What is the difference between a population doubling and a passage number?
A population doubling is a two-fold increase in the total number of cells in a culture, and is most commonly referred to during the exponential, or “log”, phase of growth. The term passage number refers to the number of times that a cell population has been removed from the culture vessel and undergone a subculture (passage) process, in order to keep the cells at a sufficiently low density to stimulate further growth. At Invitrogen, the first culture following the isolation of cells from tissue is termed the primary culture. Following the first subculture, the cells are described as a secondary culture (or passage 1). After the second subculture, the cells become a tertiary culture (or passage 2), and so on.
