Regeneration

The Fifth Doctor regenerates in The Caves of Androzani.

Regeneration is the process by which the Time Lords maintain their centuries-long lifespans. It is generally described as a "genetic reshuffling" which a Time Lord may go through if his or her body is worn out by age or has suffered some life-threatening injury or infection. Time Lords may even regenerate at will.

Regeneration in general

The regeneration process

During regeneration the body of a Time Lord is reconfigured into a new form; the new form is generally physically younger and healthier (in human terms) than the Time Lord's previous incarnation. After each regeneration there is a marked change in a Time Lord's appearance and personality. During the process of regeneration there are the genetic equivalent of 'bit errors' appear in the DNA of the regenerated cells. This is what causes the appearance of the Time Lord to change; because even the cells of the brain regenerate as well, their brain chemistry and organisation will change. Although the aspects of their personality caused by "nurture" will not change, the "nature" contribution to their personality will change. (BFA: The Sirens of Time)

The actual regeneration is a painful process as bones and tissue settle into their new forms. (DWN: The Power of the Daleks) The Time Lord will also suffer from confusion and memory loss. (DW: The Twin Dilemma) During the first few hours of the regeneration, the Time Lord will be unstable mentally and physically. If they are knocked unconscious, the whole process might be started all over again. (DWN: The Power of the Daleks) After a while, the Time Lord's body will have settled down,

Limitations

It was established that a Time Lord can regenerate successfully twelve times before permanently dying (DW:The Deadly Assassin). As with most such "rules" there were occasionally exceptions to the twelve regeneration limit and it may be circumvented through unorthodox means.

The Doctor's arch-enemy, the Master, found his own remedy for the end of his regenerative cycle. He first attempted and failed to use the Eye of Harmony on Gallifrey to renew himself, though he was able to absorb enough energy from it to extend his life for a time. (DW: The Deadly Assassin) He was finally able to gain access to the Source on Traken, and used some its powers to steal the body of Nyssa's father Tremas (DW: The Keeper of Traken). The Master was subsequently offered "regeneration: a whole new life cycle" by Lord President Borusa as an incentive to rescue the four Doctors from the Death Zone (DW: The Five Doctors) indicating that the Time Lords may bestow additional regenerations upon an individual.

Some argue that this simply implies that a cycle of regenerations can be given to non-Time Lords; the Master's physical form was at least part Trakenite when the offer was made. A similar offer was made to The Valeyard in respect of his receiving the Doctor's remaining regenerations

When this body was finally destroyed by the Daleks on Skaro, the Master's essence entered that of a human on Earth, and in this body the renegade Time Lord tried to use the powers of the Eye of Harmony's dimensional link on the Doctor's TARDIS to steal the Doctor's remaining lives. (DW: Doctor Who: The TV Movie) The Doctor was once put on trial, the punishment had he lost would have been the forfeiture of his remaining lives (DW: The Trial of A Time Lord) indicating that the regeneration cycle can be truncated as well as extended.

Rassilon is said to have achieved a cycle of perpetual regeneration, becoming immortal. It was for this secret that Lord President Borusa sent four of the Doctor's incarnations and their companions into the Death Zone, where Rassilon lay in eternal sleep in the Dark Tower. (DW: The Five Doctors) It may also have been this particular method of perpetual regeneration which Mawdryn and his team of scientists attempted to steal from the Time Lords, with the result that they became undying creatures who continually aged and regenerated until the Doctor was able to finally help them die. (DW: Mawdryn Undead) Legends suggest that Rassilon became immortal when he was turned into a vampire by the Great Vampire himself. (MA: Goth Opera)

Control over regeneration

Generally, regeneration is initiated when a Time Lord has been too badly injured to survive. However in some cases Time Lords have been known to exercise control over the process. Romana appears to regenerate on a whim, when The Doctor inquires what she is doing she replies casually 'regenerating' and emerges from the rear of the TARDIS with a new appearance (DW: Destiny of the Daleks). The Time Lord Azmael initiated a thirteenth regeneration in order to end his life (DW: The Twin Dilemma). The process can also be stopped with some difficulty (DWN: The Power of the Daleks), as the Master did in order to spite the Doctor (DW: Last of the Time Lords). The Time Lords have also been known to impose regeneration as punishment (DW: The War Games)

The degree of control that Time Lords have over their end appearance is unclear. The Master was able to make his next regeneration as young as the Tenth Doctor (DW: Utopia). Romana seemed adept enough at the process to custom design her new appearances. Romana is seen to try several bodies before finally deciding on a copy of Princess Astra (DW: Destiny of the Daleks). The initial pliability of Romana II's form is likely part of the same phenomenon that allows Time Lords to re-grow limbs within the first 15 hours of their regeneration due to having enough 'residual energy'. The Doctor is seen to exploit this quirk of the regeneration process by re-growing his arm which was severed in a sword fight (DW: The Christmas Invasion). In contrast to Romana the Doctor does not appear to have much control over his post regeneration appearance; after his fourth regeneration he commented 'thats the problem with regeneration, you never quite know what you're going to get' (DW: Castrovalva). This lack of control may be because: Control over appearance is a trait of female Time Lords, his Half-Human heritage impairs his regenerative cells, or simply due to the circumstances of his regeneration (Romana's regeneration occurred under ideal conditions, where as The Doctor has generally been seen to regenerate in order to avoid death or had a new appearance assigned to him).

