Most of the earlier answers were correct. Namekians regenerate and Saiyans become stronger after near death experiences. Cell was seriously blasted by one of the SWEETEST MOVES OF ALL ANIME the Instant Transmission Kamehameha Blast! Just like Piccolo regenerated his arm while fighting, he was down for a minute & at the same time Goku required lots of time to heal himself after each "near death experience" he incurred. Cell didn't really know what was going to happen because he'd never been pushed that far before but, he did know that he was CELL. He'd have to be hit with a CRAZY amount of Chi or Chakra, if you will, to be taken out and to completely eradicate ALL of his cells. When he self imploded he specifically left certain parts of himself behind to regenerate, even at the last second. Great question and I've never really thought about it that much but, that's why I come to this website!
Gohan was one of the biggest gangsta's of all time when fighting him.
