Cell Growth and Differentiation

What Are Stem Cells?

Stem cells are undifferentiated cells, meaning they have not developed specializations; stem cells may be totipotent (giving rise to an entire organism from a single cell), pluripotent (giving rise to many different body cell types), or multipotent (giving rise to a limited range of body cells).

Cells come in many varieties, each specialized to perform a specific function: a muscle cell is different from a nerve cell, which is again different from a skin cell. Yet all organisms begin as a single cell. Cell differentiation is the process by which a cell changes to become specialized to perform a certain function.

All animal cells come from stem cells. An embryonic stem cell (ES cell) is an undifferentiated cell that can give rise to more differentiated cells in a developing animal embryo. These cells are part of normal animal development and are the means by which the few cells of an embryo become a functioning adult organism with diverse tissues and organs. A totipotent stem cell is a stem cell that is able to give rise to an entire organism from a single cell. A pluripotent stem cell is a stem cell that can give rise to many different cell types. The developing embryo's cells undergo differentiation during gestation to form the different tissues, organs, and organ systems. Once an organism reaches maturity, some stem cells remain, in order to replace cells that become damaged or die off naturally. A multipotent stem cell is a stem cell that can give rise to a limited number of other body cell types.

Stem Cell Differentiation

The totipotent stem cell is the earliest embryonic stem cell. It gives rise to pluripotent stem cells, which differentiate into the various types of body cells. Multipotent stem cells remain in an organism throughout its life, replenishing the differentiated cells.
Embryonic stem cells exist only during early embryonic development, before the embryo is implanted in the uterus. After implantation, cells quickly differentiate, forming both the placenta and the inner cell mass. The inner cell mass is a collection of stem cells in an early embryo that will give rise to the different parts of the developed animal. The inner cell mass begins differentiation by dividing cells into endoderm, mesoderm, or ectoderm. These cells further differentiate into the body cells of the organism.

Adult stem cells persist in the body throughout the organism's life. These cells can differentiate to a limited range of cell types but are themselves not fully differentiated. They generate cells to replace damaged or aging cells. Cells within various tissues are naturally renewed at varying rates, with old cells being replaced by new ones. Multipotent stem cells provide the precursor cells that go on to fully differentiate into these specialized cells. Precursor cells may also divide, forming more precursor cells and reducing the need for stem cells. The cells that are fully differentiated and cannot divide are called terminally differentiated. They are the most specialized cells, such as nerve cells or red blood cells.