S Cells (Ｓ細胞, S Saibō) are a unique part of a Saiyan's biology. They are what determine whether a Saiyan is capable of transforming into the vaunted Super Saiyan transformation. All Saiyans possess these S Cells, but not many Saiyans have them in large amounts.
A gentle spirit plays into one's S Cells, and can increase their quantity. Due to the planet Vegeta being a rougher place to live, the Super Saiyan form became the stuff of legend, as the Saiyans no longer unlocked the transformation. A gentle spirit is not everything, however, as a Saiyan does need to be sufficiently strong before they can unlock the form. Due to these factors, someone like Gokū was able to achieve the form with ease, compared to his ancestors and pure-blooded relatives. In addition, Gokū's and Vegeta's children were all able to achieve Super Saiyan at a fairly young age because they grew up in the peaceful environment of Earth, rather than the violent home planet of their fathers'. The environmental factor can be seen when comparing Trunks and his future counterpart. Due to growing up in an apocalyptic earth, the adult Trunks did not achieve the transformation until he was a teenager. In contrast, his present day counterpart, growing up in a peaceful earth, achieve the transformation at the age of 8 (at the latest).
Inheritance also plays a factor; if a Saiyan that has obtained the Super Saiyan transformation conceives a child, they will pass on their S cell quantity to their offspring, enabling the offspring to transform at a much younger age. The inheritance factor can be seen most clearly with Goten, when contrasted with Gohan. Gohan, who was conceived and born before his father could become a Super Saiyan, transformed at the age of 9 (11 in the anime). Goten, who was conceived and born after his father had become a Super Saiyan, achieved the transformation at the age of 7, making him the youngest Super Saiyan in history.
- The concept of "S Cells" bears resemblance to the Midi-chlorians of Star Wars. The number of Midi-chlorians within the cells of living creatures determines their level of Force sensitivity, similar to how the amount of S Cells within a Saiyans body determines whether or not they can become a Super Saiyan.
- ↑ Saikyō Jump, January 2018 - We asked Master Akira Toriyama! Special Saiyan Q & A