S-Cells

S-Cells are microscopic cells found in most Saiyans. The amount of S-Cells depends on the spirit of the Saiyan in question: the more gentle the spirit, the greater the amount of S-Cells; alternatively, when a Saiyan increases their battle power, it can also increase the amount of S-Cells in their body. S-Cells are essential to becoming a Super Saiyan, and when a Saiyan has gained enough they can transform by feeling intense levels of anger or sadness; alternatively (as demonstrated by Caulifla), by focusing their power into a tingling sensation in their upper-back (directly below their neck) the Saiyan in question can trigger the transformation into a Super Saiyan without the need for emotional stress. S-Cells can be passed down through genetics, making it easier for even partial Saiyans, such as Goku Jr. and Vegeta Jr., to become Super Saiyans.

Trivia

• Presumably, artificial Saiyan hybrids such as Cell, Cell Jr., Mira, and Bio-Broly also possess S-Cells. If so, it can be assumed that Mira and Bio-Broly require them for their Super Saiyan forms, though it is unknown how they function for Cell, though presumably they have some influence in the form of Cell's Super Saiyan state.
• Gentle Saiyans having more S-Cells may explain why Super Saiyans were rare for a long time in Universe 7 as the Saiyans there were violent and lived in a society that valued aggression and ruthlessness, thus gentle Saiyans like Tarble and Gine were considered low class due to their gentle natures (and why Tarble was viewed as an embarrassment by King Vegeta) though ironically their gentle natures meant they had more S-Cells and could conceivably become Super Saiyans much more easily if they became strong enough and experienced the correct stimuli to transform. Goku, Gohan/Future Gohan, Trunks/Future Trunks, Goten, and Goku Jr. all had more S-Cells as they were of a gentle disposition with Goten (through Goku) and Goku Jr. (through his ancestor Gohan) being able to transform due to being descended from one who had obtained the Super Saiyan form.
  • However, as shown by the Saiyans in Universe 6, having gentle natures doesn't guaruntee that Saiyans are more likely to obtain it without training and stimuli as Cabba, despite his power and gentle nature, had not acquired it until after he encountered Vegeta; plus, it is heavily implied that none of the Saiyans in Universe 6 were aware of the transformation itself. It should also be noted that a gentle nature may make Saiyans more pacifistic which results in them being less eager to engage in battle and lacking the typical Saiyan ruthlessness, which means they may allow their enemies to live (which Goku has done several times, only to be attacked by said enemy), though some like Goku retain their love of fighting and training thus explaining why Goku obtained the form: he had a perfect mixture of gentle nature, strength, and fighting spirit. This would also explain why Saiyans like Tarble and Gine were considered weak as gentle natures made them unsuitable for conquering planets and caused them to be viewed as a liability on the battlefield, which may explain Raditz's decision to kill Goku once he realized that Goku's gentle personality would make him a liability on the battlefield much like how their mother was before she retired from the army, as she often had to be rescued by their father. Thus it is likely that a gentle Saiyan's nature could just as easily end up getting them killed as it would allow them to acquire enough S-Cells to transform. Of course had they lived in a different society such as Earth's, which has a martial arts culture where competitive fighting is a sport and allows a gentle Saiyan to train and fight in a more peaceful setting (as it did for Goku as well as his descendants), then they might have been able to achieve it through training and/or correct emotional stimuli (prominent examples are Goten, Trunks, Goku Jr., and Vegeta Jr., they all lived on Earth in relatively peaceful eras yet still acquired the form).
• S-Cells also explain why Goten and Trunks had an easier time of acquiring the form as opposed to Goku, Vegeta, Gohan, Future Gohan and Future Trunks: they were conceived sometime after their fathers became Super Saiyans; the friendly rivalry they have with one another likely played a role as well.
• Acquiring S-Cells via battle power explains how Vegeta and Broly were able to become Super Saiyans, as Vegeta had trained himself intensely in order to obtain it, while Broly was a Saiyan born with an immensely high power level. Bardock likely acquired his S-Cells through his high battle power after surviving the destruction of Planet Vegeta after being transported to the past, as well as have a slightly gentle and caring side to his personality which became more prominent after his experiences during the Genocide of the Saiyans. Gohan and Future Gohan also gained S-Cells due to having high battle power and gentle natures, though their rage was triggered under different circumstances. Trunks and Future Trunks presumably achieved there S-Cells for much the same reasons as they are both powerful (Trunks due to being trained by his father, while Future Trunks trained under Future Gohan to combat the Androids) and somewhat slightly gentle natures (though to a lesser extent than Goten and Future Gohan), they may have also inherited S-Cells from their fathers as Vegeta and Future Vegeta were presumably in the middle of training to become Super Saiyans when both Trunks and Future Trunks were conceived.
• Also, gentler planetary conditions (like those found on Earth) can influence the amount of S-Cells produced by a Saiyan.