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The bone marrow

Bone marrow is the spongy, liquid substance in the centre of some bones. Its main function is to produce blood cells. It constantly produces blood cells to meet the body’s needs.

There are 2 main types of bone marrow. Yellow bone marrow stores fatty tissue. Red bone marrow is where stem cells develop into red blood cells, white blood cells and platelets.

Stem cells

Blood cells are formed through a process called hematopoiesis. All blood cells develop from young, immature cells called stem cells. Stem cells can change into the different types of cells, including blood cells, depending on what the body needs. Stem cells develop and mature into the various types of cells by a process called differentiation.

Stem cells develop into blood cells in the bone marrow. When blood cells are mature and able to function, they leave the bone marrow and move into the blood.

Stem cells can be found in the bone marrow, bloodstream and umbilical cord blood.

  • In adults, most stem cells are found in the bone marrow. The largest numbers of stem cells are in the hips (pelvic bones), ribs, breastbone (sternum), spine and collarbones.
  • The bone marrow has about 10–100 times more stem cells than the bloodstream (peripheral blood stem cells).
  • A smaller number of stem cells are in cord blood. A cord blood stem cell can form more blood cells than a stem cell from adult bone marrow. The number of stem cells harvested from an umbilical cord is usually only enough to do a transplant for a child or very small adult.

Types of blood cells

The 3 main types of blood cells are:

  • Red blood cells (RBCs) carry oxygen to all cells in the body. They are also called erythrocytes.
  • Platelets make the blood clot and help stop bleeding. They are also called thrombocytes.
  • White blood cells (WBCs) defend the body against bacteria and viruses and fight infection. They are also called leukocytes. The main types of white blood cells are:
    • Neutrophils and monocytes are infection-fighting cells that eat (ingest) germs.
    • Lymphocytes are immune system cells that help make antibodies and fight infection.
    • Eosinophils and basophils are involved in allergic responses.


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