Stem cells differentiate into the specialized cells. That is, stem cells stop being stem cells and start a process — called differentiation — that turns off gene expression of stem cell genes and turns on gene expression for the specialized cell. The cell becomes a specialized cell.
Typically, a stem cell divides and just one of the daughter cells begins to differentiate. The remaining cell stays a stem cell.
This is how, during embryonic development, the various tissues and organs form.
In the child and adult, progenitor stem cells in each tissue differentiate to the cell types of that tissue. For instance, just outside skeletal muscle cells reside “satellite” cells, which are stem cell for skeletal muscle During our lifetime, the satellite cells divide and a daughter cell becomes part of the skeletal muscle cell.
The heart of an adult is much bigger than that that of a newborn. There are cardiac stem cells (CSCs) that divide and then differentiate to cardiomyocytes (heart muscle cells) as we grow.
There are osteoblast progenitor cells that repair broken bones.
Some adult stem cells seem to be more primitive and are able to differentiate into many cell types. The hematopoietic stem cells form all the cells in the blood. During our lifetime, HSCs are activated and proceed to divide and then differentiate to replenish the cells in the blood.