Regeneration is the common name given to a Gallifreyan's ability to replace a dying or unwanted body in favour of a new physical form. It is named after a more general description to describe the healing process or the regrowth of body parts (lost limbs, severed nerve connections, and other wounds).

Regeneration on-screen

To date there have been ten regenerations seen on-screen, seven of which have involved the Doctor. These are:

1. The first Doctor's regeneration into the second (The Tenth Planet)
2. The third Doctor's regeneration into the fourth (Planet of Spiders/Robot)
3. The Minyan regeneration process (Underworld)
4. The first Romana's regeneration into the second (Destiny of the Daleks)
5. The fourth Doctor's regeneration into the fifth (Logopolis/Castrovalva)
6. Mawdryn's regeneration (Mawdryn Undead)
7. The fifth Doctor's regeneration into the sixth (Caves of Androzani/The Twin Dilemma)
8. The sixth Doctor's regeneration into the seventh (Time and the Rani)
9. The seventh Doctor's regeneration into the eighth (The TV Movie)
10. The ninth Doctor's regeneration into the tenth (Parting of the Ways)

The History of Regeneration

Regeneration isn't a natural process – it's artificial. It was created as a means of extending Gallifreyan life to overcome sterility caused by the Curse of the Pythias (Marc Platt's Time's Crucible). This sterility came in the form of a high incidence of molar pregnancies, which suggests genetic damage to the Gallifreyan species (Lawrence Miles' Interference). Without extended lives and a means of repairing genetic damage, extinction would have been the only option.

Rassilon commissioned a biogeneticist called Thremix to come up with a solution (John Peel's Gallifrey Chronicles). Early attempts to gene splice Gallifreyan DNA with compatible species was eventually curtailed when Thremix isolated the Immortality Virus, which was based on genetic material found in their enemy, the vampires. Legend has it that Rassilon himself became a vampire to enable Thremix to synthesize the virus(Paul Cornell's Goth Opera).

Administration of the Immortality Virus was controlled by Metamorphic Symbiosis Regenerators, early versions of which allowed unlimited rejuvenation of the same body (Underworld). This process was found to be flawed, and led to madness.

The first generation of Time Lords were culled during the later part of Rassilon's reign (scattered hints of a "Night of the Long Knives"), and the second generation of Time Lords were subjected to a new process – biogenesis. This process involved the weaving together of thirteen sets of genetic coding into a single biodata sequence. This was done by the Looms (Marc Platt's Lungbarrow).

The early biogenesis process was also flawed. Without assistance a regenerating Time Lord would absorb the DNA of foreign organic material – clothes, digested food, etc – resulting in progressive mutations. In extreme cases a Time Lord could regenerate into the same species as his dinner (Lawrence Miles' Interference, and Paul Cornell's Circular Time).

Eventually a more complex system was developed, in which a Matrix-TARDIS link was used to control the process, which came to be called telebiogenesis (Keeper of Traken). The modern form of regeneration, which requires a 15 hour stabilisation period (The Christmas Invasion).

At the Academy, loomed Time Lords would play a game, "Eighth Man Bound", in which they tried to read their own biodata, visualising their future lives. They rarely got beyond their eighth incarnations (Lawrence Miles' Christmas on a Rational Planet).

The Older Time Lord Houses, called Oldbloods, used the biodata from the first Time Lords, whose bodies were closer to the original Gallifreyan, and had only one heart. Newer Houses – the Newbloods – received looms containing the genetic data of later generation Time Lords, whose bodies had been genetically altered by telebiogenesis, and therefore had two hearts (Lawrence Miles' Alien Bodies).

The first Doctor, and possibly the second, was an Oldblood, who had only one heart. He may have acquired a second heart when he first regenerated, but it's by no means certain. What is certain is that he acquired a second heart in his third body.

The Doctor's third body was imposed upon him against his will, and for an as-yet undisclosed reason the body he should have had was replaced with another. This was a transplant, and with it came a legacy from the body's past. These legacies included:

• Extra memories – the third Doctor seemed to think he had been a
  scientist for "thousands of years"
• Past or future lives – during his mental duel with Morbius, the
  Doctor recalled other lives and bodies. They could either have been
  the past lives of his third body, or the future lives taken from him
  when his new body was provided.
• A criminal's tattoo. Possibly a symbol of his exile, or else
  evidence of a previous life in which the Doctor's body had belonged
  to someone else.

The transplantation may well have been achieved by tampering with the Doctor's biodata. It is possible to sever a biodata extract and graft one half onto the biodata extract of another Time Lord (Big Finish's Gallifrey mini-series, Season 1). This may be how the Master could be promised a new regenerative cycle, and how the Valeyard expected to be given the Doctor's future bodies. It may also explain how the Doctor has two sets of conflicting memories about his early life.

As a Newblood, Romana was born with two hearts and the ability to try out template bodies from a wider section than would be available to Oldbloods. There are 1000 Time Lords at a time, so a cull would produce only 1000 Oldblood template bodies, while there are up to a million Time Lords stored in the Matrix (Craig Hinton's Quantum Archangel), although after the War in Heaven only 153,841 remain (Lance Parkin's The Gallifrey Chronicles).

During the Time War, War Looms are introduced, allowing Time Lords to again adopt alien forms by choice (Mark Clapham & Simon Bucher Jones' Taking of Planet 5).

Long after the Doctor's death and the fall of the Time Lords, the life stolen from the Doctor by the CIA is returned to him, and he is resurrected in his original third body (John Ostrander's The Inheritors of Time). This may explain how the ninth Doctor survived the Time War (Rose), or how an alternate ninth Doctor came to exist (Paul Cornell's Scream of the Shalka).

The Biology of Regeneration

Regeneration in role-playing

D20 Modern

Time Lord