Pathologists frequently calculate the mitotic index of a tumour biopsy to evaluate how rapidly a cancer is growing and to determine the grade of the tumour.
The formula for the mitotic index is:
Mitotic Index (MI)=number of cells undergoing mitosistotal number of cells×100 \text{Mitotic Index (MI)} = \frac{\text{number of cells undergoing mitosis}}{\text{total number of cells}} \times 100 Mitotic Index (MI)=total number of cellsnumber of cells undergoing mitosis×100A clinical scientist examines a biopsy sample of breast tissue under a high-power microscope. They count 18 cells actively undergoing mitosis and 282 cells in interphase. Calculate the mitotic index for this tissue sample.
In a different biopsy slide from a lung tumour, the scientist calculates a mitotic index of 4.5%4.5\%4.5%. Given that the total number of cells counted in this tissue sample was 800, calculate the number of cells that were undergoing mitosis.
Explain one practical difficulty a scientist might encounter when trying to accurately distinguish and count cells to calculate the mitotic index from a tissue section.
3 exam-style questions on Edexcel IGCSE Biology Cell structure. Each one has a worked solution and a mark scheme showing where the marks go.