Image A shows two conjoined cells forming a dumbbell shape; the fertilization envelope has been removed so that the mesh-like outer layer can be seen. Image B shows the sea urchin embryo when it has divided into 16 conjoined cells; the overall shape is rounder than in image A. Image C shows a “water melon” sea urchin which appears as a peach-colored ball covered in white protruding spines.
Figure 10.1  A sea urchin begins life as a single cell that (a) divides to form two cells, visible by scanning electron microscopy. After four rounds of cell division, (b) there are 16 cells, as seen in this SEM image. After many rounds of cell division, the individual develops into a complex, multicellular organism, as seen in this (c) mature sea urchin. (credit a: modification of work by Evelyn Spiegel, Louisa Howard; credit b: modification of work by Evelyn Spiegel, Louisa Howard; credit c: modification of work by Marco Busdraghi; scale-bar data from Matt Russell)
 

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.