As one of the longest-running anime series, Naruto was chock-full of surprising twists and mysteries, leading to many elaborate fan theories. The strangest of all was perhaps the use of the late Hashirama’s cells to give other ninjas his powers and regrow limbs.
In a world where ninjas can summon shadow clones of themselves and manipulate the elements to their will, few powers are more coveted than Hashirama’s ability called Wood Release.
He was the first Hokage, leader of the Konohagakure village, which is a title that Naruto himself aspires to obtain (and, by the end, does). Hashirama was known as one of the most powerful shinobi in the world but was also the only one naturally capable of using his chakra to manipulate wood. But after his death, his cells are mysteriously used to pass on his abilities to others.
Hashirama’s cells became important for their Wood Release and regenerative properties
Throughout his life, Hashirama consistently worked toward achieving peace between the ninja clans. He was an immensely powerful shinobi, who used his Wood Release power to create wood in a large variety of shapes, including clones of himself, an entire forest, and giant dragons.
He was one of a few people able to control the tailed beasts, through his Wood Release powers, and actually founded Konohagakure village with his friend-turned-rival Madara.
His descendants, however, do not have his Wood Release abilities, so his elusive powers are highly coveted. Both Konoha and Orochimaru experiment with his DNA to try and recreate his power. Orochimaru manages to be successful in only one of his test subjects when he injects a young Yamato with cells taken from Hashirama’s body.
Yamato is able to use Wood Release, though not to the level of Hashirama. From there several other characters use his cells to grant them a modicum of his Wood Release ability. Madara uses Hashirama’s DNA to heal himself, creating a strange face on his chest and is able to create a full clone of Hashirama, which he then manipulates to regenerate lost limbs. After Naruto loses his arm in one of the most epic anime fights of the last decade, he’s injected with Hashirama’s cells to regrow it.
The show never truly explains how Hashirama’s cells are able to pass on Wood Release powers or regeneration abilities. In the real world, mammals with regenerative properties are not unheard of, but simply injecting their cells into a person won’t regrow an arm.
The Naruto writers may have some explanation regarding chakras, but it was never conveyed in the show.