Hematopoiesis

What is hematopoiesis?

Hematopoiesis (pronounced “heh-ma-tuh-poy-EE-sus”) is blood cell production. Your body continually makes new blood cells to replace old blood cells so you have a steady blood supply. Hematopoiesis starts before birth and continues as a cycle throughout life.

It’s easier to remember what hematopoiesis is when you consider its roots. Hematopoiesis is derived from two Greek words:

• Haîma: Blood.
• Poiēsis: To make something.

Put these words together, and you get hematopoiesis, the process of making blood. Hematopoiesis is also called hemopoiesis, hematogenesis and hemogenesis.

What blood cells get made during hematopoiesis?

Your blood cells are the building blocks of your blood. Hematopoiesis includes the production of all blood cell types, including:

Red blood cells (erythrocytes)

Red blood cells, or erythrocytes, carry oxygen from your lungs to organs throughout your body. They also carry carbon dioxide to your lungs so you can get rid of it by exhaling it. Your blood has more red blood cells than any other type of blood cell. The production of red blood cells is called erythropoiesis.

White blood cells (leukocytes)

White blood cells, or leukocytes, fight infection and protect your body from harmful invaders, or germs. They also destroy abnormal cells. The production of white blood cells is called leukopoiesis.

Broadly, the types of white blood cells are:

• Neutrophils.
• Basophils.
• Eosinophils.
• Monocytes.
• Lymphocytes (B-cells, T-cells and natural killer cells).

Neutrophils, basophils and eosinophils have similar functions and can be grouped together and called granulocytes. The other types of white blood cells are monocytes and lymphocytes.

Platelets (thrombocytes)

Platelets, or thrombocytes, are sticky cell fragments that clump together to form a clot if you’re injured. They create a seal in damaged tissue that prevents you from losing too much blood. The production of platelets is called thrombopoiesis.

Where does hematopoiesis occur?

The most common site of blood cell production is the spongy tissue inside of your bones called bone marrow. Hematopoiesis that occurs in your bone marrow is called medullary hematopoiesis. Blood cells get made in your bone marrow and released into your bloodstream.

Less often, hematopoiesis takes place in other parts of your body, like your liver and spleen. Hematopoiesis that occurs outside of your bone marrow is called extramedullary hematopoiesis.

Hematopoiesis takes place in different locations before birth than it does after you’re born.

Before birth

Blood cell production starts when you’re still in the uterus. It begins in the yolk sac, a structure that surrounds an embryo at the beginning of pregnancy. Toward the end of pregnancy, most blood cell production happens in your bone marrow. Key milestones in hematopoiesis during pregnancy are:

• Week 3: A type of red blood cell slightly less developed than the red blood cells that get made during adulthood is made in the yolk sac.
• Months 2 & 3: Red blood cells and platelets get made in your liver and spleen. White blood cells get made in your liver, spleen and thymus.
• Month 5: Most blood cell production happens in your bone marrow. The thymus, spleen and other lymph tissue also make some types of white blood cells.

After birth

Most blood cell production happens in your bone marrow from infancy and into adulthood. Certain types of white blood cells called lymphocytes develop in your thymus, too.

Disease creates the exception. If you have a condition that prevents your bone marrow from making enough blood cells, hematopoiesis may shift to your blood cell production sites before birth. Blood cell production may shift to your liver, spleen or lymph nodes.

What happens during hematopoiesis?

Hematopoiesis begins with an originator cell common to all blood cell types. It’s called a hematopoietic stem cell (HSC). An HSC develops into a precursor cell, or “blast” cell. A precursor cell is on track to become a specific type of blood cell, but it’s still in the early stages. A precursor cell goes through several cell divisions and changes before it becomes a fully mature blood cell.

The specific types of hematopoiesis include:

• Erythropoiesis: Red blood cell production.
• Leukopoiesis: White blood cell production.
• Thrombopoiesis: Platelet production.

With each change, an originator cell becomes more specialized — less like a stem cell and more like a red blood cell, white blood cell or platelet.

What happens during red blood cell production (erythropoiesis)?

Red blood cell production occurs in your bone marrow. An HSC matures into a precursor cell called an erythroblast. An erythroblast becomes an immature red blood cell called a reticulocyte. Finally, a reticulocyte develops into a mature red blood cell.

