What is subculture or passaging of cells?
Subculture or passaging is the process of transferring cells from an old culture vessel to a fresh one to ensure their continued growth and survival. When cells proliferate and form a monolayer, they become crowded, which can be unfavourable to their growth and may lead to cell death.
During sub-culturing, cells are detached from the surface using enzymes like trypsin or mechanical means such as vigorous pipetting or cell scrapers. The detached cells are then centrifuged and transferred to a fresh flask with new growth medium. This process is commonly called 'splitting' as cells from one flask are equally divided into two flasks for culturing.
Explanation
The textbook defines subculture/passaging as the process of detaching cells from an old medium using enzymes like trypsin and transferring them to a fresh flask after centrifugation and addition of fresh media. This is necessary when cells become crowded after forming a monolayer. The passage number refers to the number of times a cell line has been sub-cultured. The process is also called 'splitting' as cells are equally split into multiple flasks.