Regulation of Cell Proliferation and Cancer

Abstract

As discussed briefly in Chapter 1, there is a fundamental difference between unicellular and multicellular organisms in the factors which determine their rate of cell growth and division. Growth of unicellular organisms is usually limited by the availability of nutrients, and in rich medium they divide exponentially. By contrast, cells in a multicellular organism are normally bathed continually in a nutrient-rich medium (the extracellular fluid), yet their division must be strictly controlled. In healthy animals, cells tend to grow in organized layers or sheets, and cell proliferation generally occurs at well-defined locations (e.g. at the base of the dermis in skin) where there are stem cells capable of proliferation. In adult animals, proliferation generally only occurs to replace cells which have a limited lifetime (e.g. skin, gut and blood), and to repair wounds. Clearly there has to be a balance between production of new cells and loss of cells due to death or terminal differentiation. Cancer is caused by a breakdown in the systems which control this balance. A tumour is thought to be usually derived from a single aberrant stem cell, and it is interesting to note that a 1 g tumour, containing perhaps 109 cells, could result from only 30 division cycles. If it stops at that stage, a single tumour may be harmless or benign. However, malignant tumours go a stage further and metastasize: cells become detached and invade other parts of the body, where they form new tumours, with devastating results. Leukaemias are caused by uncontrolled division of leukocytes (white blood cells), and are also very serious.