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It has been estimated that the various tissues and organs of the adult human contain more than 100,000,000,000,000 cells!
All these cells are derived initially from a single cell (i.e., the fertilized egg cell) through growth and division.
In many tissues (e.g., the epithelial lining of the digestive organs and the blood-forming tissues), the cells continue to grow and divide for most of the person’s life; however, in some tissues, such as muscle and nerve, cell division ceases some time after birth and subsequent tissue growth results from individual cell growth without division.
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The ongoing cell growth and division that characterizes most of the body’s tissues accounts for the growth of the organism as a whole and the replacement of dying cells. With the exceptions of muscle and nerve, replacement of old tissue cells with new ones results in a complete turnover of the body’s cellular composition every few years.
A portion of the body’s total mass is represented by non-cellular materials that are secreted by cells; for example, most of the mass of bone and cartilage is represented by secreted calcium salts and proteins.
Other animals and plants also grow by the processes of cellular division followed by individual cell growth. When the term “growth” is applied to microbial organisms such as bacteria, it can refer either to an increase in the size of the individual organism (e.g., the individual bacterial cell) or to an increase in the numbers of organisms (e.g., in a culture of bacteria).
If microorganisms such as bacteria or protozoa are placed in an appropriate nutrient medium, they may grow and divide until the medium is teeming with these cells. Such a population is called a cell culture. Similarly, cells that are removed from certain tissues of higher animals and plants can be made to grow and divide in an artificial nutrient medium, thereby forming a tissue culture.
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In most cultures of microorganisms and in some tissue cultures (e.g., cultures of white blood cells), the cells undergo optimum growth when they are suspended in the nutrient solution; such cultures are called suspension or anchorage- independent cell cultures. In most tissue cultures, however, the cells only grow when they are attached to a surface; these are known as anchorage-dependent cell cultures.
Occasionally, transformed or neoplastic tissue cells (e.g., tumor cells) can be adapted to grow in suspension. In theory, at least, cells from virtually any tissue can be cultured, but in practice the degree of success varies widely with the organism and tissue that is used.
Nevertheless, it is fair to say that much of our present understanding of the regulation, kinetics, and mechanics of cell growth and division has been obtained from studies using cultures of microorganisms and tissue cells, and cell-culturing techniques have become increasingly important research tools of the cell biologist.
