A haematopoietic stem cell is a cell isolated from the blood or bone marrow that can renew itself, can differentiate to a variety of specialized cells, can mobilize out of the bone marrow into circulating blood, and can undergo programmed cell death, called apoptosis.
They are ultimately responsible for the constant renewal of blood cells. Out of billions of new blood cells produced each day, 1 in every 10,000 to 15,000 bone marrow cells is thought to be a stem cell.
In the blood stream the proportion falls to 1 in 100,000 blood cells. The classic source of haematopoietic stem cells (HSCs) is bone marrow. With regard to morphology, haematopoietic stem cells resemble lymphocytes.
They are non-adherent, and round, with a rounded nucleus and low cytoplasm-to-nucleus ratio.