Essentially, cell culture involves the distribution of cells in an artificial environment (in vitro) which is composed of the necessary nutrients, ideal temperature, gases, pH and humidity to allow the cells to grow and proliferate. In vivo - When the study involves living biological entities within the organism. In vitro - When the study is conducted using biological entities (cells, tissue etc) that has been isolated from their natural biological environment. E.g. tissue or cells isolated from the liver or kidney. Whereas pieces of tissue can be put in the appropriate culture to produce cells that can then be used for culture (explant culture), cells from tissues (soft tissue) can be obtained through enzymatic reactions. Here, such enzymes as trypsin and proname are used to break down the tissue and release the desired cells. When cells have been obtained directly from the organism/animal tissue (or even plant tissue) through enzymatic or mechanical techniques, such cells are referred to as primary cells. However, cells that continue to proliferate indefinitely (after the first subculture) under special conditions are referred to as cell lines. These particular cells tend to have been passaged for a long period of time, which causes them to acquire homogenous (similar) genotypic and phonotypical traits.