The Siberian salamander is an extraordinary animal: According to some reports, it can survive at temperatures of 50 degrees below freezing in a state of hibernation and for several decades in permafrost. As soon as the outside temperature rises, the salamander returns to normal activity.
They owe this ability to their liver: It can produce the alcohol glycerol, which acts as a kind of antifreeze in the animal’s body. This lowers the freezing point and helps protect cells and tissues from damage during freezing and thawing. “The formation of ice crystals is the reason why extreme cold is usually so harmful to living beings,” explains Dr. Alexander German from the Department of Molecular Neurology (Director: Prof. Dr. Jürgen Winkler) at Uniklinikum Erlangen. “This is because the crystals can mechanically damage cells, thereby destroying the sensitive nanostructure of the tissue.”
Human embryos can also be preserved for many years through extreme deep freezing. To do this, the cells are treated with chemicals that, like glycerol, prevent the formation of ice crystals. “The tissue also solidifies when cooled to below -130 degrees,” says German. “However, the water in and between the cells transitions into a glass-like state.” Glass is as solid as ice, but its molecules are arranged randomly – not regularly as in a crystal.
The process is called “vitrification”. However, it has not yet been possible to freeze nerve tissue or even entire parts of the brain in such a way that they can resume their function after thawing. One reason for this is that the “antifreeze agents” used are themselves toxic for sensitive cells. In addition, brain tissue is particularly sensitive: it contains hundreds of millions of nerve cells that are linked together by countless tiny contacts called synapses. The neurons exchange information via these connections.
Previous vitrification methods tear apart this highly complex network and also damage the synapses. Even if the individual cells survive, the frozen structure is no longer functional. “However, we have optimized the composition of the preservatives and the cooling process so that the neural tissue remains intact,” emphasizes German.
The team tested the success of its method on brain sections. Researchers used this method to cool part of a rodent brain, the hippocampus, to -130 degrees Celsius. This plays an important role in the storage of memory content. “We were able to use electron microscopy images to prove that the nanostructure of the tissue was not altered by the freezing process,” says German. “After thawing, electrical signals spontaneously formed again in the hippocampus, propagating normally through the neural networks.”
However, the neurons did not just start exchanging information again. Brain researcher Dr. Fang Zheng from the Institute of Physiology and Pathophysiology (Director: Prof. Dr. Christian Alzheimer) at FAU was able to show that something known as long-term potentiation could also be triggered at the synapses of the nerve cells. This refers to a key cellular process that ensures frequently used synapses are strengthened, enabling them to transmit information particularly well. “This mechanism is of central importance for learning processes and the storage of new memory content,” says German.
The method developed in the study apparently makes it possible to preserve brain tissue in a functional state over a long period of time and to examine it again later for functionality. For example, in some people with epilepsy, nerve cells are removed during surgery. Such samples could be used years later for testing medications. Cryopreservation of pathologically altered tissue is also important for research into neurodegenerative diseases.
Alexander German also hopes that in the future it will be possible to put entire organisms into a kind of artificial hibernation and revive them after an extended period of time. “This could be an option for space travel, for example, or for people suffering from a currently incurable disease,” he says. “Because at a later date, there may be a treatment option that can help the person affected.”
Proceedings of the National Academy of Sciences
Functional recovery of the adult murine hippocampus after cryopreservation by vitrification
3-Mar-2026