Koro-sensei and tentacle-having humans (e.g. Itona) have a drastic difference in their bodies: Koro-sensei's body is made up of tentacles wholly, whereas Itona has his human body along with a tentacle.
Are there any differences between the no. of the tentacle cells infused? If yes, it's a fact that even a single tentacle cell can cause death like pain, then how did Koro-sensei survive that much pain?