Background
The Uzumaki and Senju clans, being distant relatives have always had close ties. Mito ended up marrying Hashirama Senju, who would help found Konohagakure and became the village's First Hokage.
During her husband's battle against Madara Uchiha at the Valley of the End, Hashirama at some point gained control of Kurama. To gain power for her husband, Mito used a fūinjutsu to seal Kurama within herself. During her husband's battle against Madara Uchiha at the Valley of the End, Hashirama at some point gained control of Kurama. To gain power for her husband, Mito used a fūinjutsu to seal Kurama within herself. Despite the seal's strength, it almost broke when she was going through childbirth.
Towards the final years of her life, it was decided that a new host for Kurama would have to be found. Kushina Uzumaki, one of Mito's clansmen, was then selected for the role due to her special chakra. Terrified by the idea of having been taken from her home to become a jinchūriki, Mito comforted Kushina, and told her that in order to counter the fox's hatred, she needed to fill herself with love. Some time after this, the beast was presumably extracted from within Mito and sealed into Kushina.