Live cells and tissues can be maintained and studied outside the body. In a complex organism, tissues and organs are formed by several kinds of cells. These cells are bathed in blood plasma, which contains hundreds of different molecules. Cell and tissue culture has been very helpful in isolating the effect of a single molecule on one type of cell or tissue. It also allows the direct observation of the behavior of living cells under a microscope. Several experiments that cannot be performed in the living animal can be reproduced in vitro.
The cells and tissues are grown in complex solutions of known composition (salts, amino acids, vitamins) to which serum components are frequently added. In preparing cultures from a tissue or organ, cells must be initially dispersed either mechanically or by treating the tissue with enzymes. Once isolated, the cells can be cultivated in a suspension or spread out on a Petri dish or glass slide, to which they adhere, usually as a single layer of cells (Figure 1–3). Cultures of cells that are isolated in this way are called primary cell cultures. Many cell types were once isolated in this way from normal or pathological tissue and have been maintained in vitro ever since because they have been immortalized and now constitute a permanent cell line. Most cells obtained from normal tissues have a finite, genetically programmed life span. Certain changes, however (mainly related to oncogenes; see Chapter 3: The Cell Nucleus), can promote cell immortality, a process called transformation, which may be a first step in transforming a normal cell into a cancer cell. Because of transformation and other improvements in culture technology, most cell types can now be maintained in the laboratory indefinitely. All procedures with living cells and tissues must be performed in a sterile area, using sterile solutions and equipment.
Cell culture has been widely used for the study of the metabolism of normal and cancerous cells and for the development of new drugs. This technique is also useful in the study of parasites that grow only within cells, such as viruses, mycoplasma, and some protozoa (Figure 1–13). In cytogenetic research, determination of human karyotypes (the number and morphology of an individual's chromosomes) is accomplished by the short-term cultivation of blood lymphocytes or of skin fibroblasts. By examining cells during mitotic division in tissue cultures, it is possible to detect anomalies in the number and morphology of the chromosomes that have been shown to be related and are diagnostic of numerous diseases collectively called genetic disorders. In addition, cell culture is central to contemporary techniques of molecular biology and recombinant DNA technology.
Photomicrograph of chicken fibroblasts that were grown in tissue culture and infected by the protozoan Trypanosoma cruzi. Although the borders of the cells are not readily visible, their nuclei (N) can be easily seen. Many trypanosomes are present within each cell (arrows). High magnification. (Courtesy of S Yoneda.)