Cryopreservation of Embryos

Cryopreservation is the process where embryos are preserved (frozen). Embryos are drawn up into straws and loaded into a cryo chamber of the cryo bath. The cryo machine is temperature controlled and freezes at the gradual rate to ensure steady crystallization. After the cryo machine reaches -35°C, the embryos are plunged into liquid nitrogen at -196°C and stored in long-term storage tanks until transfer or export date. Embryos can be stored indefinitely at these conditions, with very little or no change in conception due to long storage.

Embryo quality when freezing is very important, with grade 1 embryos having the best chance of development after thawing and transfer. Grade 1 and 2 embryos preserve well, but it is preferred to transfer both grade 2 and 3 embryos fresh. Grade 3 embryos do not freeze well and conception rates are low.

With IVF embryo production we preserve blastocyst embryos with a three-step freezing protocol. The advantages of the media we use, is that the IVF embryo can be thawed and transferred directly with decent results. Only grade 1 embryos are selected for preservation or fresh transfer.

Embryo preservation after flushing is a much easier process with a one-step freezing media, with direct thaw and transfer. It is important to use the correct thaw protocol, depending on media and protocol used during the freeze process.

Embryo development stages are also determined during embryo grading to ensure the right developed stage embryo, be placed in the same stage recipient depending on reaction and day of heat during the recipient preparation program.

Embryos are classified as follows:

1. Development stage:

  • Morula (M) – is an embryo with 16 to 32 cells. At this stage the cells form a solid ball contained within the zona pellucida (capsule). Morula’s are great for embryo freezing in the case of embryo flushing and ideal for fresh transfer.
  • Early Blastocyst (EB) – is an embryo with 32 to 64 cells. At this stage the embryo starts forming a cavity, called the blastocoel, that occupies less than 50% of the embryo inside the zona pellucida. Early Blastocysts are great for embryo freezing in the case of embryo flushing and ideal for fresh transfer.
  • Blastocyst (B) – is an embryo with more than 64 and up to 100 cells. At this stage the embryo has formed a large blastocoel, that occupies more than 50% of the embryo inside the zona pellucida. If the blastocyst starts expanding, it is indicated as an Expanded Blastocyst (XB). Blastocysts are great for embryo freezing in the case of embryo flushing and IVF production and ideal for fresh transfer.
  • Hatch (H) – is an embryo with more than 100 cells. At this stage the embryonic cells have escape the zona pellucida, which is called hatched. This embryo is great for fresh transfer, but because of the protective capsule not being there we prefer not to freeze it. Not long after this, the embryo will aim to attach to the uterine wall in the uterus.

2. Embryo Grading:

  • Grade 1 – have more than 85% of the total cellular material inside the embryo being alive and normal.
  • Grade 2 – have between 50% and 85% of the total cellular material inside the embryo being alive and normal.
  • Grade 3 – have less 50% of the total cellular material inside the embryo being alive and normal.
  • Degenerates – are embryos where all the cells have died off or development stopped. These embryos are discarded.
  • Oocytes – are unfertilized and will also be discarded.

Embryo grading has a direct correlation with conception rate, with grade 1 having the best pregnancy rate. The transfer veterinarian also grades the reaction of the surrogates to fit the embryo development stage and grade to a suitable recipient.