Boss Stem Cells

Boss Stem Cells are difficulty modifiers. The first one is acquired by killing The Hand of the King. Once you get it, you need to beat him with one Boss Stem Cell inoculated to get the second, with two cells to get the third, etc. They can be disposed at the start of the game in the Tube.

When activated, they also give you access to bonus doors scattered in the levels. The doors show the number of cells required and shine blue if you can enter them.