The ability to freeze immortal cell cultures is a breakthrough because it allows the cell cultures to be used again at a later date. It's extremely easy for a cell culture to become contaminated by microbes and no longer useable. Hence, freezing cell cultures allows us to take a portion of the healthy, living culture and freeze it so that in the event that the rest of the cell culture does become contaminated and no longer useable, scientists still have a backup supply of the cell line. The ability to freeze cell cultures would not have been developed without the achievement of an immortal cell line because other cell lines died before they could be frozen.
According to Rebecca Skloot in her book The Immortal Life of Henrietta Lacks, it was scientist George Gey, head of tissue culture research at Johns Hopkins University Medical Center, who first realized the necessity to achieve growing "immortal human cells." Gey defined immortal cells as a "continuously dividing line of cells all descended from one original sample, cells that would constantly replenish themselves and never die. If a cell line becomes contaminated and dies, it detains a scientist's ability conduct research on the line, making studies much more costly and difficult. Hence, having a cell line that does not die makes conducting research much easier. Having an immortal cell line, or continuously living cell line, ensures a scientist's ability to continue to preserve and study the cells.
Of course, one of the first scientific breakthroughs that stemmed from growing the immortal cell line from Henrietta Lack's cells, referred to as the HeLa cells, was the developed ability to continue to preserve cell cultures through freezing them. We also used the cells to figure out how to immunize against polio, were able to mass produce the cell line, and even clone the cells. The cell culture also led to the ability to isolate stem cells, the ability to fertilize cells in vitro, and new discoveries in human genetics, especially an exact count of chromosomes.
In The Immortal Life of Henrietta Lacks, Rebecca Skloot addresses the important developments in cell culture. More specifically, Skloot discusses methods for freezing cells. This is extremely important for the scientists, such as George Gey, who are attempting to grow an immortal cell line. Freezing cells allows the scientists to keep the cells alive, so that the cells can be monitored and cultured. The freezing techniques can also “freeze” the cells in time, so that they can be used at a later date. After George Gey cultures the first immortal cell line, using Henrietta Lacks’s cells, he utilizes the freezing technique to keep the cells alive, to grow more cells, and to ship the cells across the world. Unlike many scientists of the time, George Gey is not after a profit in this business. He does not sell the HeLa cell line, named after Henrietta Lacks. Instead, he gives away the immortal cells to other scientists around the world. The methods of freezing cells allow George Gey to ship the cells via airplane and, ultimately, through the postal service. Thus, the developments of freezing cells are highly important to major strains, such as the HeLa line, which will be shipped and shared with other scientists worldwide.