How is the mitotic index defined?

The mitotic index is defined as the proportion of cells undergoing mitosis at a given time compared to the total number of cells observed in a sample. This metric is calculated by taking the number of cells actively engaged in the process of mitosis and dividing it by the total number of cells that are visible under the microscope.

This measurement is particularly useful in understanding cell proliferation rates in various contexts, such as during tissue growth, development, or the progression of diseases like cancer. A higher mitotic index indicates a greater number of cells are dividing, which can be associated with rapid growth or increased activity in tissues.

The other options do not accurately represent the definition of the mitotic index. For example, some refer to meiosis or specific conditions like cancerous cells without encompassing the general aspect of mitotic activity across all cells. Thus, option C is the accurate definition of the mitotic index.