Developmental potential of frozen-thawed human blastocysts

Vitrification of oocytes, embryos and blastocysts

In assisted reproductive technology, cryopreservation of human oocytes and embryos has been significantly improved by refined slow-cooling and the new vitrification method. The slow-cooling method requires a programmed cryo-machine, and usually takes several hours. It is, however, difficult to eliminate injuries resulting from ice formation completely. Vitrification has become a reliable strategy because it is simple, can lead to high survival rates and viability, and has better clinical outcome. Vitrification transforms cells into an amorphous glassy state inside and outside the vitrified cell with ultra-rapid cooling and warming steps by plunging the oocytes and embryos into liquid nitrogen, instead of ice-crystal formation. Over the past decade, several advances in vitrification technologies have improved clinical efficiency and outcome. In this chapter, we focus on vitrification technologies for cryopreservation in human assisted reproductive technology.

Current aspects of blastocyst cryopreservation

Cryopreservation of human gametes and embryos has become an essential part of assisted reproduction. Successful cryopreservation of human blastocysts is increasingly relevant as extended in-vitro culture of human embryos becomes more common, permitting routine use of blastocyst transfer in IVF programmes. This reduces the number of embryos transferred, thereby reducing multiple pregnancies and maximizing cumulative pregnancy rates per oocyte retrieval. The superiority of blastocyst freezing over earlier stage freezing in terms of implantation per thawed embryo transferred improves overall expectations for the cryopreservation programme. Therefore, a reliable procedure for the cryopreservation of blastocysts is needed because, after transfer, only a small number of supernumerary blastocysts are likely to be available for cryopreservation. Since the early 1980s, two common techniques have been used in cryopreservation: the conventional slow cooling method and the more recent rapid procedure known as vitrification. Vitrification has become an attractive alternative to slow freezing, since it appears to result in significantly higher survival and pregnancy rates. The aim of this review is to focus on the cryopreservation of human blastocysts using slow and rapid protocols and to assess the impact of the crypreservation protocol used on the survival, implantation and pregnancy rates.

Ultrastructure of preimplantation genetic diagnosis-derived human blastocysts grown in a coculture system after vitrification

OBJECTIVE

To evaluate ultrastructural features of preimplantation genetic diagnosis (PGD) blastocysts before and after vitrification.

DESIGN

Descriptive study of both vitrified and fresh hatching blastocysts.

SETTING

PGD program at the Instituto Universitario, Instituto Valenciano de Infertilidad.

PATIENT(S)

Patients undergoing PGD donated their abnormal embryos for research (n = 26).

INTERVENTION(S)

Biopsied embryos were cultured in the presence of human endometrial cells until day 6. Sixteen blastocysts were vitrified. A total of 11 high-scored hatching blastocysts, 6 warmed and 5 fresh, were fixed for ultrastructure.

MAIN OUTCOME MEASURE(S)

The cytoskeleton structure, type of intercellular junctions, and basic intracellular organelles in trophoectoderm cells and the inner cell mass were analyzed.

RESULT(S)

Ten of 16 blastocysts (62%) survived the warming process. Six of these showed no signs of cell degeneration and light microscopy revealed similar ultrastructural characteristics to those of controls. However, in trophoectoderm cells from both fresh and cryopreserved blastocysts, a reduced number of tight junctions and the presence of degradation bodies were detected.

CONCLUSION(S)

The particular ultrastructural features observed in PGD-derived blastocysts could be related to embryo manipulation and culture conditions. Vitrification does not seem to alter blastocysts, as those that survive hatching do not display detectable cellular alterations when observed through electron microscopy.

Vitrification demonstrates significant improvement versus slow freezing of human blastocysts

Blastocyst culture has reduced the number of embryos transferred per cycle, whilst simultaneously creating new quandaries regarding supernumerary blastocyst cryopreservation. This retrospective study was undertaken to compare a slow freezing protocol to a vitrification protocol for cryopreservation of day 5 and day 6 human blastocysts. To demonstrate this, the survival, implantation rate and pregnancy rates were compared after thawing, assessment and embryo transfer of 86 consecutive day 5 and day 6 thawed blastocyst transfer cycles from January 1, 2002 to December 31, 2003. Seventy-one day 5 slow-frozen (SF) blastocysts were thawed and 59 embryos survived the thawing (83.1%). An average of 2.5 SF blastocysts was replaced per embryo transfer, resulting in a pregnancy rate of 16.7% (4/24). Concurrently, 41 vitrified (VIT) blastocysts were thawed and all 41 survived the thawing process (100%). An average of 2.0 VIT blastocysts was replaced per embryo transfer, resulting in a pregnancy rate of 50% (10/20). Survival, pregnancy and implantation rates of day 5 VIT blastocysts have significantly increased (P < 0.01, P < 0.02 and P < 0.01 respectively) over day 5 SF blastocysts. A similar trend was observed with day 6 blastocysts.

Improved outcome of frozen-thawed blastocyst transfer with Menezo's two-step thawing compared to the stepwise thawing protocol

PURPOSE

To compare the effects of two different blastocyst thawing protocols on implantation, pregnancy, and live birth rates.

