Simplify the following text for a Grade 12 native English-speaking person.
Original: In adult organisms , stem cells and progenitor cells act as a repair system for the body , replenished in adult tissues . In a developing embryo , stem cells can differentiate into all the specialized cells , but also maintain the normal turnover of regenerative organs , such as blood , skin , or intestinal tissues .
Simplified: In adult organisms , stem cells act as a repair system for the body , replenishing specialized cells , but also maintain the normal turnover of blood , skin , and intestinal tissues .