ANIMAL CELL CULTURE
One important approach to studying the cells of multicellular organisms is to grow isolated cells in culture, where they can be manipulated under controlled laboratory conditions. The use of cultured cells has allowed studies of many aspects of mammalian cell biology, including experiments that have elucidated the mechanisms of DNA replication, gene expression, protein synthesis and processing, and cell division. Moreover, the ability to grow animal cells in culture has allowed studies of the signaling mechanisms that control cell growth and differentiation within the intact organism.
Figure 1.22 Culture of animal cells Cells obtained from a tissue are grown on culture dishes in nutrient medium.
Although the process is technically far more difficult than the culture of bacteria or yeasts, a wide variety of animal cells can be grown and manipulated in culture. Cultures are initiated by the dispersion of a piece of tissue into a suspension of its component cells, which is then added to a culture dish containing nutrient media (Figure 1.22). Most animal cell types, such as fibroblasts and epithelial cells, attach to and grow on the plastic surface of dishes used for cell culture. Because they contain rapidly growing cells, embryos or tumors are frequently used as starting material. Embryo fibroblasts grow particularly well in culture and consequently are one of the most widely studied types of animal cells. Under appropriate conditions, however, many specialized cell types can also be grown in culture, allowing their differentiated properties to be studied in a controlled experimental environment. Embryonic stem (ES) cells are a particularly notable example. These cells are established in culture from early embryos and maintain their ability to differentiate into all of the cell types present in adult organisms. Consequently, embryonic stem cells have played an important role in studying development and differentiation, as well as offering the possibility of contributing to the treatment of human diseases by providing a source of tissue for transplantation therapies.
The initial cell cultures established from a tissue are called primary cultures (see Figure 1.22). The cells in a primary culture usually grow until they cover the culture dish surface. They can then be removed from the dish and replated at a lower density to form secondary cultures. This process can be repeated many times, but most normal cells cannot be grown in culture indefinitely. For example, normal human fibroblasts can usually be cultured for 50–100 population doublings, after which they stop growing and die. In contrast, embryonic stem cells and cells derived from tumors frequently proliferate indefinitely in culture and are referred to as immortal cell lines. In addition, a number of immortalized rodent cell lines have been isolated from cultures of normal fibroblasts. Instead of dying as most of their counterparts do, a few cells in these cultures continue proliferating indefinitely, forming cell lines like those derived from tumors. Such permanent cell lines have been particularly useful for many types of experiments because they provide a continuous and uniform source of cells that can be manipulated, cloned, and indefinitely propagated in the laboratory. The first human cell line to be established were HeLa cells, which were isolated from a cervical cancer in 1951 and have been used in thousands of laboratories studying many aspects of human cell biology (see Key Experiment).