METHODS

Ninety four consecutive frozen-thawed blastocyst transfers from 1996 to 2002 were retrospectively analyzed. Blastocysts were cryopreserved using Menezo's two-step slow freezing protocol. Frozen blastocysts were thawed for transfer according to Menezo's stepwise or two-step protocol. Immediately after thawing, assisted hatching was performed and all embryos were cultured for 3 to 5 h before transfer. Only viable embryos were selected for transfer.

RESULTS

Implantation, pregnancy (determined by the presence of fetal cardiac activity), and live birth rates were significantly higher with two-step (25, 45.7, and 42.9%) than with stepwise thawing protocol (9.2,18.6, and 16.9%, P < 0.01). The percentage of gestational sacs resulting in live babies was higher with two-step thawing (76.6%) than with stepwise thawing (50%, P = 0.04).

CONCLUSIONS

We conclude that the two-step thawing protocol offers advantages over the stepwise method. The two-step thawing protocol dramatically increased embryo implantation potential, resulting in higher pregnancy rate, and subsequent live birth rate, after frozen blastocyst transfer.

Vitrification versus programmable rate freezing of late stage murine embryos: a randomized comparison prior to application in clinical IVF

A prospective randomized trial was performed to compare post-thaw development of murine blastocysts following programmable rate freezing and two methods of vitrification. Frozen 2-cell murine embryos (n = 429) thawed and cultured for 48 h, were randomly allocated by stage of development into four groups: control (not refrozen), programmable rate freezing (PR) in 0.25 ml straws, vitrification in flexible micropipettes by immersion in super-cooled (VSC) liquid nitrogen (LN2), and vitrification in flexible micropipettes by immersion in LN2 (VLN). Survival, developmental stage progression, presence or absence of an inner cell mass (ICM), and cell counts were recorded 24 h post-thaw. All measured outcomes were different between embryos from the control group and all freezing methods. Controlled-rate freezing resulted in the lowest total cell counts and fewest embryos with a distinct ICM. A higher percentage of embryos survived 24 h post-thaw, progressed to more advanced developmental stages and had higher total cell counts after VLN compared with PR. Moreover, fewer embryos, frozen by either PR or VSC, contained a detectable ICM compared with VLN. These data demonstrate that vitrification may be a better method for freezing murine blastocysts than PR, and may prove to be a superior method for freezing human blastocysts.

Successful freezing and thawing of blastocysts cultured in sequential media using a modified method

OBJECTIVE

To evaluate the clinical role of blastocyst freezing and thawing after prolonged culturing in sequential media.

DESIGN

Retrospective analysis of 293 blastocyst freeze-thawing cycles.

SETTING

University hospital infertility unit.

PATIENT(S)

Nonselected couples undergoing IVF.

INTERVENTION(S)

Blastocysts were frozen and thawed by a modified method.

MAIN OUTCOME MEASURES

Blastocyst recovery after freeze-thawing and pregnancy rates after the transfer. Evaluation of the effect of the number of transferred blastocysts, the method of IVF, and of the woman's age on the results achieved by frozen-thawed blastocysts.

RESULT(S)

Frozen-thawed blastocysts provided a 29.5% clinical pregnancy rate per transfer. After the transfer of three blastocysts the pregnancy rate was 42.0%, and after the transfer of one or two blastocysts it was approximately the same (25.0% and 28.0%, respectively). The method of IVF did not affect pregnancy rates, but the increasing age of the woman did. Pregnancies were characterized by a low abortion rate (8.0%) regardless of the age of the woman.

CONCLUSION(S)

A modified method for blastocyst freeze-thawing provides good clinical results. It offers the possibility for a single-thawed blastocyst transfer and represents a good alternative for older women because of its lower risk of spontaneous abortion.

Initial experience with extended culture and blastocyst transfer of cryopreserved embryos

OBJECTIVE

Our purpose was to evaluate the viability and transfer efficiency of cryopreserved embryos allowed to develop into blastocysts in extended culture for in vitro fertilization.

STUDY DESIGN

The embryos for in vitro fertilization that had been cryopreserved at either 2 PN (pronuclear) or cleaving stage (day 1-3) were thawed and cultured for uterine transfer on day 5. Outcome for day 5 embryo transfer was prospectively compared with previous outcomes from embryos transferred on day 2 or 3.

RESULTS

For embryos thawed and transferred on day 2 or 3 (n = 99), the pregnancy rate was 33%, the implantation rate per embryo transferred was 15.2%, and the rate of multiple gestations was 42.4% (14/33) with 35.7% of pregnancies having >/=3 gestational sacs. For extended culture embryos transferred on day 5 (n = 25), the pregnancy rate was 36%, the implantation rate per embryo transferred was 16.7%, and the rate of multiple gestations was 33.3% (3/9) with all of these being twins. For embryo transfers performed on day 5 in which only blastocysts were transferred (n = 9), the pregnancy rate was 66.7%, the implantation rate per blastocyst was 44.4% (greater than the rate for the day 2 or 3 embryos, P =.0043), and the rate of multiple gestations was 33.3% (2/6) with all of these being twins. In extended culture 29.8% of cryopreserved embryos progressed to the blastocyst stage. In this series 4 subjects (15.4%) did not have blastocysts by day 5.

CONCLUSION

Acceptable pregnancy rates can be obtained from cryopreserved embryos cultured to the blastocyst stage with a significantly higher implantation rate. Transfer of embryos that have "self-selected" to blastocysts results in reduced risk of higher-order (>2) multiple gestations, because only 1 or 2 embryos are transferred.