposure to 40 °C for 1 week. Chromosome damage accumulated in spermatozoa during storage at 25 °C.

Risk of chromosomal aberration in spermatozoa during intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) has become critical for the treatment of severe male infertility. The principal feature of ICSI is the direct injection of spermatozoon into an oocyte, which facilitates the production of fertilized embryos regardless of semen characteristics, such as sperm concentration and motility. However, the chromosomal integrity of ICSI zygotes is degraded compared to that of zygotes obtained via in vitro fertilization. This chromosomal damage may occur due to the injection of non-capacitated, acrosome-intact spermatozoa, which never enter the oocytes under natural fertilization. Furthermore, it is possible that the in vitro incubation and pre-treatment of spermatozoa during ICSI results in DNA damage. Chromosomal aberrations in embryos induce early pregnancy losses. However, these issues may be overcome by embryo production using gametes with guaranteed chromosomal integrity. Because conventional chromosome analysis requires fixing cells to obtain the chromosome spreads, embryos cannot be produced using the nucleus that has been analyzed. On the other hand, genome cloning using androgenic or gynogenic embryos provides an additional nucleus for chromosome analysis following embryo production. Thus, this review aims to highlight the hazardous nature of chromosomal aberrations in sperm during ICSI and to introduce a method for the prezygotic examination for chromosomal aberrations.

Preservation of chromosomal integrity in murine spermatozoa derived from gonocytes and spermatogonial stem cells surviving prenatal and postnatal exposure to γ-rays in mice

Pre- and postnatal male mice were acutely (659-690 mGy/min) and continuously (0.303 mGy/min) exposed to 2 Gy γ-rays to evaluate spermatogenic potential and chromosome damage in their germ cells as adults. Acute irradiation on Days 15.5, 16.5, and 17.5 post-coitus affected testicular development, as a result of massive quiescent gonocyte loss; the majority of the seminiferous tubules in these testes were devoid of germ cells. Acute irradiation on Days 18.5 and 19.5 post-coitus had less effect on testicular development and spermatogenesis, even though germ cells were quiescent gonocytes on these days. Adverse effects on testicular development and spermatogenesis were observed following continuous irradiation between Days 14.5 and 19.5 post-coitus. Exposure to acute and continuous postnatal irradiation after the differentiation of spermatogonial stem cells and spermatogonia resulted in nearly all of the seminiferous tubules exhibiting spermatogenesis. Neither acute nor continuous irradiation was responsible for the increased number of multivalent chromosomes in primary-spermatocyte descendents of the exposed gonocytes. In contrast, a significant increase in cells with multivalent chromosomes was observed following acute irradiation on Days 4 and 11 post-partum. No significant increases in unstable structural chromosomal aberrations or aneuploidy in spermatozoa were observed, regardless of cell stage at irradiation or the radiation dose-rate. Thus, murine germ cells that survive prenatal and postnatal irradiation can restore spermatogenesis and produce viable spermatozoa without chromosome damage. These findings may provide a better understanding of reproductive potential following accidental, environmental, or therapeutic irradiation during the prenatal and postnatal periods in humans.

Two cases of reciprocal chromosomal translocation (4; 7)(p+; q-) (2; 8)(q-; q+) in piglets produced by ICSI

In this study, the karyotypes of 14 piglets from four different litters produced by intracytoplasmic sperm injection (ICSI) and embryo transfer were analysed. The chromosome analysis was based on a classical cytogenetic examination following the standard protocols of lymphocyte cultures. Two cases of reciprocal translocation [(4; 7)(p+; q-) and (2; 8)(q-; q+)] were detected in two female transgenic piglets. These animals showed neither anatomical nor physiological alterations and had normal growth. To our knowledge, this is the first karyotype study of piglets produced by ICSI.

