Stem cell

A stem cell has two defining abilities. It can self-renew, dividing again and again to make more stem cells, and it can differentiate, producing daughter cells that mature into specialized types. When a stem cell divides asymmetrically, one daughter stays a stem cell while the other heads toward specialization, which keeps the size of the stem cell pool steady.

Stem cells are ranked by potency, the range of cell types they can make. The fertilized egg and its earliest descendants are totipotent. Embryonic stem cells, taken from the inner cell mass of an early embryo, are pluripotent and can form any body tissue. Adult, or somatic, stem cells are usually multipotent and limited to the cell types of their own tissue; hematopoietic stem cells in bone marrow, for instance, make red cells, white cells and platelets throughout life. Induced pluripotent stem cells are adult cells reprogrammed in the lab to behave like embryonic ones.

Stem cells maintain and repair tissues, which is why bone marrow transplants can rebuild a patient's blood. Researchers also use them to model disease, test drugs and grow replacement tissue.