Symbiotic nucleus
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Symbiotic nuclei were a part of Time Lord cells that allowed for safe time travel. They had some connection to the Rassilon Imprimatur, with which all time vessels needed to be primed for time travel. (TV: The Two Doctors)
Symbiotic nuclei also enabled a Time Lord's ability to regenerate. (PROSE: Love and War) It was believed that a Time Lord could only regenerate twelve times because the nuclei could only divide so much. The Valeyard, however, claimed this was imposed by Rassilon to prevent a thirteenth regeneration and keep Time Lords from immortality. (AUDIO: Trial of the Valeyard)
