Cancer is a result of the uncontrollable proliferation of abnormal cells. In humans, there exists more than 150 types of cancer. We also find cancer in plants, fish and other animals. The cancerous cell becomes " aggressive " and progressively invades neighbouring tissue and can migrate through the blood and lymphatic system, commonly referred to as metastasis.

The cells do not always divide rapidly. Certain cancers are less aggressive and at times do not metastasize and continue to invade neighbouring tissue (for example basal cell carcinoma of the skin).

There are two main categories of cancer : carcinomas and sarcomas. Carcinomas originate from epithelial tissue (skin and grandular epithelium) and those covering the interior cavity structures of body organs. The sarcomas originate from supporting tissue such as fat, muscles, blood vessels, bone and cartilage. There are also cancers comprised of the two types: carcinosarcomas. Cancers originating from blood (lymphomas, leukemias), nervous system (brain) and melanoma are classified seperately.

For cancer to develop there must be several mutations in the genes of cellular DNA . These mutations are caused by environmental factors and errors occuring during cell division, a process which takes place over long periods of time. Mutations, predisposing an individual to develop certain cancers, are rarely inherited.

While mutations often occur, the cell possesses a mechanism for self-repair. In effect, the oncosuppressor gene, namely p53, triggers the death (apoptosis) of a mutated cell except during cell division where it appears that altered DNA is impossible to repair. Apoptosis essentially eleminates those cells that possess damaged or altered chromosomes. The cells which survive damaged DNA and are deprived of normal p53 function, can multiply abnormally and accumulate lesions in the genome thus forming a malignant tumor. Once the altered DNA or errors in transcription accumulate and the oncosuppressor becomes ineffective, the cell reproduces itself hence creating a population of malignant cells. This abnormal proliferation is the basic characteristic of the cancer process. As the cell divides in an uncontrollable fashion, it will become different in it's form, size, internal components and these transformations will be transferred to the descendant cellular line.

Mutations are induced by initiator agents such as the benzopyrene chemicals of tobacco smoke, polycyclic hydrocarbons and viruses, to name a few. Several steps are necessary to arrive at the cancer stage. A chronic infection or a series of infections gives rise to a reparative process in the tissue whose cells become particularly susceptible to carcinogenic agents.

The invasion of the basal membrane is a major occurrence in the development of cancer. When a carcinoma is labelled " in situ " the process of malignancy is limited to the surface of the epithelium and by the basal membrane. As soon as the basal membrane is penetrated by the abnormal cells then these cells can spread in the supportive and vascular tissue beginning the process of metastasis.

In order to survive, tumor cells need oxygen and nutrients and in turn stimulate the growth of new blood vessels. The immunological system however does not always recognize the tumor cells as foreign entities and therefore are not destroyed. Some researchers hope to counter these phenomena by developing a somewhat transformed virus which will restore the damaged cells, copies of p53 which were lost or inactive, thus to introducing into the tumor a protein which will prevent attachment of the tumor to blood vessels or provide the body a defense mechanism capable of fighting against malignant cells.

Pour en savoir plus:

Gloor E, Shu YJ: Comprehensive Cancer Cytopathology of the Cervix Uteri. Color atlas of cancer cytopathology, vol 4. McGraw-Hill, New-York 1995.

Meisels A, Morin C: Cytopathology of the Uterine Cervix. ASCP Press, Chicago 1990.

http://www.baclesse.fr/cours/fondamentale/ différents aspects du cancer