Contact inhibition, lost.

Normal cells stop dividing on contact; a transformed cell ignores contact, piles into a tumor and, if malignant, sheds cells into blood.

Show which cell type fills the dish in a single layer and stops, and which one seeds a second tumor elsewhere.

Saying a benign tumor metastasises.

• Cancer is a major cause of death worldwide; more than a million Indians suffer from it. In cancer cells, the normal controls on cell growth and differentiation break down.
• Normal cells show contact inhibition: touching other cells stops their uncontrolled growth. Cancer cells seem to lose this property, so they keep dividing into masses called tumors.
• Benign tumors stay in their original location, do not spread and cause little damage. Malignant tumors are masses of rapidly proliferating neoplastic (tumor) cells that invade and damage the surrounding tissue and starve normal cells by competing for nutrients.
• Metastasis, the most feared property of malignant tumors: cells shed from the tumor travel in the blood and start new tumors wherever they lodge.
• Carcinogens, the agents that turn normal cells neoplastic, are physical, chemical or biological. Ionising radiations (X-rays, gamma rays) and non-ionising UV damage DNA; chemicals in tobacco smoke are a major cause of lung cancer.
• Oncogenic viruses carry viral oncogenes. Normal cells carry cellular oncogenes (c-onc), or proto-oncogenes, which can cause oncogenic transformation when activated under certain conditions.
• Early detection lets many cancers be treated successfully. Detection uses biopsy with histopathology (thin stained sections of suspected tissue examined by a pathologist) and, for leukemias, blood and bone marrow tests for raised cell counts.
• Radiography (X-rays), CT (X-rays building a three-dimensional image) and MRI (strong magnetic fields and non-ionising radiations) detect internal cancers. Antibodies against cancer-specific antigens detect certain cancers, and molecular tests find genes for inherited susceptibility, so carriers can avoid their carcinogen (for example tobacco smoke for lung cancer).
• Treatment: surgery, radiation therapy, chemotherapy and immunotherapy; most cancers get a combination of surgery, radiotherapy and chemotherapy. Radiotherapy irradiates tumor cells lethally while sparing nearby normal tissue; most chemotherapy drugs have side effects such as hair loss and anaemia.
• Tumor cells can avoid detection by the immune system, so patients are given biological response modifiers such as α-interferon, which activate the immune system to destroy the tumor.

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