Science

The exact mechanism that makes regeneration possible has not been stated, though many theories have been made. These varying explanations may or may not be compatible with each other.

• One explanation was Cardinal Rassilon had been investigating a method of regenerating decaying and diseased tissue via a series of permanently carried self-replicating biogenic molecules. The cells of a Gallifreyan body can be repaired, restored and reorganised. This will result in a wholly new physical form. The brain cells would similarly be rearranged, though to a lesser degree, thus ensuring the new incarnation will replicate the memories and personality of the former incarnation. Cardinal Rassilon intended for this mechanism only to be used upon the Gallifreyan elite. He also input a parameter of 12 regenerative cycles to avoid decaying biogenic molecules. (BFA: Zagreus)

• Another theory attributes regeneration to a "nanomolecular virus" that rebuilds the body much like the "self-replicating biogenic molecules". (EDA: The Gallifrey Chronicles)

• A third theory is that Time Lords have triple-helix DNA: the third strand was added by Rassilon to make regeneration possible. (MA: The Crystal Bucephalus)

• Another explanation is that regeneration may be linked to what is known as the "Rassilon Imprimatur", the symbiotic nuclei of a Time Lord that bonds him or her to a TARDIS, and allows his or her body to withstand the molecular stresses of time travel. (DW: The Two Doctors)

• One partial explanation of the process links it to the release of massive amounts of a hormone known as lindos at moments of extreme trauma, with the hormone triggering the regeneration itself. Recently-regenerated Time Lords can be identified by the raised levels of lindos in their system. (DWN: The Twin Dilemma, BFA: Unregenerate!)

Aborted Regeneration

Occasionally, a regeneration will fail and the regeneration will abort. Though Time Lord technology can treat this, in some occasions the damage will be too severe to fix.

After being shot by the War Lords, the War Chief was barely able to survive. While being taken back to the War Lords' planet, his body attempted to regenerate. Due to the massive injuries and the lack of medical care, this regeneration aborted. The result was that he turned out looking like two individuals poorly fused together. (NA: Timewyrm: Exodus)

Time Lords can choose whether to regenerate or not and in one case the Master chose not to, although the Doctor begged him to do so. (DW: Last of the Time Lords)

The Doctor's Regenerations

Following is a list of how each Doctor has regenerated, thus far.

1. First Doctor: apparently succumbs to old age and/or possibly from the energy draining effects of the planet Mondas (DW: The Tenth Planet)
2. Second Doctor: a forced regeneration and exile to Earth by the Time Lords. (DW: The War Games)
3. Third Doctor: radiation poisoning from the Great One's cave of crystals on Metebelis III. (DW: Planet of the Spiders)
4. Fourth Doctor: falls from the Pharos Project radio telescope. (DW: Logopolis)
5. Fifth Doctor: spectrox toxaemia. (DW: The Caves of Androzani)
6. Sixth Doctor: injured as the Rani attacks the Doctor's TARDIS (DW: Time and the Rani)
7. Seventh Doctor: dies on the operating table while undergoing surgery for gunshot wounds. (DW: Doctor Who: The TV Movie)
8. Eighth Doctor: unknown as of yet, possibly as a result of the Last Great Time War.
9. Ninth Doctor: cellular degeneration caused by absorbing the energies of the time vortex from Rose Tyler, which she in turn had absorbed from the heart of the TARDIS. (DW: The Parting of the Ways)

The Eighth Doctor unofficially regenerated into the Alternative Ninth Doctor.

Behind the Scenes

Regeneration was first introduced when the First Doctor (William Hartnell) changed into the Second Doctor (Patrick Troughton) in the closing moments of The Tenth Planet. Tradition has it that Hartnell himself thought up the idea of the Doctor being able to change into a new body as a means of keeping the series going after his departure. In The Power of the Daleks (the first story featuring the Second Doctor), the Doctor described himself as having been "renewed", and also said that the change was "part of the TARDIS."

The change in the Doctor's appearance was meant to occur several stories earlier, during The Celestial Toymaker, with the Toymaker capriciously changing the Doctor's appearance out of spite (the Doctor is invisible for most of the story and would have reappeared in the final episode played by a different actor). The reason for the change of plan is unknown - either it was decided to retain Hartnell in the role for a few more stories, or the actor was accidentally issued with a new contract by mistake, making it impossible to recast the Doctor at that moment in time.

The process was not actually referred to as "regeneration" until the end of Planet of the Spiders, when the Third Doctor (Jon Pertwee) changed into the Fourth (Tom Baker). Prior to this, the Doctor was simply described as having "changed his appearance."