Cells were a part of many living organisms' bodies.
As the Tenth Doctor explained to Rose Tyler, regeneration changed "every single cell" in his body. (TV: Children in Need Special) The Ninth Doctor was forced to regenerate after absorbing the energy of the Time Vortex, which caused every cell in his body to die. (TV: The Parting of the Ways)
The process of progenation involved taking a sample of diploid cells, splitting them into haploids, recombining them in a different arrangement and growing the result into an offspring. (TV: The Doctor's Daughter)