Cell culture is the complex process by which cells are grown under controlled physiochemical environment (pH, temperature, oxygen, carbon dioxide, osmotic pressure, etc.), outside of their natural environment in special medium. There are two different ways to keep cells in culture:
One way is via immortal cell lines that are mostly derived from cancer. The oldest and most commonly used human cell line is the HeLa cell line (cervical cancer). These cells were taken in vitro shortly before Henrietta Lacks died from her cancer in 1951. Because of the hardiness, proliferating nature and their adaptation to growth in tissue culture plates HeLa cells are a very common tool in biological science.
The other way is to take cells in culture is directly from living tissue (e.g. rats or mice) and established for growth in vitro: these are known as primary cells. Most of the primary cell cultures do have a limited lifespan. In scientific papers, the ‘age’ of these primary cells is given as days in vitro (DIV), counted from the day, the tissue is taken into culture.