TISSUE CULTURE

a method of biological research in which fragments of tissue from an animal or plant are transferred to an artificial environment in which they can continue to survive and function. The cultured tissue may consist of a single cell, a population of cells, or a whole or part of an organ. Cells in culture may multiply; change size, form, or function; exhibit specialized activity (muscle cells, for example, may contract); or interact with other cells. Tissue culture is a relatively recent development. In 1907 the American zoologist Ross G. Harrison successfully demonstrated the growth of frog nerve cell processes in a medium of clotted lymph. Thereafter, a number of experimenters succeeded in cultivating animal cells, using as culture media a variety of biological fluids, such as lymph, blood serum, plasma, and tissue extracts. Tissue culture permits control of the cellular environment, allowing the behaviour of cells to be both examined and manipulated. Tissue cultures have revealed basic information about cells regarding their composition and form; their biochemical, genetic, and reproductive activity; their nutrition, metabolism, specialized functions, and processes of aging and healing; the effects on cells of physical, chemical, and biological agents (drugs and viruses, for example); and the differences between normal cells and abnormal cells such as cancers. Work with tissue cultures has helped to identify infections, enzyme deficiencies, and chromosomal abnormalities, to classify brain tumours, and to formulate and test drugs and vaccines. Cells may be grown in a culture medium of biological origin such as blood serum or tissue extract, in a chemically defined synthetic medium, or in a mixture of the two. A medium must contain proper proportions of the necessary nutrients for the cells to be studied and must be appropriately acid or alkaline. Cultures are usually grown either as single layers of cells on a glass or plastic surface or as a suspension in a liquid or semisolid medium. To initiate a culture, a tiny sample of the tissue is dispersed on or in the medium, and the flask, tube, or plate containing the culture is then incubated, usually at a temperature close to that of the tissue's normal environment. Sterile conditions are maintained to prevent contamination with microorganisms. Live cultured cells may be examined directly with a microscope or observed by means of photographs and motion pictures taken through the microscope. Cells may also be killed, preserved, and stained for further examination or cut into thin sections to disclose additional details under a light or electron microscope. Cells in tissue culture are subjected to a broad range of experimental treatment. For example, viruses, drugs, hormones, vitamins, disease-causing microorganisms, or suspected cancer-producing chemicals may be added to the culture. Sometimes cultures are grown from single cells, producing uniform biological populations called clones. Cultures have been used to investigate fundamental processes of growth and development in both normal and abnormal tissues. One finding has been that normal cells undergo an aging process, retaining their ability to multiply readily for only 50 to 100 generations, after which the rate decreases markedly. Many cancer cells, on the other hand, apparently can be perpetuated forever. Since the discovery that certain viruses will grow in tissue cultures, the technique has been used to produce vaccines against poliomyelitis, influenza, measles, mumps, and other infectious diseases. Cell cultures have also produced viral inhibitors, including interferon. Hormones are now being produced from cultures of cells or organs. The cultured white blood cells from two individuals can be used to determine compatibility between potential donors and recipients of tissue transplants. By removing and culturing cells from a pregnant woman, it is possible to tell whether her fetus has the chromosome defect associated with Down syndrome (mongolism). The identification and diagnosis of chromosome abnormalities and inherited disorders has been greatly enhanced by the development of somatic cell genetics. Tissue culture techniques have been used to culture many kinds of hybrid cells that contain chromosomes from different species in the same cell, allowing the functions of individual chromosomes to be separately defined. Tissue-culture studies have clarified the genetic causes of certain hereditary diseases, and methods have been developed for detecting environmental substances that may cause gene damage. The nature of certain cancers has been elucidated by the discovery of specific genes and chromosomal aberrations that are associated with the disease. Studies of cell cultures have revealed the existence of a so-called cytoskeleton in mammalian cells, which gives the cell its shape and regulates a variety of biochemical activities. The methods of somatic cell genetics are being applied to plant cells in an effort to develop new strains of cereal crops with improved nutritional properties.