We are going to look at hematopoiesis, the process of creating the blood cells and platelets that we rely on every day. Where and how this process occurs will be described in this lesson.
Uncontrollable bleeding, dead internal organs and continuous bacterial infections: that would be the state of our bodies if we didn’t have blood cells.
Lucky for us, we do! They carry oxygen, defend the body against pathogens, and stop bleeding.Blood cells are created throughout our life time in order to make sure that we have healthy ones at all times. Can you imagine how worn out our blood cells would be if the ones we were born with were still working in our bodies? New blood cells are created at the same rate as the ones that die.The process of producing new blood cells is called hematopoiesis. Hematopoiesis started taking place long before you ever able to read this lesson – it began when you were a developing fetus inside of your mom. The process took place in your liver and spleen during that time.
Once you were born – and ever since – it takes place in your bone marrow, found at the ends of your long bones.
Interestingly, hematopoiesis forms three different types of blood cells from the same beginning cell, which is able to become more than one type of a cell. Cells with that ability are called stem cells. The beginning stem cell in hematopoiesis is classified as a hematopoietic stem cell, meaning it is a stem cell that can become any type of blood cell, and it’s called a hemocytoblast. The process to make each type of blood cell is different.
Erythrocytes, our red blood cells, are by far the most numerous of the blood cells. Our bodies work best when we have between 4.2 and 5.2 million red blood cells per mcL of blood. If the number of red blood cells drops too low, then our blood oxygen level will also drop since red blood cells carry oxygen. The kidneys will detect this drop and release erythropoietin, a protein that prompts red blood cell production, or erythropoiesis.
Erythropoiesis begins with the hemocytoblast that divides to make one hemocytoblast and one cell that will begin to differentiate by becoming a proerythroblast, which is a slightly differentiated erythrocyte. The proerythroblast will get smaller as it continues to differentiate, losing its organelles during the process.The proerythroblast will enter the blood from the bone marrow and become a reticulocyte, which is a premature erythrocyte that does not have a nucleus, but still has small amounts of organelles. Within two days in the blood, the reticulocyte will eject the rest of the organelles, display hemoglobin on its surface and finally become a mature red blood cell.
The process of producing white blood cells is called leukopoiesis, which makes sense when you think about the fact that the formal name for a white blood cell is leukocyte. We should have around 5,000 to 9,000 leukocytes per mcL of blood to defend our bodies against pathogens. However, not all leukocytes are the same, and so the process to produce them is also not the same.
The large white blood cell with an oval nucleus that carries out phagocytosis is called a monocyte. The hemocytoblast will divide into another hemocytoblast and a myeloid progenitor cell, which is a cell that can become one of two different leukocytes. It continues to differentiate and decrease in size until it becomes a monoblast, an immature monocyte. It will remain in the bone marrow until it becomes a mature monocyte, then be released into the blood.
A granulocyte, a leukocyte that has granules in its cytoplasm, also develops from a myeloid progenitor cell.
That cell will differentiate into a myeloblast, which is a slightly differentiated granulocyte. It will begin to develop granules in the cytoplasm, at which point it can become either a promyelocyte, which is a neutrophil precursor, immature basophil, or immature eosinophil. The promyelocyte will develop a multi-lobed nucleus and mature into a neutrophil. The immature basophil will develop a large nucleus and become a mature basophil. The immature eosinophil will develop a 2-lobed nucleus and become a mature eosinophil. Each of these are released as mature cells into the blood.
Lymphocytes are small white blood cells that contain one large nucleus.
The hemocytoblast will divide and begin to differentiate to produce a lymphoblast, the lymphocyte precursor cell. That cell has one of a few options from that point. It can develop granules and become a natural killer cell, a lymphocyte that can kill tumor or virus-infected cells by injecting granules into them. Or it can become a pro-B cell and continue to differentiate into a B cell.
B cells are lymphocytes that produce antibodies; they don’t mature until they go through several processes after leaving the bone marrow and entering the spleen. The spleen will release them once they are fully mature.The last option is for the lymphoblast to exit the bone marrow and go to the thymus where it will become a pro T cell and further differentiate into a T cell, a leukocyte that actively carries out the immune response.
At this point it will go through a differentiation and maturation process in the thymus that will ultimately determine whether it will become a CD4+ or CD8+ T cell.
It’s time to produce the cells that stop us from bleeding after an injury. Thrombocytes, often called platelets, are fragments of a large cell. You should note that platelets are not blood cells, but they form from a hemocytoblast like blood cells. As a matter of fact, they are derived from the same myeloid progenitor cell as monocytes and granulocytes. The myeloid progenitor cell will become a very large, granule-filled cell called a megakaryocyte. The cell membrane will fragment off with granules inside.
These are the platelets and the process of their production is called thrombopoiesis.
We covered a lot, so here is a quick recap of the main points from this lesson. The process of producing new blood cells is called hematopoiesis.
Cells that can become more than one type of cell are called stem cells. The beginning stem cell for hematopoiesis is a hematopoietic stem cell. The hematopoietic stem cell is called a hemocytoblast.Erythropoiesis includes:
- Erythrocytes– red blood cells
- Erythropoietin– protein that prompts red blood cell production
- Erythropoiesis – process of producing red blood cells
- Proerythroblast – a slightly differentiated erythrocyte
- Reticulocyte – a premature erythrocyte
- Leukopoiesis– process of producing white blood cells
- Leukocyte – white blood cell
Monocyte production includes:
- Monocyte – large phagocytic white blood cell with an oval nucleus
- Myeloid progenitor cell – cell that can become any leukocyte except the lymphocytes
- Monoblast – immature monocyte
Granulocyte production includes:
- Granulocyte – granular leukocyte
- Myeloblast – slightly differentiated granulocyte
- Promyelocyte – a neutrophil precursor
Lymphocyte production includes:
- Lymphocytes – small white blood cells that contain one large nucleus
- Lymphoblast – a lymphocyte precursor cell
- Natural Killer cell – lymphocyte that can kill tumor or virus-infected cells using granules
- B cells – lymphocytes that produce antibodies
- T cell – leukocyte that carries out the immune response
- Thrombocytes, also called platelets – fragments of a large cell
- Megakaryocyte – very large, granule filled cell
- Thrombopoiesis – production of platelets