Lack of cytokinesis results in a huge cell with many nuclei. Human cells usually divide up to 50 times. Shrinking telomeres track the number of divisions a cell has undergone, and when telomeres reach a certain length, division ceases. Cancer cells and certain rapidly dividing somatic cells retain long telomeres and divide continually. Crowding prevents normal cells from dividing.
Apoptosis is a form of programmed cell death.
It selects certain cells to die during prenatal development, particularly in the thymus and brain. An apoptotic cell rounds up, the cell membrane forms blebs, the nuclear membrane breaks down, chromatin condenses, and DNA is cut into many equal-sized pieces. Stem cells actively divide, replenishing tissues. Different cell populations include specific proportions of cells in different stages of the cell cycle. Cancer can result from excess cell division or deficient apoptosis.