Bone marrow is the body's primary manufacturing plant for red blood cells, white blood cells, and platelets. It is a soft fatty tissue found inside the hollow shafts of bones such as the middle of the chest (sternum), hip bone (pelvis), and thigh bone (femur). In the marrow, fibrous tissue forms a sponge-like network that supports "primitive" cells called hematopoietic stem cells. Stem cells are essentially "blanks" that can be used by the body to produce any type of blood cell. As needed, stem cells in the marrow differentiate and then go through a maturation process to become one of five different types of white blood cell, or a red blood cell, or a platelet. These processes take place in a series of steps, and bone marrow will typically contain not only fully mature cells but cells at each stage of development – immature precursors. Normally, only fully mature cells are released from the bone marrow into the bloodstream.
White Blood Cells (WBCs)
There are five different types of white blood cells: lymphocytes, neutrophils (also called granulocytes), eosinophils, basophils, and monocytes. Each plays a different role in protecting the body from infection. Neutrophils, basophils, and eosinophils kill and digest bacteria. Monocytes, which live much longer than neutrophils, also ingest and digest bacteria.
Lymphocytes exist in the blood and lymphatic system. There are two main types of lymphocytes, T cells and B cells. T cells finish maturing in the thymus gland. They help the body distinguish between self and foreign agents. B cells circulate in the blood and produce antibodies, immune proteins that target and attach to specific antigens.
Red Blood Cells (RBCs)
Shaped like donuts with depressions instead of holes in the middle, red blood cells (RBCs) transport oxygen in a continuous cycle of pickup and delivery. Hemoglobin inside RBCs binds to oxygen in the lungs. The RBCs are then carried throughout the body in the bloodstream and hemoglobin releases oxygen to the tissues. Hemoglobin molecules consist of iron-containing heme pigment and globin proteins. There must be a sufficient amount of iron available in the bone marrow for normal hemoglobin and RBC production.
Platelets, also called thrombocytes, are cytoplasm fragments of very big cells seen in bone marrow called megakaryocytes and are essential for normal blood clotting. They are produced by the bone marrow and released to circulate in the blood. When there is an injury to a blood vessel or tissue and bleeding begins, platelets help to stop bleeding by adhering to the injury site, clumping together (aggregation), and releasing chemical compounds that stimulate further aggregation. These steps result in the formation of a loose platelet plug at the site of the injury. At the same time, platelets support the coagulation cascade, a series of steps that involves the sequential activation of proteins called clotting factors. These processes result in the formation of a clot that remains in place until the injury has healed.