Hayflick limit in the context of Cellular differentiation

Immortality is the concept of eternal life and permanent resistance to death from natural causes. Some species possess "biological immortality" due to an apparent lack of the Hayflick limit.

From at least the time of the ancient Mesopotamians, there has been a conviction that gods may be physically immortal, and that this is also a state that the gods at times offer humans. In Christianity, the conviction that God may offer physical immortality with the resurrection of the flesh at the end of time has traditionally been at the center of its beliefs. What form an unending human life would take, or whether an immaterial soul exists and possesses immortality, has been a major point of focus of religion, as well as the subject of speculation and debate. In religious contexts, immortality is often stated to be one of the promises of divinities to human beings who perform virtue or follow divine law.

Cellular senescence is a phenomenon characterized by the cessation of cell division. In their experiments during the early 1960s, Leonard Hayflick and Paul Moorhead found that normal human fetal fibroblasts in culture reach a maximum of approximately 50 cell population doublings before becoming senescent. This process called the Hayflick limit is also known as "replicative senescence", since it is brought about through replication. Hayflick's discovery of mortal cells paved the path for the discovery and understanding of cellular aging molecular pathways. Cellular senescence can be initiated by a wide variety of stress-inducing factors. These stress factors include both environmental and internal damaging events, abnormal cellular growth, oxidative stress, autophagy factors, among many other things.

The physiological importance of cell senescence has been attributed to prevention of carcinogenesis, and more recently, aging, development, and tissue repair. Senescent cells contribute to the aging phenotype, including frailty syndrome, sarcopenia, and aging-associated diseases. Senescent astrocytes and microglia contribute to neurodegeneration.

Maximum life span (or, for humans, maximum reported age at death) is a measure of the maximum amount of time one or more members of a population have been observed to survive between birth and death. The term can also denote an estimate of the maximum amount of time that a member of a given species could survive between birth and death, provided circumstances that are optimal to that member's longevity.

Most living species have an upper limit on the number of times somatic cells not expressing telomerase can divide. This is called the Hayflick limit, although this number of cell divisions does not strictly control lifespan.

Leonard Hayflick (May 20, 1928 – August 1, 2024) was an American anatomist who was Professor of Anatomy at the UCSF School of Medicine, and was Professor of Medical Microbiology at Stanford University School of Medicine. He was also past president of the Gerontological Society of America and was a founding member of the council of the National Institute on Aging (NIA). The recipient of a number of research prizes and awards, including the 1991 Sandoz Prize for Gerontological Research, he studied the ageing process for more than fifty years. He is known for discovering that normal human cells divide for a limited number of times in vitro (refuting the contention by Alexis Carrel that normal body cells are immortal). This is known as the Hayflick limit. His discoveries overturned a 60-year old dogma that all cultured cells are immortal. Hayflick demonstrated that normal cells have a memory and can remember what doubling level they have reached. He demonstrated that his normal human cell strains were free from contaminating viruses. His cell strain WI-38 soon replaced primary monkey kidney cells and became the substrate for the production of most of the world's human virus vaccines. Hayflick discovered that the etiological agent of primary atypical pneumonia (also called "walking pneumonia") was not a virus as previously believed. He was the first to cultivate the causative organism called a mycoplasma, the smallest free-living organism, which Hayflick isolated on a unique culture medium that bears his name. He named the organism Mycoplasma pneumoniae.

In 1959, Hayflick developed the first inverted microscope for use in cell culture research. To this day, all inverted microscopes used in cell culture laboratories worldwide are descended from this prototype. His microscope was accessioned by the Smithsonian Institution in 2009.