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What is bone marrow?

Bone marrow is a soft fatty tissue found inside the central spongy part of bones throughout the skeleton, and is the body’s primary producer of white blood cells, red blood cells and platelets.

The bone marrow contains hematopoietic stem cells, which are immature cells that can differentiate and mature into the different types of white blood cells, red blood cells, and platelets as needed. This process takes 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 blood.

The Blood Cells

White Blood Cells (WBCs)
There are five different types of white blood cells:

  • Lymphocytes
  • Neutrophils
  • Eosinophils
  • Basophils
  • Monocytes

Each plays a different role in protecting the body from infection.

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 its own cells and foreign cells like bacteria. B cells circulate in the blood and produce antibodies, immune proteins that target and attach to specific antigens.

Neutrophils, basophils, and eosinophils kill and digest bacteria. Monocytes, which live much longer than neutrophils, also ingest and digest bacteria.

Red Blood Cells (RBCs)
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 as well as globin proteins. There must be a sufficient amount of iron available in the bone marrow for normal hemoglobin and RBC production.

Platelets (Thrombocytes)
Platelets, also called thrombocytes, are cytoplasm fragments of very large cells 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.

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(Apr 25, 2016) Nagalla S. Bone Marrow Failure. Medscape Reference. Available online at https://emedicine.medscape.com/article/199003-overview. Accessed March 2019.

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