35 Mitotic Index

Mitotic Index is a calculation that can be done to determine how actively a group of cells is dividing. In order to calculate the Mitotic Index of a sample, you first count the number of cells that are actively dividing (cells that are in Prophase, Metaphase, Anaphase, or Telophase). Next, you divide the number of cells that are dividing by the total number of cells. You’re basically calculating the percentage of cells that are actively dividing within the sample.

Figure 1 shows an image of cells from the root tip of an onion, viewed under a microscope. Onion root tips are regions that are undergoing a lot of mitosis because the tip of the root is actively growing downward into the soil. Black boxes have been put around cells that are actively dividing (NOTE: if you think there are additional cells that are dividing, that’s ok – I put a box around the really obvious ones).

Figure 2 shows an area of an onion root that is further away from the tip of the root, so it is less actively dividing. In this image, there are 24 cells that are in active division (in prophase, metaphase, anaphase, or telophase). There are approximately 86 cells total that are visible in the image. For this sample, the Mitotic Index would be calculated as 24 (cells in active division / 86 (total cells) = 0.28

In this image, there is only one cell that is in active division. There are approximately 25 cells total that are visible in the image. For this sample, the Mitotic Index would be calculated as 1 (cells in active division / 25 (total cells) = 0.04

The root tip cells in Figure 1 have a higher Mitotic Index than the cells higher up the root in Figure 2. That is because there are more actively dividing cells in Figure 1 than there are in Figure 2.