The Hippocampal Slice Cryopreservation Project

by Ben Best




The Hippocampal Slice Cryopreservation Project (HSCP) was a research project conducted by cryobiologists Dr. Gregory Fahy and Dr. Yuri Pichugin at the University of California in Los Angeles (UCLA) with funds provided by The Institute for Neural Cryobiology (INC) and UCLA. The objective of the project was to cryopreserve hippocampal slices through vitrification at −130ºC, with complete viability upon rewarming.

The hippocampus is the area of the brain thought to be most critical in learning new information through a process known as LTP (Long-Term Potentiation). More neurophysiological experiments are performed on hippocampal slices than on any other brain tissue.The hippocampus is also the brain tissue most easily damaged by oxygen deprivation (ischemia) and is one of the first areas of the brain affected by Alzheimer's Disease. In addition to the medical and neurophysiological significance of the hippocampus is the fact that hippocampal slices are not only easy to study & manipulate, but are so widely studied by others that considerable literature & equipment exists to assist the study.

The HSCP used hippocampal slices which Dr. Pichugin prepared from rat brains and evaluated for viability in an Oslo Chamber. An Oslo Chamber is a plastic container that looks like a casserole dish with aerated partitions where hippocampal slices can be maintained in an environment controlled for temperature, oxygenation and humidity. Hippocampal slices from an adult rat can live in Oslo Chambers for 12 hours or longer. Injuries to slices occurring during surgical removal from a rat brain may even heal in the Oslo Chamber.

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An initial experiment compared introduction of various cryoprotectants with stepped increases (and washouts) of cryoprotectants so as to reach peak concentration at 22ºC, 2ºC and −22ºC. Mannitol was added on washout to buffer against osmotic jolts. In this initial experiment the dye MTT (which measures the ability of mitochondria to transport electrons) was used to indicate viability.

The cryoprotectants used in this initial experiment were ethylene glycol (used in automobile antifreeze), DMSO (DiMethylSulfOxide), glycerol and VEG (pronounced "Vee Ee Gee", not "veg"). VEG is a modification of the cryoprotectant cocktail VS41A (VS41A is a Vitrification Solution which is a mixture of DMSO, formamide and propylene glycol) in which the propylene glycol is replaced with an equal weight of ethylene glycol (the "EG" of VEG). [VEG is patent 6,395,467 -- 21st Century Medicine (21CM) -- composed of 16.84% w/v ethylene glycol, 13.96% w/v formamide and 24.2% w/v DMSO, totalling 55% w/v cryoprotective solution.] DMSO showed the worst viability, ethylene glycol the next worst, and glycerol the third worst at two of the 3 temperatures studied, with VEG being the best overall. The superiority of glycerol to DMSO was reassuring in view of unpublished experimental results of Isamu Suda showing the same superiority for whole brains (see next paragraph).

Decades ago the Japanese experimentalist I. Suda had exposed whole cat brains to 15% glycerol, stored them for 5 days at −20ºC and then demonstrated that the cat brains had EEG patterns similar to those of cat brains that had not been subject to cooling [NATURE 212:268-270 (1966) and BRAIN RESEARCH 70:527-531 (1974)]. In the spirit of Suda, hippocampal slices were subjected to 30% glycerol and then either stored for 12 hours at −20ºC, −40ºC and −76ºC or cooled to these temperatures and rewarmed without storage. In these and all subsequent experiments, the viability assay was based on the potassium/sodium ratio (K+/Na+ ratio). For neural tissue, especially, the ability of cells to maintain membrane potential with the sodium pump is an easily-measured & reliable indicator of viability. Based on this assay, although the controls exposed to 30% glycerol showed 70% viability, those cooled and stored at −20ºC were essentially dead. Those stored at −40ºC and −76ºC had slightly better viability, but not more than 15% viability, and the effects of cooling to −76ºC were not improved by warming immediately from that temperature (no storage). Using 30% VEG rather than glycerol produced even worse results -- possibly because the temperature of introduction of VEG was too high. VEG needs to be added at a lower temperature than glycerol.

In addition to the ice crystal damage of freezing and the toxicity of cryoprotectant, attempts to cryopreserve organs & tissues are also hampered by a phenomenon known as chilling injury. To investigate this phenomenon hippocampal slices were cooled to 0ºC for one hour (without cryoprotectant) and then rewarmed & assayed. The slices showed 30% of the viability of controls which had been maintained at 37ºC. Hippocampal slices held at 0ºC in artificial CerebroSpinal Fluid (aCSF) for 50 minutes also showed 30% viability. This result was not changed by using modified aCSF containing mannitol (maCSF) instead of the standard aCSF, at 50 min of exposure to 0ºC. But adding 10% glycerol seemed to reduce the chilling injury somewhat -- and adding 10% VEG resulted in a 40% viability. 25% VEG (with & without mannitol) resulted in 50% viability. These results indicate a protective effect of VEG against the 0ºC chilling injury.

The experiment was repeated with controls at 37ºC and 10ºC as well as with 25% VEG slices at 10ºC and 0ºC. There was little significant difference between the 37ºC controls, the 10ºC controls or the 10ºC 25% VEG slices, but the 0ºC 25% VEG slices lost 30% of viability. This indicates that chilling injury occurs between 10ºC and 0ºC. Worse results were obtained with VEG addition at 15ºC -- indicating that 10ºC is the optimum temperature for introduction of VEG. Moreover, at 10ºC introduction of VEG at 5-minute steps of increasing concentration produced better viability than introduction of VEG at 10-minute steps -- indicating that toxicity damage is more important than osmotic damage at this temperature and for these times & concentrations. Experiments also indicated that 300mM mannitol is the optimum concentration for buffering osmotic damage during removal of VEG.

