Cell culture is the cultivation of cells (outside the body) from a multicellular organism. This technique is very important to biotechnology processes because most research programs depend on the ability to grow cells outside the parent animal. Cells grown in culture usually require very special conditions (for example, specific pH, temperature, nutrients, and growth factors). Cells can be grown in a variety of containers, ranging from a simple petri dish to large-scale cultures in roller bottles (bottles that are rolled gently to keep culture medium flowing over the cells).