The procedure of cryopreservation is complicated and involves a lot of sophisticated instruments and cryoprotectants. The embryos are initially exposed to cryoprotectants followed by computerised cooling and seeding and then plunged into - 196 degree centigrade liquid nitrogen where it can be preserved indefinitely.
Need for freezing embryos:
1. Surplus embryos which can be used for the same patient at a later date.
2. Danger of ovarian hyperstimulation syndrome where the cycle is cancelled.
3. Patient takes ill after oocyte aspiration.
4. Donor oocyte programme.
5. When the progestrone level is high (more than 1) at the time of HCG
6. When the implantation site is not favourable.
The most important criteria in cryopreservation is that the cell must maintain the structural integrity throughout the cryopreservation procedure. The whole aim is to reduce damage caused by intra-cellular ice formation. This can be achieved by dehydrating the cell before or during cooling procedure, failing which the large intra-cellular crystals can damage the cells.
There are two methods of cryopreservation of embryos:
1. Rapid cooling.
2. Slow cooling.
Rapid cooling protocol involved embryos which are briefly exposed to liquid nitrogen vapour and directly plunged into liquid nitrogen dewars. Here, the solution used for freezing contain higher concentration of cryoprotectants.
Slow cooling involves seeding where the cryopreservation tube is manually touched by a cold forceps dipped in liquid nitrogen further away from the embryo to initiate ice formation which gradually spreads to the rest of the solution containing the embryos. This prevents the damage to the embryos.
Thawing: At the time of embryo replacement, during a natural or HRT cycle, the straws are removed from the liquid nitrogen and placed in a water bath at room temperature before the rehydration procedure.
At the end of rehydration, the embryo is washed and placed in the carbon dioxide incubator till the time of embryo transfer.
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