A human, as well as every sexually reproducing organism, begins life as a fertilized egg (embryo) or zygote. Trillions of cell divisions subsequently occur in a controlled manner to produce a complex, multicellular human. In other words, that original single cell is the ancestor of every other cell in the body. Once a being is fully grown, cell reproduction is still necessary to repair or regenerate tissues. For example, new blood and skin cells are constantly being produced. All multicellular organisms use cell division for growth, maintenance, and repair of tissues. Cell division is tightly regulated, and the occasional failure of regulation can have life-threatening consequences. Single-celled organisms use cell division as their method of reproduction.
Not all cells in the body reproduce to repair tissues. Most nerve tissues, for example, are not capable of regeneration. This means people who have damaged their nerves or nervous system are often left paralyzed.
However, this may change in the future; scientists have discovered a new drug called intracellular signal peptide (ISP), which helps nerve cells regenerate in rats. It works by blocking an enzyme that causes scar tissue in damaged nerve cells allowing the nervous system a chance to repair itself. The full research study is located here.