There are two main types of cell cultures: primary cell cultures and continuous cell lines. Primary cell cultures are cells that can not survive forever outside of the original organism. Examples of primary cell cultures include cells from a mammalian embryo. Continuous cell lines (also known as "immoral cell cultures") are cell cultures that can survive indefinitely without being in an organism. Continuous cell lines are generally considered abnormal, and they include cell cultures from tumors.
Near the end of the 19th century, the German scientist Wilhelm Roux showed that cell cultures were possible when he took some cells from a chicken embryo and kept them preserved in saline for a few days. Pure cell lines were introduced later in the 20th century, thanks in part to the work of Harry Eagle, who, in 1955, showed that naturally occurring cells could be mixed with synthetic nutrients in order to preserve them for an indefinite period of time.
The methods for preserving a cell culture vary on the type of cells, but there are certain techniques that are common to many cells. Cell cultures must be stored in a sterile environment in order to avoid contamination in the sample. The culture is then normally stored in a liquid suspension, with one of the most common suspensions being saline. In order to keep a cell culture alive, experimenters provide it with a mix of appropriate nutrients that it would normally receive from the host organism (such as glucose for animal cells and carbon dioxide for plant cells).
There are several practical uses for cell cultures. The most common use of cell cultures is to grow less potent forms of viruses so that they can be used in vaccines. Cell cultures also can be used to observe the DNA of an organism in order to better understand it at the rudimentary level, and thus cell cultures are an essential aspect of genetic engineering. This can be especially useful for agriculture, as cell cultures can be used to help create new crops of plants that are resistant to drought or disease.