Another way to study cells that are at the same stage of their cell cycle involves a technique called culture fractionation.
This procedure avoids the potential problems of synchronization techniques, in which the inducing factor may itself alter or distort the normal events of the cycle.
In culture fractionation, a random culture of cells (i.e., cells that are at various stages of the cell cycle) is sorted into subpopulations of varying cell size by sedimentation through a liquid density gradient and is followed by the collection of the gradient and the entrained cells as a series of fractions.
During sedimentation, the larger cells travel farther through the density gradient than the smaller cells.
Cell sedimentation through the gradient may be achieved by simply using gravity (i.e., the cells fall through a stationary, cylindrical column of gradient) or may be accelerated using centrifugal techniques.
The rationale behind this approach is that a relationship exists between cell size and cell age (i.e., they are directly related to one another), so that the population is sorted into a linear sequence of fractions of increasing cell age.
By studying and comparing chemical, physiological, or morphological properties of the cells in each of the separated fractions, specific effects occurring during the cell cycle can be assigned an age. Thus, changes taking place during cell growth may be followed even though a synchronous cell population is not being employed.
Culture fractionation requires the processing of very large quantities of material so that all of the collected fractions from the density gradient contain enough cells of the same age (i.e., size) for valid physical and/or chemical measurements.
Sedimentation of large numbers of cells through a density gradient under the influence of the earth’s gravity alone involves the use of large cylindrical chambers called “sta-put” devices; centrifugal separations of large numbers of cells usually require “zonal rotors.”