Heals for a Half Red Heart upon traveling to the next floor.
Heals 50% of Isaac's empty Red Hearts and Bone Hearts (rounded down), but at minimum a half heart, upon traveling to the next floor.
Synergies
Blood Oath: The healing will occur before any health is drained, allowing for a greater stat bonus.
Glowing Hourglass: Returning to a previous floor will keep the health gained from Stem Cell, as well as granting another half-red heart after going down to the next floor again.
Tainted Magdalene: Since all red heart healing is doubled, Stem Cell will bring her to full health.
A Stem Cell is an undifferentiated cell of a multicellular organism that is capable of giving rise to indefinitely more cells of the same type, and from which certain other kinds of cells arise by differentiation.
Heals for a 
Half Red Heart upon traveling to the next floor.
Heals 50% of Isaac's empty 
Red Hearts and 
Bone Hearts (rounded down), but at minimum a half heart, upon traveling to the next floor.
Blood Oath: The healing will occur before any health is drained, allowing for a greater stat bonus.
Glowing Hourglass: Returning to a previous floor will keep the health gained from Stem Cell, as well as granting another half-red heart after going down to the next floor again.
Tainted Magdalene: Since all red heart healing is doubled, Stem Cell will bring her to full health.
Golden Trinket / 
Mom's Box: No effect.
Tainted Bethany: Adds 1 blood charge.
Samson Unlocks 
Ultra Greedier Unlocks 
The Binding of Isaac: Rebirth
