Regulation of Cell Growth and Replication
Most cell types in the body can grow beyond normal size (hypertrophy). However, not all cell types in the mature animal have the same ability to replicate and produce two new daughter cells.
some cell types, for example the epithelium lining the small intestine and certain blood cells, are continuously replicating to replace cells that are lost from the body or die. other cell types, for example cardiac and skeletal muscle cells, do not normally replicate and produce new daughter cells. What determines the ability of cells to replicate and divide is not fully understood. However, a variety of chemical signals stimulate cell growth and, in some cases, cell division. Chemical signals with these capabilities are said to be growth factors. Typically, growth factors stimulate only certain populations of cells. For example, colony-stimulating growth factors stimulate bone marrow cells to produce blood cells, while insulin-like growth factors stimulate cartilage proliferation at growth plates in bone to promote an increase in body size.Unregulated cell growth and replication are a factor in the development of cancer. A mass of cells undergoing uncontrolled growth is a tumor. A tumor is said to be benign if it is local and does not invade other tissues. Malignant tumors are capable of invading surrounding tissues and spreading to other sites throughout the body (metastasis).
sometimes normal cells must undergo spontaneous, or programmed, death and be removed without an inflammatory response so that normal function of a tissue or organ can proceed. For example a corpus luteum formed in an ovary during an estrus cycle must be removed before another cycle can begin. The term apoptosis is applied to spontaneous or programmed death of normal cells. in some cases the development of tumors may also include a decrease in the rate of normal apoptosis.