5.1.6—Tumour formation

A tumour is an abnormal mass of cells formed when changes in a cell’s control of division or survival allow repeated, unregulated cell proliferation. The altered cell passes the change to descendants as the clone expands.

• Control failure: A mutation alters a gene or control system that normally limits cell division, promotes appropriate cell death or checks abnormal cells.
• Abnormal proliferation: The affected cell continues dividing when normal controls would stop division or remove the cell. Its descendants inherit the relevant change and form an expanding clone.
• Tumour mass: Repeated abnormal mitosis produces an accumulation of cells rather than the controlled replacement or growth produced by normal tissue division.
• Benign boundary: A benign tumour remains local and does not invade nearby tissue in the supported comparison.
• Malignant boundary: A malignant tumour invades nearby tissue and can spread through blood or lymph to establish secondary growths; this spread is metastasis.
• Normal/stem-cell contrast: Normal mitosis is regulated and supplies matching cells for growth, replacement or repair. Stem-cell self-renewal maintains a controlled reservoir with differentiation potential; tumour proliferation is a loss of that control, not normal self-renewal.

The key transition is regulatory: a cell that should pause, differentiate, die or remain within tissue instead keeps contributing descendants. Whether the resulting growth remains local or becomes invasive depends on the tumour behaviour, so “uncontrolled division” is the starting mechanism rather than a claim that every tumour spreads.

A carcinogen or mutation can increase risk but does not guarantee a tumour, and not every mutation affects division control. Do not invent named genes, cancer types or a specific carcinogenic pathway here; distinguish regulated mitosis and stem-cell self-renewal from pathological loss of control.