The term 'line' is used when a culture has undergone an intentional experimental process to establish a more uniform and stable population of cells (see 'establishing cell line'). This will require one or more passages, but may involve additional selection processes. Through such passaging and/or selection processes, the resulting 'line' attains some level of genetic stability and compositional homogeneity which is typically absent in primary cultures. Because of their relative homogeneity, ‘lines’ are capable of being characterized and stably propagated over a period of time. A new type of cell line can be established not only through the passaging/selection of a primary culture, but also through experimental modifications of existing lines (e.g. immortalization, stable genetic modifications, drug selection for a resistant subset, etc.).
The definition provided here establishes the 'scale' of cell populations that qualify as cell lines - specifically those with a shared propagation history in culture. In this way, the 'cell line' class demarcates populations that represent what researchers actually use in the practice of science - e.g. as inputs to culturing, experimentation, and sharing. The definition is such that cell lines will exhibit important attributes. For example, they will have a relatively homogenous cell type composition as they have experienced similar selective pressures due to their continuous co-propagation. In addition, these populations can also be characterized by a passage number, again owing to their common passaging history. As defined here, 'cell line' can refer to a population of cells in active culture, applied experimentally, or stored in a quiescent state for future use.