Simplify the following text for a Grade 8 native English-speaking person.
Original: Because its main function is to nurture the developing sperm cells through the stages of spermatogenesis , the Sertoli cell has also been called the `` mother '' or `` nurse '' cell .
Simplified: A Sertoli cell is a cell found in the testes . Its main function is to help the developing sperm cells through the stages of spermatogenesis - the process of turning stem cells into sperm .