One of the most important discoveries in cellular biology, a discovery equally important in our understanding of the aging process, was made in 1961 by two cell biologists, Dr. Hayflick and Dr. Moorehead, while working with an experiment demonstrating the senescence, or aging, of cells.

Dr. Hayflick postulated that the aging process was determined by a biological clock that existed in every living cell. The two scientists, carried out experiments somewhat along the lines of Dr. Carrel's experiment, using fibroblast cells (found in lung, skin, muscle, and heart tissue).

These experiments demonstrated conclusively that the life span of a cell is limited. It could divide approximately 50 times over a period of years, at which point it ceased dividing.

The rate at which these cell divisions took place appeared to be governed by the amount of nutrient available. Cells that had an oversupply of nutrient completed the 50 cell divisions in about a year. Cells that had an under supply of nutrient took three times longer than did normal cells to complete their 50 divisions. While going through this division process, some cells went through changes and degenerated prior to reaching the 50 cell division limit. The parts of the cell most affected by the degenerative process were the cell organelles (like mitochondria), membranes, and genetic material (like DNA).

Given the degeneration that befalls some cells, and their eventual loss in organs and tissues, it is a very reasonable conclusion that this loss plays a large part in the aging process.