Platelets (Thrombocytes)
Platelets (plat'lets) (thrombocytes) result from fragmentation of certain large cells, called megakaryocytes, in the red bone marrow. The platelet count in blood is normally 150,000 to 300,000 per cubic millimeter. Thrombocytopenia is an insufficient number of platelets (less than 50,000 per cubic millimeter).
Platelets do not have nuclei and are about half the size of red blood cells. They live only about 10 days and are involved in repairing damaged blood vessels and in initiating the process of blood clotting.
Platelets are fragments of cells that are involved in vessel repair and blood clotting.
Hematopoiesis
The process by which blood cells are formed is called hematopoiesis. Blood cells are continuously produced in the red bone marrow of the skull, ribs, and vertebrae, and in the ends of long bones. The bone marrow contains stem cells that constantly divide, producing cells that eventually become the various types of formed elements. At the top is a multipotent stem cell (hemocytoblast) that divides, producing two other types of stem cells. The myeloid stem cell gives rise to the cells that go through a number of stages to become red blood cells, platelets, granular leukocytes, and monocytes. The lymphoid stem cell produces the lymphocytes.
As red blood cells pass through a number of stages, they lose their nuclei and gain hemoglobin. About 2.5 million erythrocytes are produced every second, and an equal number is continuously destroyed by the spleen and liver. Red blood cell production increases when a growth factor called erythropoietin is released by the kidneys (and probably other organs as well). Erythropoietin stimulates stem cells to divide and produce cells that become red bloar! cells.
Ordinarily, only mature blood cells enter the bloodstream. Blood entering the bone goes into blood sinuses, where the mature blood cells collect and from which they enter the circulatory system.
Each type of white blood cell seems capable of producing specific growth factors that circulate back to the bone marrow and stimulate increased production of white blood cells. The best known growth factor is Gm-CSH (granulocyte- macrophage colony-stimulating hormone).
Blood cells are formed through the process of hematopoiesis. Stem cells in the red bone marrovv continuously divide, giving rise to cells that become the formed elements.
