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Mitotic index is a measure for the proliferation status of a cell population. It is defined as the ratio between the number of cells in mitosis and the total number of cells. Cells in the cell cycle can be identified using antibodies against the nuclear antigen Ki-67. The mitotic index can be worked out from a slide, even with light microscopy. It is the number of cells containing visible chromosomes divided by the total number of cells in the field of view.

If you administer colchicine or other colchicine-derivative medications (i.e. colcemid) you can arrest the cell cycle at this point leaving the chromosomes in their visible form. Colchicine disrupts the microtubule formation which is necessary for the spindle fibers to separate the chromosomes during anaphase.

Cell population growth occurs as cells pass through interphase and mitosis to complete the cell cycle. Many cells lose the capacity to divide as they mature or divide only rarely. Other cells are capable of rapid cell division. For example, as plant roots grow, cells near the tip of the root, in the apical meristem, divide rapidly to push the root through the soil. The root cap detects the pull of gravity and directs the rapid growth of cells near the tip.

By quantifying aspects of a dividing cell population, we can examine how cells differ in their capability to divide. Experimentally, we can change properties of the cell's environment and quantify the effects on cell division.

For a group of cells that rarely complete the cell cycle, we expect a high proportion of cells to be in the resting stage of the cell cycle (G1). However, in a rapidly dividing cell population, we expect a high proportion of cells to be in the stage of mitosis. One way to quantify cell division is by using the mitotic index:

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