Over the past decade, pluripotent stem cells have provided researchers in diverse fields with a new tool to probe developmental biology, define the underlying pathology of diseases, and develop cell-based therapies for genetic disorders. Whatever the source of the stem cells used in a lab—harvested from IVF embryos, garnered from another lab’s cell lines, or reprogrammed from adult cells using chemical factors—they’ll likely all have one destination in common: the freezer.
Whether a lab is managing large collections of individual patient samples or simply saving cell lines to be used for research down the road, freezing cells offers a solution to long-term storage problems and a source for replicating experiments in the future.
But if you’re adding stem cells to your research repertoire, you will need to acquaint yourself with some cryoprotection stumbling blocks. Stem cells, it turns out, generally aren’t as easy to freeze as differentiated cells. You can’t just take your favorite cell-freezing reagent, apply your usual methods, and stick the cells in a box in the freezer.
When scientists first began working with human embryonic stem cells—and putting cell colonies on ice for future use—they found that only around 5 percent of cells in a given sample were alive and pluripotent after a single freeze-thaw cycle. For one thing, stress, including that brought on by cold temperatures, encourages differentiation, says Alexandra Stolzing of the Fraunhofer Institute for Cell Therapy and Immunology in Leipzig, Germany. “If you push them toward a random tissue state, you lose the value of having stem cells in the first place.”
Freezing also seems to activate a programmed cell-death pathway, but adding apoptosis inhibitors to the medium only slightly improves yield in most cases. Researchers are still trying to understand exactly how low temperatures affect pluripotent stem cells and are working to create new protocols and reagents that enable more cells to survive. Slowly, their discoveries are making it easier to pick up the tricks of stem cell cryopreservation.