Cell division is a normal process in multi-cellular organisms. Growth and repair (replacement of dead cells) take place as a result of cell division (mitosis). Except for cells like the liver and brain cells, which rarely divide in the mature adult, most cells undergo frequent division. Sometimes, however, cell division becomes very rapid and uncontrolled, leading to cancer. It should be clearly understood that rapid growth means a high rate of cell division for a particular cell type. It is possible for perfectly normal cells, e.g. the bold- forming cells, to have a higher rate of division than some cancerous cells.

Cancer is not one single disease but a complex of many diseases. About two hundred distinct types of cancer have been recognized. These can be grouped into four main types: carcinomas, sarcomas, lymphomas and leukemias.
Carcinomas are tumors mode up principally of epithelial cells of ecto-dermal or endodermal origin. The solid tumors innerve tissue and in tissues of body surfaces, or their attached glands, are examples of carcinomas. These include cervical, breast, skin and brain carcinomas. About 85 per cent of cancers are carcinomas.
Sarcomas are tumors made up principally of connective tissue cells, which are of mesodermal origin. They are solid tumors growing from connective tissue, cartilage, bone and muscle. Although they account for most of the cancers studies in laboratory animals, they constitute only about 2 per cent of human cancers.
Lymphomas are cancers in which there is excessive production of lymphocytes by the lymph nodes and spleen. Hodgkin’s disease is an example of a lymphoma. Lymphomas constitute about 5 percent of human cancers.
Leukemias are neo-plastic growths of leucocytes (W.B.C), and are characterized by excessive production of the cells. They constitute about 4 per cent of human cancers.
In addition to the types of cancer mentioned above there may be mixed malignant tumors, e.g. tumors arising from both ecto-dermal and meso-dermal and meso-dermal tissues.
CHARACTERISTICS OF CANCER CELLS
Almost all types of differentiated cells can become neo-plastic or cancerous. The process of cell change in which a cell lose its ability to control its rate of division, and thus becomes a tumor cell, is called cell transformation. The cancerous cell generally retains the-structural and functional characteristics of the normal cell type from which it is derived. Thus cancerous cells of the thyroid gland continue to secrete thyroxin. Neo-plastic cells, however, differ from their normal counter parts in several respects.
Immortalization Normal cell cultures do not survive indefinitely. For example, human cell cultures die after about 50 generations, and chicken cell cultures have a much can grow indefinitely. Cell cultures infected with mouse sarcoma virus can be maintained as long as nutrition is provided and overcrowding avoided.
Loss of contact inhibition Normal cells in a culture stop growing when their plasma membranes come into contact with one another. When two normal cells come into contact, one or both will stop moving and then being to move in another direction. This inhibition of growth after contact is called contact inhibition.