List of the 100 most densely populated cities in the United Kingdom, measured in people per square miles. 1  Portsmouth, Hampshire â 15,336. 2  London â 13,870. 3  Leicester, Leicestershire â 11,800. 4  Glasgow, Glasgow â 8,542. 5  Bristol, Bristol â 13,398. 6  Liverpool, Merseyside â 12,790. 7  Stoke-on-Trent, Staffordshire â 11,939.
In 1961 at the Wistar Institute in the US, Hayflick researched a phenomenon later called the Hayflick Limit, or the claim that normal human cells can only divide forty to sixty times before they cannot divide any further.
Biologists chose the word immortal to designate cells that are not subject to the Hayflick limit, the point at which cells can no longer divide due to DNA damage or shortened telomeres. Prior to Leonard Hayflick's theory, Alexis Carrel hypothesized that all normal somatic cells were immortal.