The closed vitrification system Rapid-i™ gives equally good results compared to an open system

In 2009, a study by Seki and Mazur1 showed the dominance of warming rate over cooling rate for successful vitrification. Before this study was published it was common belief 2 that vitrification requires extremely fast cooling rates of at least 
-10 000°C/minute.

Since such high cooling rates are difficult to attain with closed vitrification systems, open systems have been favoured, despite concerns regarding viral contamination risks of embryos in direct contact with liquid nitrogen.


The study3 “A closed system supports the developmental competence of human embryos after vitrification” has shown that a closed vitrification system can perform at least as well as an open system. The following is an abbreviation of this paper.

Material and Methods

The vitrification systems used in the study were Rapid-i (closed system by Vitrolife) and CryoTop (open system by Kitazato). The same vitrification and warming solutions were used with both systems. The study consists of three parts:

1. Assessment of in vitro development. Vitrified, warmed and re-vitrified zygotes were used. After second warming the zygotes were cultured for 120 hours and blastocyst cell number was counted. Quality was assessed using the Gardner system4.

2. Possible cell membrane damage was studied. Vitrified blastocysts were warmed and divided into three groups, open system vitrification, closed system vitrification and non-revitrified. After revitrification and warming, the proportion of dead cells was determined by nuclei staining.

3. Investigation of implantation competence after embryo transfer. Patients scheduled for transfer of single vitrified-warmed blastocysts in hormone replacement cycles were divided into two groups, transfer of one blastocyst either vitrified with a closed or open vitrification system.

Results

Developmental competence, study part 1.Graph

There were no significant differences between the vitrification systems.

Cell damage, study part 2.
There were no differences in quality of embryos before vitrification.

Graph

There were no significant differences in cell number or percentage of dead cells between the study groups.

Developmental competence after transfer of single vitrified –warmed blastocysts, study part 3.
There were no differences in quality of embryos before vitrification.

Graph
There were no significant differences in results between the two vitrification systems.

Discussion

As shown above, there were no significant differences in survival rates, blastocyst cell numbers, cell damage following vitrification and warming or developmental capacity between Rapid-i and CryoTop.

There have been concerns with the closed vitrification systems that a reduction in cooling rate compared to open vitrification systems would cause ice crystal formation inside and outside cells resulting in cell death and reduced developmental capacity. The results of this study support the findings of Seki and Mazur1 demonstrating that a very high cooling rate is less important for embryo vitrification compared with high warming rates. In this study, warming rates for both systems were higher than the respective cooling rates, possibly explaining the similarities in results.

Thus, the closed vitrification system Rapid-i, which enables aseptic vitrification, can be used to obtain good vitrification results without the risk of impairing the developmental capacity of human embryos. 

REF

  1. Seki S, Mazur P. The dominance of warming rate over cooling rate in the survival of mouse oocytes subjected to a vitrification procedure. Cryobiology. 2009; 59: (75-82).
  2. Nagy NP, Vajta G et al. The human embryo: vitrification. In: Gardner DK et al, editors. Textbook of assisted reproduction technologies. 3rd ed. London: Informa Healthcare; 2009.
  3. Hashimoto S et al. A closed system supports the developmental competence of human embryos after vitrification. J Ass Reprod and Genet. 2013; Published Online Jan 2013.
  4. Gardner DK, Lane M. Culture and selection of viable human blastocysts: a feasible proposition for human IVF: Human Reprod Update. 1997; 3: 367-54.