passaging

Passaging is the procedure of harvesting cells from a culture, transferring the cells to one or more culture vessels with fresh growth medium, and using those cells to start new cultures. It is also referred to as subculturing. As is relates to the phrase "passage number", it is the number of times the cells have been passaged in culture.
Cells increase their number as they divide in culture, eventually filling up the culture vessel and/or depleting the nutrients in the culture medium. The state where the culture vessel is filled up is called "confluent". When cells come into contact with each other, growth is greatly reduced or stopped altogether. If the confluent state is prolonged, cells change their phenotype to abnormal characteristics or cells may die. In order to maintain cells in culture, they need to be passaged.

In order to continue culturing while maintaining the state of the cells as much as possible, it is essential to passage/subculture before reaching confluence. The timing of passage is determined based on cell density, culture duration, and cell distribution.

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