Stem cells are the body’s self-renewing, unspecialized cells that possess the capacity to differentiate into various specialized cell types, such as blood, bone, or nerve cells. This ability places them at the forefront of regenerative medicine, offering therapeutic potential for a wide range of diseases. Banking stem cells, particularly those collected from umbilical cord blood or adult tissues, requires long-term preservation to ensure they remain viable until a future medical need arises. The challenge lies in halting all cellular activity indefinitely without causing damage to the delicate biological structures.
The Science of Long-Term Cryopreservation
Preserving stem cells for decades relies on a process called cryopreservation, which is the storage of biological material at ultra-low temperatures to achieve a state of suspended animation. The temperature required to halt all metabolic processes is typically below the glass transition temperature of water, which is approximately -130°C. Standard practice involves cooling cells down to the temperature of liquid nitrogen, which is -196°C, or storing them in the vapor phase above it, generally between -150°C and -190°C.
Achieving this frozen state without destroying the cells requires the use of cryoprotective agents (CPAs), such as Dimethyl Sulfoxide (DMSO). CPAs work by penetrating the cells and lowering the freezing point of the water inside, which minimizes the formation of damaging ice crystals that would otherwise puncture cell membranes. The cooling itself must be precisely controlled using a slow, programmed rate, often around -1°C per minute, to allow water to exit the cells gradually and prevent osmotic shock. The combination of CPAs and controlled-rate freezing ensures the cells are safely cooled to the temperature where biological activity is stopped.
Documented Viability: How Long Is “Indefinite”?
Empirical evidence from decades of research suggests that the storage duration for properly cryopreserved stem cells is extremely long, leading to the scientific consensus that storage is “indefinite” under ideal conditions. This concept is based on the principle that at temperatures below -130°C, all enzymatic and metabolic activity is effectively arrested. For hematopoietic stem cells (HSCs), which are used in bone marrow and cord blood transplants, studies have demonstrated successful retrieval and function after storage for over 20 years.
Umbilical cord blood stem cells have remained viable and functional even after 27 years of cryopreservation. Research consistently shows that the length of cryopreservation does not significantly diminish the recovery of viable cells or their ability to engraft. An analysis of autologous hematopoietic stem cells cryopreserved for up to 34 years showed that the cells still retained functional capacity. These findings suggest that the practical limit is determined less by the cells themselves and more by the integrity of the storage facility and the consistency of the ultra-low temperature.
Post-Storage Quality and Thawing Protocols
While long-term storage is possible, a cell sample’s quality and usability are ultimately determined by the process of retrieval and revival. The most important step in retrieving cells is the rapid thawing process, typically accomplished by plunging the cryovial into a 37°C water bath. This rapid warming prevents the formation of large, damaging ice crystals that can occur if the cells are warmed slowly, a phenomenon called recrystallization.
Once thawed, the viability of the cells must be immediately assessed to ensure they are suitable for clinical use. Standard quality control protocols involve a post-thaw viability test, often using dyes like Trypan Blue or 7-Amino Actinomycin D to distinguish between live and dead cells. A sample is generally considered viable if a specific percentage of cells, often over 70% or 80%, are living. Before the stem cells can be administered, the cryoprotective agents, especially DMSO, must be removed quickly because they are toxic to the patient. This is done through a washing process that dilutes and removes the CPA, completing the transition to a functional therapeutic product.