Type of Intracellular Ice Formation: DISCUSSION


The purpose of this study was to determine whether the temperature at which intracellular ice is formed in mouse preimplantation embryos is influenced by the developmental stage. Knowledge of those temperatures is essential for computing the likelihood of IIF as a function of cooling rate. At the cooling rate used here (predominantly 20°C/min), modeling shows that mouse oocytes and embryos lose only — 12% of their water osmotically during cooling to —70°C [16, Fig. 1.9]. That means that the chemical potential of water inside the cells will become increasingly higher than that of the ice and solution outside the cell. Put differently, it means that the cell water becomes increasingly supercooled. At some temperature, the supercooling can no longer be maintained. http://cheap-asthma-inhalers.com/

The internal contents will be nucleated, and the cells will freeze internally. There are two forms of ice nucleation. One of them is heterogeneous. It occurs as a consequence of the presence or appearance of a nucleating agent, the most effective nucleating agent being an externally added ice crystal. The other form of nucleation is homogeneous. It occurs when, by random fluctuations, enough liquid water molecules assume the configuration of an ice crystal of sufficient size to grow spontaneously. Such a crystal is often referred to as a critical size ice embryo. Both theory and experiment show that the homogenous nucleation temperature (Th) of water is near —40°C. The exact temperature at which it occurs is affected primarily by two factors. One is the thermodynamic freezing/ melting point of the system.

If that is suppressed colligatively by added solutes, Th is suppressed approximately twice as much. The second factor is the volume of the supercooled element of water. The smaller it is, the lower is the probability per unit time of the formation of a critically sized ice embryo. The volume of water in mouse oocytes and embryos is 1.9 X 105 im3, a volume that remains nearly constant at the different developmental stages studied here. The effect of water volume on Th is significant but fairly weak; but for that volume, the computed Th of a 1 M EG solution is —40°C.

This entry was posted in Intracellular Ice Formation and tagged developmental stages, nucleation, oocytes.