lifespan of a cell "Cell Differentiation Creates New Types of Cells" "Cells Die by Suicide" "Cell Differentiation Creates New Types of Cells" "Cells Die by Suicide" "Cells Die by Suicide" quiescent "Cells Die by Suicide" "Cell Differentiation Creates New Types of Cells" "Cells Die by Suicide" "Cell Differentiation Creates New Types of Cells" "Cell Differentiation Creates New Types of Cells" PubMed lifespan,