Originally Posted by Shino'sDad
Remember that the Uchiha received the Eyes of the Sage of Six Paths, while the Senju received the Body of the Sage of Six Paths. So it makes sense that there be some sort of special form the Senju can take. I assume that Tsunade's Miotic Regeneration is a lessened form of the First's Form. Madara even mentions that her version is much weaker than Hashirama's when they were fighting, so this wasn't a complete pull out of no where. We were informed he had some sort of special form already.
I must have skipped over that fact during Madara's absolute slaughter of the living Kage Rangers.