Subculturing, also referred to as passaging, is the removal of the medium and transfer
of cells from a previous culture into fresh growth medium, a procedure that enables the
further propagation of the cell line or cell strain.
The growth of cells in culture proceeds from the lag phase following seeding to the log
phase, where the cells proliferate exponentially. When the cells in adherent cultures
occupy all the available substrate and have no room left for expansion, or when the cells
in suspension cultures exceed the capacity of the medium to support further growth, cell
proliferation is greatly reduced or ceases entirely (see Figure 4.1 below). To keep them at
an optimal density for continued growth and to stimulate further proliferation, the culture
has to divided and fresh medium supplied.