Cord blood is the small volume of blood that remains in the umbilical cord and placenta following a baby’s birth. This blood is rich in hematopoietic stem cells, which are immature cells capable of developing into all types of mature blood cells. Cord blood banking is the process of collecting, testing, and cryogenically preserving these valuable stem cells for potential medical use. The primary purpose of banking is to store these cells for treating various life-threatening diseases, including certain cancers, blood disorders, and immune system deficiencies.
The Cryopreservation Process
The long-term viability of cord blood is achieved through cryopreservation, a specialized technique designed to halt all biological activity without destroying the cellular structures. Before freezing, the blood sample is processed to isolate and concentrate the stem cells, and a cryoprotectant solution is introduced.
The most common cryoprotectant used is dimethyl sulfoxide (DMSO), which is typically added to achieve a final concentration of 10%. DMSO penetrates the cells and reduces free water, preventing the formation of damaging ice crystals during cooling. The sample is then cooled slowly at a controlled rate, down to approximately -80°C. This controlled-rate cooling minimizes cellular stress before the final deep-freeze.
Once the sample reaches this intermediate temperature, it is transferred for long-term storage in specialized cryogenic tanks. These tanks are maintained at ultra-low temperatures, typically below -150°C, using the vapor phase of liquid nitrogen. At this extremely low temperature, all metabolic and biological processes within the stem cells effectively stop, placing the cells into a state of suspended animation.
Viability Under Long-Term Storage
The theoretical lifespan of properly cryopreserved cord blood is indefinite. This is because all biological and chemical activity ceases at the ultra-low storage temperature, meaning the cells do not age or degrade.
While the theoretical lifespan is limitless, proven longevity is constrained by the history of cord blood banking, which began in 1988. Studies demonstrate that cord blood remains fully viable and functional after many years in cryogenic storage. Researchers have successfully analyzed and transplanted units stored for over 20 years, with some showing viability maintained for up to 27 years.
For example, a patient with leukemia was successfully treated using a unit cryopreserved for two decades. The key finding from these studies is that storage duration does not significantly impact the recovery rate or function of the stem cells upon thawing. As long as the storage temperature is maintained without fluctuation, cells retrieved after decades are functionally equivalent to those stored for a short period.
Factors Affecting Post-Thaw Utility
While cryopreservation ensures long-term viability, the ultimate utility of the cord blood depends on its condition when retrieved for clinical use. The initial quality of the collected unit is a significant factor, particularly the total nucleated cell (TNC) count and the CD34+ cell count, which indicate the number of blood-forming stem cells present. A low initial cell count can limit therapeutic options, regardless of storage perfection.
The thawing process is a delicate procedure that directly affects cell survival. Unlike the slow cooling, thawing must be performed rapidly, often by immersing the unit in a 37°C water bath. This quick warming minimizes the toxic effects of the cryoprotectant DMSO on the cells.
Even with perfect technique, some cell loss is inevitable; studies show that the median TNC recovery after thawing is around 70% of the pre-freeze count. Consistent monitoring of the storage tank temperature is also a factor, as any significant temperature excursion above the required ultra-low range can degrade the cells. Viability is measured post-thaw to ensure a sufficient percentage of cells are alive and functional before the unit is used for transplantation.