Leonard Hayflick (1928–2024)

Pioneer of cell aging research and discoverer of the Hayflick limit.

All human somatic cells come to the end of their finite lifespans. Sadly, the same can now be said for the scientist who first discovered this phenomenon. On 1 August 2024, Leonard Hayflick, a pioneer in cell aging research and discoverer of the ‘Hayflick limit’, met the end of his own spectacular life at his home in The Sea Ranch, California.

Credit: Michael D. West

Len was the recipient of more than twenty-five prestigious awards from a variety of organizations, which reflects a broad range of achievements across multiple scientific disciplines. Most notable among them was the John Scott Award in 2014 awarded to Len and his colleague Paul Moorhead. This award, established in the USA in 1823 in honor of Benjamin Franklin, has previously been awarded to Marie Curie, Thomas Edison, Jonas Salk, Nikola Tesla and the Wright Brothers, among others.

Born in Philadelphia on 20 May 1928, Len earned his PhD at the University of Pennsylvania and subsequently took a postdoctoral position at the University of Texas Medical Branch in Galveston, studying cell culture under Charles M. Pomerat. In 1958 he was recruited by virologist and director Hilary Koprowski to join the Wistar Institute in charge of the cell culture core facility. It was at the Wistar Institute that Len’s most notable contributions were made, including the creation of human in vitro cell strains that are widely used in aging and cancer research, as well as the culture of viruses for research and vaccine production. In 1968 Len took the position of Professor of Medical Microbiology at Stanford University School of Medicine. In 1982 he relocated to the University of Florida, Gainesville as director of their center for gerontological studies. Len served on numerous editorial boards and was the Editor-in-Chief of Experimental Gerontology for 13 years. From 1982 to 1983 he was the President of the Gerontological Society of America. He participated in the founding of the National Institute on Aging, and consulted with the World Health Organization, the National Cancer Institute, the American Federation for Aging Research and biotechnology companies such as Genentech (where he helped to design large-scale cell culture technologies) and Geron Corporation (which cloned the components of the immortalizing enzyme telomerase, demonstrating the potential to manipulate cell lifespan). In 1988 Len relocated to the University of California, San Francisco, where he was, until his death, a professor of anatomy.

Len’s most notable discovery was his demonstration of the finite replicative capacity of normal human diploid cell strains. Len remarked that he was unaware at the time of the prediction of August Weismann in the late 19th century that “death takes place because a worn-out tissue cannot forever renew itself, and because a capacity for increase by means of cell division is not everlasting but finite” (personal communication with M.D.W.). But he was, however, quite aware of the then-prevailing consensus that cultured cells — if provided the proper conditions for growth — would proliferate endlessly. This view was supported by research that demonstrated the long-term continuous passage of cells in vitro, including the tumor-derived HeLa cell strain and chick heart-derived cells (which had been shown to continuously proliferate for up to seven years in culture, during which time the authors Carrel and Ebeling calculated that if all cells produced during this expansion had been retained, the total mass of the cells would have exceeded the mass of the sun¹).

While culturing human fetal lung-derived fibroblasts in the Wistar Institute’s cell culture core facility, Len was puzzled by the unexpected arrest of cell growth on his new cell strains after approximately 10 months of continuous passaging. Vincent Cristofalo related to M.D.W. the story that one day Len entered the Institute’s lunchroom complaining about his problem and one of the faculty members responded “Well, Len, maybe your cells are just growing old!”. The correlation of proliferative arrest with the date of initiation of culture, and subsequent mixing of early and later-passage cells convinced Len that he was observing the effect of an intrinsic ‘replicometer’ that imparted to fibroblasts cultured in vitro a finite replicative capacity later designated by Macfarlane Burnett as the Hayflick limit.