Chromosomal integrity and DNA damage in freeze-dried spermatozoa

Freeze-drying technology may one day be used to preserve mammalian spermatozoa indefinitely without cryopreservation. Freeze-dried mouse spermatozoa stored below 4°C for up to 1 year have maintained the ability to fertilize oocytes and support normal development. The maximum storage period for spermatozoa increases at lower storage temperatures. Freeze-drying, per se, may reduce the integrity of chromosomes in freeze-dried mouse spermatozoa, but induction of chromosomal damage is suppressed if spermatozoa are incubated with divalent cation chelating agents prior to freeze-drying. Nevertheless, chromosomal damage does accumulate in spermatozoa stored at temperatures above 4°C. Currently, no established methods or strategies can prevent or reduce damage accumulation, and damage accumulation during storage is a serious obstacle to advances in freeze-drying technology. Chromosomal integrity of freeze-dried human spermatozoa have roughly background levels of chromosomal damage after storage at 4°C for 1 month, but whether these spermatozoa can produce healthy newborns is unknown. The safety of using freeze-dried human spermatozoa must be evaluated based on the risks of heritable chromosome and DNA damage that accumulates during storage.

Chromosome analysis of mouse zygotes produced by intracytoplasmic injection of spermatozoa exposed to acrosome reaction inducing agents methyl-beta-cyclodextrin and calcium ionophore A23187

PURPOSE

This study was performed to investigate whether removal of cholesterol from the plasma membrane and collapse of the acrosome can prevent structural chromosome aberrations of paternal origin in mouse zygotes produced by intracytoplasmic sperm injection (ICSI).

METHODS

Mouse spermatozoa were treated with methyl-beta-cyclodextrin (M beta CD) to remove cholesterol from the plasma membrane and with calcium ionophore A23187 to collapse the acrosome. Chromosomes of zygotes derived from M beta CD- and ionophore-treated spermatozoa were analyzed at the first mitotic metaphase.

RESULTS

Both chemical agents effectively induced the acrosome reaction. Incidence of structural chromosome aberrations in ICSI zygotes derived from M beta CD-treated spermatozoa was similar to that in zygotes produced by in vitro fertilization (IVF) with the same spermatozoa, but significantly lower compared to ICSI zygotes derived from acrosome-intact spermatozoa. Chromosome aberration rates in ICSI zygotes derived from ionophore-treated spermatozoa were evidently high compared to IVF zygotes.

CONCLUSIONS

Induction of the acrosome reaction through cholesterol efflux by M beta CD can prevent chromosome aberrations of paternal origin, while use of ionophore to induce the acrosome reaction exerts detrimental effect on paternal chromosomes in ICSI zygotes.

Possible causal factors of structural chromosome aberrations in intracytoplasmic sperm injection of the mouse

Incidence of structural chromosome aberrations in mouse one-cell embryos produced by intracytoplasmic sperm injection (ICSI) with mature epididymal spermatozoa were influenced by sperm incubation medium and time. When spermatozoa were incubated in bicarbonate-buffered TYH for ≤0.5 h, the embryo aberration rates were significantly higher than in vitro fertilization (IVF) embryos. However, after the incubation of spermatozoa in the same medium for ≥2 h, the aberration rates were close to the IVF embryo level. When spermatozoa were incubated in bicarbonate-buffered mCZB, hepes-buffered H-TYH and H-mCZB, and phosphate-buffered PB1, the increased incidences of aberrations were observed at any incubation time. In the case of sperm incubation in H-TYH, H-mCZB and PB1, the aberration rates increased in a time-dependent manner. Chromosome aberrations generated by ICSI were transmissible to offspring. On the other hand, the aberration rate in embryos derived from testicular spermatozoa was independent of the medium type and incubation time. Thus, the incubation media appears to have no effect on sperm chromatin. TYH can effectively induce capacitation and acrosome reaction, while H-TYH, H-mCZB and PB1 never induce these spermatozoal events. It is probable that the cholesterol-rich plasma membrane and intact acrosome injected into the ooplasm affect sperm chromatin remodeling, thus resulting in the generation of chromosome damage in ICSI embryos.