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Hayflick Limit is a biological theory that proves why the human body cannot surpass 125 years of age.
We, as humans, undergo aging, but our cells behave differently. Throughout history, humanity has sought answers to the aging process through various religions, cultures, and histories. Until 1961, when a biomedical expert, Leonard Hayflick, made a groundbreaking discovery that changed the medical world forever.
Who is Leonard Hayflick?
Born in Philadelphia, Pennsylvania, on May 20, 1928, Leonard Hayflick's passion for science and biology was instilled by his parents, Edna and Nathan Hayflick. On his ninth birthday, his uncle gifted him a chemistry set, sparking a lifelong scientific journey.
During adolescence, Hayflick's parents built a small biology and chemistry lab in the basement of their home. While attending John Bartram High School in Philadelphia, Hayflick displayed a profound understanding of chemistry, even correcting his chemistry teacher.
Hayflick's Educational Journey: In 1946, he commenced studies at the University of Pennsylvania, but military service delayed his academic pursuits. Returning in 1948, he resumed studies and, after graduating in 1951, became a research assistant in bacteriology. Despite a rewarding job, his preference for the University of Pennsylvania environment led him back for a master's degree. Soon after graduating, he earned a doctoral scholarship in medical microbiology and chemistry, obtaining his Ph.D. in 1956.
A Serendipitous Discovery!
At the Wistar Institute in 1958, Hayflick delved into researching whether viruses could induce cancer in humans. Extracting presumed cancer-causing viruses and introducing them to healthy human cells, he aimed to validate his findings. To maintain unbiased research, multiple samples were necessary, resulting in the cultivation of numerous cells. Amid the cell culturing process, Hayflick observed an intriguing phenomenon: a group of older cells ceased division, leaving him puzzled.
While these cells continued metabolic activities, they refrained from further division. Upon examining other cultured cells, he noted that the majority would cease division after around 50 population doubling cycles.
Contrary to prior beliefs that all our cells continuously divide, an unstoppable process, Hayflick discovered that after each division, telomeres found at the end of each chromosome progressively shortened. Upon reaching a critical limit, cells ceased division.
Until then, scientists speculated that the natural aging process was linked to the origin of life, a concept still beyond our understanding today. Uncovering this about cells led Hayflick to cease cancer cell research and shift focus to what is now known as gerontology, the study of aging processes.
In two years of research, it was discovered that cellular aging is linked to the human body's age, explaining why we can only live around 125 years. His paper, titled 'Continuous Cultivation of Human Diploid Cell Strains,' was published in 1961. In another study, cells collected from different body parts were examined, comparing cells from adults and fetuses.
The results revealed that cells divide about 40 to a maximum of 60 times before stopping. Once they stop, they undergo senescence and die. Similarly, as humans age, the body undergoes senescence, and over time, we die. This theory is intricately described in his paper, where he mentions that the length of telomeres presented in various cells may take varying times to shorten to the point of cell division cessation.
Scientific Significance Behind the Discovery
Some cells only divide 40 times before stopping due to telomere length, indicating each DNA will have unique characteristics. This means why some people age faster than others, all attributed to genetics. When correlating with a person's age, when cells divide up to the 60th time, it correlates to the age of 125, and thus, if their genes contain longer telomeres, they theoretically have a longer lifespan.
A cell can complete mitosis, or double and divide cells, only forty to sixty times before undergoing apoptosis and dying afterward. Since our bodies are composed of cells, this explains why death due to aging is inevitable. Furthermore, the paper shows that with each doubling and cell division, the cells themselves become more fragile, weaker, and less efficient in the mitotic process.
Above, we see the research conducted by Heyflick in 1961, where he attempted to observe how many times a cell could double and divide in the cell culturing process. Once the 50th mitosis completes, the cell will initiate the apoptosis process and gradually die off.
This serves as a perfect representation of the human aging process. As time progresses, our bodies weaken, all senses such as vision and hearing follow suit, and most importantly, the healing process for wounds slows down as cells take more time to regenerate. Over time, everything becomes slower and more challenging.