Carrier solutions can enhance cryoprotection. RPS−2 (Renal Preservation Solution number 2) was developed by Dr. Fahy in 1981 as a result of studies on kidney slices. RPS−2 actually resulted in hippocampal slice viability at 10ºC which was 50% greater than that of control slices at 37ºC after one hour in the Oslo Chamber (although the difference disappeared after 2 hours). A combination of 25% VEG with RPS−2 at 10ºC resulted in viability fully the same as that of 37ºC controls -- although 25% VEG is insufficient concentration for vitrification. Pushing the VEG concentration to 50% in RPS−2 at 10ºC brought the slice viability back down to 50% that of the 37ºC controls.

The next experiment was based on the idea that both chilling injury and toxicity should be reduced at sub-zero temperatures. VEG was added at 10ºC in steps up to 25% concentration, and a final 25% VEG (bringing the total to 50%) was added (and washed-out) at −10ºC. These 50% VEG slices were only slightly less viable (and not statistically less viable) than the 37ºC controls. The concentration of VEG required to vitrify is estimated to be 53%. Adding (and later washing-out) 30% rather than 25% VEG at −10ºC (bringing the total to 55%) resulted in viability 77% of that of controls.

In the next experiment one group of slices with 53% VEG was held at −10ºC while the other group with 53% VEG was cooled to vitrification temperature (−130ºC) and then rewarmed. The non-vitrified 53% VEG slices had 60% viability and the vitrified 53% VEG slices had 56% viability as compared to untreated controls. The difference between the VEG-treated slices and the slices that were exposed to VEG and then vitrified was not statistically significant. But when the experiment was repeated, the vitrified group was significantly less viable.

The following experiment added VEG at 5-minute intervals at 10ºC, but the final addition (and washout) at −10ºC was given 10 minutes for diffusion. Viability of the non-vitrified slices exposed to 53% VEG was 65%, but when slices treated in this way were vitrified, the K/Na ratio was only 42% of control K/Na, perhaps due to inadequate equilibration. In conclusion, using 10-minute steps throughout all phases of the addition and washout procedure was able to protect completely against vitrification/devitrification injury, but was associated with increased toxicity. Nonetheless, the viability after vitrification was up to 56% of untreated control viability.

Subsequent experiments made use of ice blockers and cryoprotectant cocktails less toxic than VEG as well as longer equilibration times prior to vitrification. The worst of these newer experiments gave results equal to the best results obtained with VEG, and the best results of the newer experiments indicated no difference between vitrified slices and untreated controls. However, the variability of the results were very high -- up to plus or minus 25%, with an average of 75% viability of hippocampal slices which have been vitrified at −130ºC. Efforts were devoted to reducing this variability and moving the average viability closer to 100%.

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  1. Vitrification currently results in at least five times higher recovery of K/Na ratio than could be obtained after freezing, even though freezing involved higher concentrations of glycerol than were used by Suda.
  2. Chilling injury can be minimized by minimizing time spent at 0ºC. Chilling injury is confined to a window between +10ºC and −10ºC, being absent at both of these temperatures.
  3. Chilling injury can be moderated but not eliminated by cryoprotectants. VEG reduces chilling injury better than glycerol.
  4. RPS−2 can be used with VEG to significantly increase the viability of hippocampal slices.
  5. Although it is apparently possible to achieve 100% recovery of K/Na ratio after vitrification and warming, it is presently not possible to do this on a consistent basis. Perhaps by eliminating sources of experimental variation and continuing to refine procedures and cryoprotectants, this final barrier can be overcome.

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The final results of the HSCP were published in the April 2006 issue of the peer-reviewed journal CRYOBIOLOGY -- [CRYOBIOLOGY 52(2):228-40 (2006)]. (Also available as a PDF file: Cryopreservation of rat hippocampal slices by vitrification )

When hippocampal slices were treated with 50% w/v VEG in aCSF carrier solution at 0ºC there was considerable damage, which was mostly eliminated by the use of RPS−2 carrier solution instead of aCSF. But 53% w/v VEG was the minimum concentration which would vitrify at the cooling rates used in the study. An increase of VEG concentration from 50% w/v to 53% w/v caused a considerable reduction of viability above −10ºC (but no further injury from −10ºC to −130ºC).

The viability problems were eliminated by using VM3 rather than VEG as the cryoprotectant solution. VM3 is non-toxic (>90% K/Na ratio recovery upon exposure of tissues to VM3). Slices treated with VM3 cooled to −130ºC (vitrified) and rewarmed were as viable as slices treated with VM3 without any cooling. VM3 is composed of 16.84% w/v ethylene glycol, 12.86% w/v formamide, 22.3% w/v DMSO, 7% w/v polyvinylpyrrolidine K12, 1% w/v "Supercool X−100" ice blocker and 1% w/v "Supercool Z−100" ice blocker in a carrier solution of LM5 rather than RPS−2. LM5 maximizes the effectiveness of ice blockers better than RPS−2.

Electron Micrographs (EMs) of the CA1 and CA3 regions of the hippocampus in VM3 after cooling to −130ºC and rewarming showed complete preservation of ultrastructure, in agreement with the >90% viability assays obtained from the same treatment with VM3. Glass transition temperature (Tg) of both VEG and VM3 are 3−4ºC above −130ºC, so the samples had vitrified (solidified).

These results demonstrate the ability to cool hippocampal slices to −130ºC and rewarm them without loss of viability.


For further discussion of cryoprotective agents, see A Summary of the First 21CM Seminars .

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