What happens during white blood cell production (leukopoiesis)?

Although they’re all white blood cells, granulocytes (basophils, eosinophils and neutrophils) have slightly different origins from monocytes and lymphocytes. In addition, monocytes and lymphocytes follow different development paths.

Granulocyte production

Granulocytes (basophils, eosinophils and neutrophils) get made in your bone marrow. An HSC follows a development path called the myeloid cell line for granulocyte production. The word “myeloid” means “relating to the bone marrow” — where granulocytes get made.

For granulocytes to form, an HSC becomes a precursor cell called a myeloblast. A myeloblast forms a myelocyte, which later becomes a basophil, eosinophil or neutrophil.

Mononuclear cell production

Monocytes get made in your bone marrow, while lymphocytes (B-cells, T-cells and natural killer cells) get made in your bone marrow and other lymph tissue. For instance, very young forms of lymphocytes develop in the bone marrow and then travel to the thymus, where they will mature and develop into T-cell lymphocytes.

• Monocytes develop along the myeloid cell line: Monocytes originate in your bone marrow, which means that they’re part of the myeloid cell line. They become a precursor cell called a monoblast before maturing into a fully developed monocyte.
• Lymphocytes develop along the lymphoid cell line: Lymphocytes originate in lymph tissue, including bone marrow and other lymph tissue, like your thymus. As a result, HSCs that eventually become lymphocytes develop along the lymphoid cell line. HSCs become precursor cells called lymphoblasts. They eventually differentiate into T-cells, B-cells or natural killer cells.

Platelet production (thrombopoiesis)

Platelet production occurs in your bone marrow, where an HSC matures into a precursor cell called a megakaryoblast. The megakaryoblast becomes a megakaryocyte. Fragments of the megakaryocyte break off, becoming platelets.

How long does hematopoiesis take?

The rate of blood cell production depends on your needs. For example, your body may increase white blood cell production to fight germs if you’re sick. In general, hematopoiesis lasts as long as the life cycle of a blood cell. Once it’s time for a blood cell to die, a healthy body has made a new one to replace it.

On average:

• Red blood cells survive for about 120 days.
• White blood cells survive from a few hours to a few days.
• Platelets survive for about five to nine days.

What happens when there’s a problem with hematopoiesis?

Your body regulates blood cell production so that you have just the right amount of blood cells. Certain conditions can interfere with hematopoiesis, causing you to have too few or too many blood cells. Having abnormal amounts of blood cells can cause a range of symptoms and conditions.

• Red blood cells: Having too few red blood cells is called anemia. You may feel tired or weak because your body’s tissues aren’t receiving enough oxygen. Having too many blood cells is called erythrocytosis. Mild erythrocytosis may cause unpleasant symptoms. In more serious cases, too many red blood cells can cause your blood to become too thick and increase your risk of a heart attack or stroke.
• White blood cells: Having too few white blood cells is called leukopenia. A low white blood cell count raises your risk of infections. Having too many white blood cells is called leukocytosis. It’s usually a sign of an infection. A high white blood cell count may indicate a blood disorder or cancer.
• Platelets: Having too few platelets is called thrombocytopenia. This condition may put you at risk of prolonged bleeding and increased bruising. Having too many platelets is called thrombocytosis. It can put you at risk of developing serious blood clots.

What conditions interfere with hematopoiesis?

Multiple factors and conditions can disrupt hematopoiesis, with effects ranging from mild to severe. For instance, as you age, fat deposits can collect in your bone marrow so there’s less room for hematopoiesis.

Blood disorders and blood cancers, like leukemia, lymphoma and myeloma, can interfere with blood cell production. They may cause you to have too many sick blood cells that don’t function correctly.

Also, some medications can interfere with hematopoiesis, leading to low blood cell counts. For instance, chemotherapy kills cancer cells, but it also may lower your white blood cells (neutropenia). Low blood cell counts may be a side effect of taking certain medications.

A note from Cleveland Clinic

Hematopoiesis is a common, ongoing process essential for your health and survival. Many factors that compromise your health can also influence your body’s ability to make blood cells. A process that takes place (for the most part) in your bone marrow maintains the steady blood supply that keeps your tissues oxygenated and your body infection-free.