Cryopreservation

Cryopreservation of mammalian cell lines has been and still is an absolute requirement for cell culturists. The freezing and thawing processes are complex and very stressful for cells. A substantial portion of the cryo-stored cells do not survive these treatments. In addition, the surviving cells may, due to the harsh treatment, exhibit a lag phase which delays growth after thawing.

The freezing process of a cell suspension can be divided into several stages.

As the temperature is decreased below 0^oC, ice starts to form in the extracellular medium, whereas the cytosol remains unfrozen. Depending on the cooling rate, the cytosol can even become supercooled and stay liquid at temperatures as low as -15^oC. When the water of the extracellular medium begins to form ice crystals, the solutes become more concentrated, a phenomenon also referred to as "solution effect". As a consequence, the concentrated medium equilibrates with the intracellular liquid resulting in cell shrinking. At lower temperatures, ice formation also takes place inside the cell, and the same solution effect is observed in the cytosol, with the disadvantageous consequences such as changes in pH or cytotoxic solute concentration. A further decrease in temperature lets the solute concentration rise to a point which makes crystallization impossible. At a certain temperature, the highly concentrated liquid solidifies in a process called "eutectic freezing". In addition, ice crystals, occuring intra- or extracellularly may disrupt cell membranes resulting in poor cell recovery after thawing. Recrystallization of ice crystals during thawing is thought to be another detrimental factor for cell survival.

The recovery of living cells after storage is thus greatly affected by the freezing protocol, the cooling rate and the compounds in the freezing medium.

The new freezing media FILOCETH^PLUS  FILOCETH^STEM fulfill best the requirements for optimal survival rates of cryopreserved cells.