HPP_img_8507 - Kryo


Slow freezing

“Slow freezing”, probably the most commonly used freezing process worldwide, involves the addition of a cryoprotective agent to prevent the formation of ice crystals in the cells, after which the cells are cooled in steps under computer control to -196 degrees Celsius. This permits long-term cell storage as the biological processes in the cell are shut down. The added cryoprotective agents (anti-freeze media) prevent osmotic damage in the cell as far as possible; despite the low concentrations, these still have a slight cytotoxic effect (poisonous effect on the cell). Furthermore, they do not definitely prevent the formation of ice, which can damage the cell organelles.

While this procedure is relatively simple in technical terms and can be performed with multiple cells at once, it does carry with it a risk of cell loss of approximately 30 %. This means that 1/3 of these cells cannot complete the insemination process and develop into an embryo after thawing.

Even the transfer of embryos with good morphology does not result in a pregnancy rate as high as that expected in a “fresh” cycle. The pregnancy rate may be up to 30 % lower than in the “fresh cycle”.

Slow freezing is practically only advisable with inseminated egg cells, also known as pronuclear (PN) stages, as these are significantly less sensitive than non-inseminated egg cells and embryos.

Two pronuclei

A so-called PN cell. A so-called PN cell. PN stands for pronucleus. The two pronuclei are clearly visible here.

With conventional IVF treatment (including with ICSI), it is common that more egg cells are produced and retrieved than are required to achieve pregnancy in a “fresh cycle”. Shortly before the the end of insemination, which involves the fusion of the genetic material in the egg cell and the sperm cell, the cells are in what is known as the PN (Pro Nucleus) stage. Only in this stage can the cells be frozen with little risk of loss.


Straws (plastic stems) like these are used to freeze 1 - 3 egg cells. The straws are sealed with a steal ball or ideally by welding the end together.

Cryopreservation equipment

One of the cryopreservation devices. Here, the straws are lowered in stages by a computer controlled process through the nitrogen gaseous phase into the liquid (temperature = -196 deg. C).

These can be thawed in later cycles/cultured to form embryos and transferred without the need to repeat hormone treatment or perform surgery to retrieve egg cells.

If the embryo transfer in the fresh cycle has been successful, these cells can be used later on, say two years later, for treatment to conceive another child. The cells can be stored without any additional loss of quality for several years.

However, with slow freezing, which remains the most common freezing method in Germany, it is barely possible to freeze non-inseminated egg cells (such as would be used for fertility preservation prior to cancer treatment or as a “lifestyle” measure) without damage. Even for embryos this procedure is not ideal.