Physical factors are involved in the destruction of embryos and oocytes during freezing and thawing procedures.

Observations made during the freezing and thawing of mouse and human oocytes and mouse embryos with the cryomicroscope suggest that physical factors as well as physicochemical factors may play a role in the development of lethal damage upon thawing. The point of contact with the approaching ice front may predispose that area to the appearance of future cytoplasmic blebbing. The ice front distorts the oocyte and this distortion remains during its subsequent thermal history and is unrelated to desiccation distortion. Ice initiates the formation of both intra- and extracellular gas bubbles which are apparent upon thawing; with the progression of the thawing process they can be seen to grow in volume. Growth of these bubbles can give rise to expanding vesicles which can totally destroy an embryo. The consequences of these physical factors for the successful cryopreservation of oocytes, embryos, tissues and organs are discussed.