The big idea: A tumour is a mass of cells formed by uncontrolled division. Tumours differ in how fast their cells divide and whether they invade nearby tissue and spread (metastasis).
The points to remember
- Benign tumour: cells divide slowly, the tumour stays in one place and does not invade or spread. It does not cause cancer.
- Malignant tumour: cells divide fast, invade neighbouring tissue and can spread. This is cancer.
- Metastasis: cells break off and travel in the blood or lymph to other parts of the body.
- Primary tumour: where the tumour first formed. Secondary tumour: one formed by cells that spread.
Remember it as: Benign stays home; malignant moves out.
Real example: moles and melanoma. Most moles on the skin are benign growths of pigment cells. Melanoma is a malignant tumour of the same cells that can spread to the lymph nodes, lungs and other organs.
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Measuring division: the mitotic index: The mitotic index is the proportion of cells in a sample that are in mitosis:
mitotic index = number of cells in mitosis ÷ total number of cells
A high value means the cells are dividing fast, as in a growing root tip or a malignant tumour.
Worked example: the field of view above
Count
Cells in any phase of mitosis: 6. All cells: 24.
Divide
6 ÷ 24 = 0.25, or 25%.
Read it
A quarter of the cells are dividing: a fast-growing tissue.
Making it reliable: Count cells in several randomly chosen fields of view (or repeat the count) and use the mean. Divide by the total number of cells, not the number in interphase.
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How it is asked: 1A items on what all tumours share or what makes a tumour malignant; 1B data on the mitotic index: calculate it, say how to make it reliable, or draw conclusions from a bar chart with error bars.
Distinguish between benign and malignant tumours.
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