Cell Culture

Cell culture describes the laboratory growth of cells derived from plants or animals. To put cells into culture, the tissue of interest is exposed to en­zymes that dissociate the tissue to release the component cells. In some cases, for example with blood-forming tissues, suspensions can be produced more simply by mechanical means, such as forcing them through a syringe. Dispersed cells are then transferred to a suitable growth medium and al­lowed to attach to the surface of culture flasks. When cells have grown (by dividing) to cover the flasks’ surface, the process of enzymic dissociation can be repeated and the cells replanted to additional flasks. This process is re­ferred to as subcultivation or “splitting.”

Cell culture requires careful attention to the growth medium to ensure cells are given all the components they require to grow. Often the culture medium requires growth factors or hormones to stimulate growth.

The general process of cell culture has been used extensively since the early 1900s for research on tissue growth and development, virus biology, properties of cancer cells, studies relating to aging, genetics, and gene ther­apy. More recently, large-scale cell culture systems have been developed to produce biopharmaceuticals in quantities, another facet of the broad field of biotechnology.

A central advantage of the cell culture technique is its simplicity com­pared to the difficulties of studies using whole plant or animal organs, which are usually composed of many different cell types. With cell culture, it is possible to observe, in a well-defined environment, small numbers of cells of a single type derived by expanding an original population. In contrast, with an intact organ, one could be working with forty or more differing cell types, a nondefined fluid, and literally billions of cells.

The limitations of cell culture include the finite doubling potential of most normal cells, the possibilities for unexpected infection with viruses or microorganisms, or even cross-contamination with other cell types. Media used to propagate cells are rich in nutrients and, therefore, support growth of a multitude of organisms. Accordingly, most culture methods require ster­ile conditions. Often antibiotics are used to inhibit growth of unwanted mi­crobial contaminants. Another difficulty with some cultured cells is their tendency to change their morphology, functions, or the range of genes they express.

Cell culture has had a tremendous impact on human health. The abil­ity to culture cells allowed the laboratory growth of polio virus to produce vaccines that nearly eliminated polio as a disease. Two of the many areas of scientific study where uses of cell culture techniques have had major impact are human aging and cancer research. In the 1960s, biologists found that normal human fibroblasts, cells derived from connective tissue, had a pre­dictable limit in their ability to proliferate in culture. Subsequently, the ob­servation was extended to other normal cell types and species. Furthermore, the number of subcultivations that could be achieved was age related. Cells from young donors were able to divide more times than those isolated from older donors. After extensive research on this phenomenon, in the 1990s it was determined that the telomeres, small segments at the end of human chromosomes, become shorter with age both in cultured cells and in cells taken directly from individuals. An enzyme, telomerase, which acts to main­tain telomeres, decreased in activity with age. Interestingly, cells engineered to express more telomerase retained telomeres and the ability for extended proliferation. Cancer cell lines, which can grow indefinitely in culture, also retain long telomeres.

Scientists have also learned much about cancer initiation and progres­sion through the use of cells in culture. Normal fibroblasts from mouse em­bryos generally declined in proliferation rate with subcultivation. After an extended, so-called “crisis” phase, they seemed to recover and eventually re­turned to active division. However, the chromosome number of the resul­tant cell population was abnormal. Furthermore, if the cells were subcultivated extensively, they acquired malignant properties characteristic of cancer cells. This change results when normal genes are expressed under inappropriate circumstances. Their products overcome the normal controls of the cell division cycle to allow abnormal proliferation.

References

Hay, Robert J., J. G. Park, and A. Gazdar, eds. Atlas of Human Tumor Cell Lines. San Diego: Academic Press, 1994.

Hunter-Cevera, J. C., and A. Belt, eds. Preservation and Maintenance of Cultures Used in Biotechnology. San Diego: Academic Press, 1996.