According to this

Blastocyst & Embryo Freezing in IVF

Embryos can be frozen at the pronuclear stage (one cell), or at any stage after that up to and including the blastocyst stage (5-7 days after fertilization).

Given that freezing normally destroys mammalian cells1 why can early stage embryos survive being frozen?


1: Shier, W. T. (1988). Studies on the mechanisms of mammalian cell killing by a freeze-thaw cycle: conditions that prevent cell killing using nucleated freezing. Cryobiology, 25(2), 110-120.

  • $\begingroup$ I think you might find this answer to a somewhat related question of interest: biology.stackexchange.com/questions/43077/… ——— Since freezing usually kills tissues unless they are specially treated, I think a better question would be why early embryos can survive freezing! Based on this, I've suggested edits to your question. $\endgroup$
    – tyersome
    Jul 29 '19 at 22:48

The reason why cells (and tissues) die during freezing is that ice crystals rupture the cell membranes1,2, so to me it seems the question is actually why early embryos can survive this process.

The answer depends on the technique used, but it comes down to size. Small tissues (or cells) have a large surface area to volume ratio.

In techniques that use cryoprotectants (compounds that suppress ice crystal formation), the small size allows efficient replacement of water by cryoprotectant prior to freezing (and the reverse after thawing) so that the cells don't have time to die1,2.

In flash freezing (vitrification), the small size allows the entire tissue to be frozen instantaneously, which also suppresses ice crystal formation1,2.

This Scientific American article is very relevant as well.


1: Konc, J., Kanyó, K., Kriston, R., Somoskői, B., & Cseh, S. (2014). Cryopreservation of embryos and oocytes in human assisted reproduction. BioMed research international, 2014.

2: Loutradi, K. E., Kolibianakis, E. M., Venetis, C. A., Papanikolaou, E. G., Pados, G., Bontis, I., & Tarlatzis, B. C. (2008). Cryopreservation of human embryos by vitrification or slow freezing: a systematic review and meta-analysis. Fertility and sterility, 90(1), 186-193.


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