Len, together with his collaborator Paul Moorhead, faced hurdles in publishing these findings; the Journal of Regenerative Medicine rejected their findings on the basis that “the inference that death of the cells in some of the uninfected cultures is due to “senescence at the cellular level” seems notably rash. The largest fact to have come out from tissue culture in the last fifty years is that cells inherently capable of multiplying will do so indefinitely if supplied with the right milieu in vitro.” Len and Paul later published their results in Experimental Cell Research, which has become one of the most frequently cited scientific publications (10,948 so far)².

The discovery of the Hayflick limit inspired the thinking of other notable scientists. On his journey home from attending a lecture on Hayflick’s replicometer in Moscow, Alexey Olovnikov pondered the resemblance between the DNA double helix and train tracks, and how passengers entered the train near, but not at, its very ends: this prompted him to imagine parallels in the DNA replication machinery and led to his theory of ‘marginotomy’³. Later, Len collaborated with scientists associated with Geron Corporation, James Watson, Carol Greider and others to test the Olovnikov hypothesis; Len even donated a sliver of skin in order, by his own account, to have “skin in the game”. His cells were the first to be immortalized with the catalytic component of telomerase.

A second, but no less important, accomplishment during Len’s career was the development of a contamination-free cell strain known as WI-38 that has been used ever since to safely manufacture and test viral vaccines designed to prevent poliomyelitis, rubella, varicella, mumps, rabies, adenovirus infections, hepatitis A and other communicable diseases. Prior to Len’s discovery, cells from monkey kidneys were used to make vaccines, but because they were often contaminated with viruses they could not be used effectively and without considerable risk. Once WI-38 was developed, Len was inundated with requests from across the globe for starter samples. Len subsequently distributed this cell strain without charge to many scientists and vaccine manufacturers. His work constituted a critical step toward the safe production and administration of viral vaccines, which have since saved or improved the lives of over 4.5 billion people⁴.

Len is also known for his discovery of the cause of walking pneumonia. In a casual conversation with Len during his visit to the Wistar Institute, the walking pneumonia researcher Robert Chanock expressed his skepticism over the viral nature of walking pneumonia, as it was treatable with antibiotics. Recalling his dissertation work on animal diseases, Len pondered whether the cause could be a mycoplasma, and asked Chanock to send him an egg yolk containing the active agent. Len subsequently isolated the disease-causing mycoplasma⁵.

If these lifetime accomplishments were not enough, in 1959 Len also developed the first inverted microscope for use in cell culture research. All inverted microscopes used today in cell culture laboratories across the globe are descended from Len’s prototype. The original microscope developed by Len is now held at the Smithsonian Institution.

Len managed to cram into one lifetime a long series of accomplishments that would be difficult for half a dozen other prolific scientists to accumulate during their careers. On a personal note, Len was the commensurate scientist willing to admit his change of mind if the data supported it. Despite initially publishing that cellular aging was too complex to understand, let alone manipulate, upon seeing the data on telomerase-based rescue from senescence he immediately accepted the data and altered his long-held views.

Given his stature and influence on multiple fields of science, Len was surprisingly unassuming. One notable exchange one of us (J.P.M.) witnessed along these lines was related to the Hayflick limit. It happened in a big gerontology conference in Vancouver, Canada in a small room, with only a few people attending. One scientist was presenting a new theory of aging. Halfway through the talk someone from the back asked a question. The presenter went on to explain that his theory was based on the Hayflick limit and that, for some reasons irrelevant here, it led to aging of tissues. A couple of minutes later, the same voice from the back interjects again. A bit annoyed, the presenter again brings up the Hayflick limit and repeats his arguments before continuing the talk. A few minutes later and — you guessed it — yet again the same person from the back interrupts and questions the presenter. The frustration building up in the presenter’s face was visible. He stares at the back and in a patronizing voice asks: “Excuse me, sir, are you familiar with the Hayflick limit?”

And a voice roars through the room: “I am Leonard Hayflick!”

To his close friends he was jovial and warm with a profound sense of hospitality. He was quick to invite those that came to visit at The Sea Ranch to stay with him in his home. Len was active in writing and publishing right up until the time of his death — long after what many would consider the time for retirement.