even within humans and other mortal species, there are cells with the potential for immortality: cancer cells which have lost the ability to die when maintained in a cell culture such as the hela cell line, and specific stem cells such as germ cells (producing ova and spermatozoa). in artificial cloning, adult cells can be rejuvenated to embryonic status and then used to grow a new tissue or animal without aging. normal human cells however die after about 50 cell divisions in laboratory culture (the hayflick limit, discovered by leonard hayflick in 1961).