Stem cells are a special population of cells with the extraordinary ability to transform into a wide range of cell types that make up the human body. During early development and growth, these cells can mature into specialized structures, while in adult tissues they act as a built-in repair system, dividing continuously as needed to replace worn-out or damaged cells. Each time a stem cell divides, the daughter cells retain the ability either to remain stem cells or to differentiate into cells with distinct biological roles—such as neurons, muscle fibers, or blood cells.