i think it was stated that unless all of his spine was atomized he could regenerate, it aslo states that the power originates in his skull, and goes down to his tailbone
that's fair, however, another way to look at it, in the self decapitation scene, mc stretches neck so that can also be seen as stretching spine. there is also the fact that we learn later on, mc is weird when it comes to super humans, for the sake of not saying spoilers, all im gonna say is that that might have something to do with it