We are made of cells – a lot of them. They cooperate to make tissues, and those tissue make up organs. The cells take on different characteristics (it’s called DIFFERENTIATION), according to what they’re required to do (eg they may become nerve cells, or gut lining cells, or bones, or skeletal muscles etc). Generally, they behave. Indeed, there is a theory that normal cells die unless they are constantly told not to.
All cells (except red blood cells) have a nucleus and in that nucleus is a complete copy of the individual’s GENOME – that is, about 30,000 genes – which can be found in the DNA (DEOXYRIBONUCLEIC ACID). Not all of them are turned on, however. The genes that are turned on determine what the cell does and what it looks like (ie how it differentiates); in other word, different genes are active in nerve cells to those active in muscle cells.
By ‘behave’ I mean they die when they are told to (programmed cell death or APOPTOSIS), or they divide when they are told to, or they do their everyday job. When a cell divides, it has to replicate its DNA.
Cancer cells have changes in their DNA so that they ignore the messages that all other cells obey. They divide when they’re not supposed to, and don’t die when they should. This behaviour leads to a lump. The cells that don’t do as their told gradually pick up more genetic abnormalites because when a cell divides it is at its most vulnerable to mistakes in DNA replication. Thus cancers get worse and worse as they age. Malignant lumps infiltrate the surrounding tissues., perhaps getting around nerves or into thin-walled vessels
Bits of the tumour may break off (METASTASISE) and grow elsewhere; cancers of the surface cells usually go to lymph nodes, but they make also go to the liver, to the lungs, to the bones or to the brain. In fact they may go anywhere. Cancers of the cells that make up muscles, fat, fibrous tissue etc tend not to head for lymph nodes; I don’t know why.
There are many different kinds of cancer because there are many different kinds of cell. In face, at a genetic level, each cancer is unique, hence in future, genetic analysis of the tumour may lead to optimisation of treatment. That’s the hope, anyway.