The Hayflick limit. Hayflick demonstrated that a population of normal human fetal cells in a cell culture will divide between 40 and 60 times. The population then enters a senescence phase. 

Using a novel agar and fluid medium formulation he had devised, Hayflick isolated a unique mycoplasma   Mycoplasma pneumoniae, a mycoplasma shown to be the cause of some cases of primary atypical pneumonia, or "walking pneumonia." With M. F. Barile.

"The building of a connection between the Gompertz equation and the biology of ageing owes much to the work of biophysicist George Sacher [10] of the Argonne National Laboratory, whose introduction to ageing stemmed from the growing recognition during the 1950s that irradiation would shorten length of life [11]. The same recognition led physicist Leo Szilard [12] to propose the somatic mutation theory of ageing and prompted a range of studies on the effects of radiation on ageing both in animal models such as Drosophila (e.g. [13]) and also in human survivors of atomic bomb irradiation [14].

Sacher [10] used the Gompertz model to compare the patterns of increase in age-specific mortality rates across different species. By plotting age-specific mortality on a logarithmic scale against age (figure 1), he showed that a linear increase was generally observed, in accordance with the logarithmic version of equation (1.1), i.e.   

"Thomas B. L. Kirkwood, Decipherng death: a commentary on Gompertz (1825) 'On the nature of the function express of the law of human mortality and on a new mode of determining the value of life contingencies"). (http://rstb.royalsocietypublishing.org/content/370/1666/20140379,  accessed 01-2017

In 1961 "Hayflick developed the first normal human diploid cell strains for studies on human aging and for research use throughout the world. Prior to his seminal research, all cultured cell lines were immortal and aneuploid. One of these new cell strains, developed by Hayflick and his colleague Paul Moorhead at the Wistar Institute in Philadelphia, Pennsylvania, called WI-38, has become the most widely used and highly characterized normal human cell population in the world. Hayflick had found that his normal cell strain WI-38 was capable of growing all of the then known human viruses. He hypothesized that because WI-38 was free from contaminating viruses, it could replace the then widely used primary monkey kidney cells, which contained several dangerous contaminating viruses. Indeed, WI-38 became used worldwide for human virus vaccine manufacture, to the benefit of billions of people" (Wikipedia article on Leonard Hayflick).

Order of authorship in the original publication: Olshansky, Hayflick. Available from PubMedCentral at this link.

In January 2008 I collaborated with fellow ABAA member and tax lawyer Bruce Barnett on the appraisal of frozen ampoules of WI-38 that Leonard Hayflick donated to the Coriell Institute for Medical Research. This was one of the first, if not the very first, appraisals of the fair market value of living material donated to a non-profit organization.