16.3C: WBC Formation
Haematopoiesis refers to the formation of blood cells components. It is necessary for vertebrate function.
- Describe the formation of leukocytes (white blood cells, or WBCs)
Key Points
- Haematopoietic stem cells are self-renewing and reside in the medulla of the bone ( bone marrow ).
- All blood cells are divided into two main lineages, produced through lymphoid progenitor cells or myeloid progenitor cells depending on lineage type.
- Lymphoid progenitor cells differentiate into B and T cells and NK cells.
- Myeloid progenitor cells differentiate into myelocytes (granulocytes and monocytes) or non-leukocytes such as erythorocytes and megakaryocytes (which produce platelets).
- Before birth, most blood cell formation occurs in the liver or spleen, which tend to enlarge when used for hematopoiesis. In adults, most blood production occurs in the bone marrow.
Key Terms
- myelocyte : A large cell found in bone marrow that becomes a granulocyte or monocyte when mature.
- differentiation : The gradual changes that occur when a cell or tissue type changes into a different type. Cells generally become more specialized the more they differentiate, and are considered to be terminally differentiated when they cannot differentiate (and often cannot divide) any further.
- megakaryocyte : A large cell found in bone marrow, responsible for the production of platelets.
Haematopoiesis refers to the formation of blood cellular components, including both white and red blood cells. All cellular blood components are derived from haematopoietic stem cells located within the bone marrow. In a healthy adult, approximately 10 11 –10 12 new blood cells are produced daily to maintain equilibrium levels in peripheral circulation.
Leukocyte Haematopoiesis
Haematopoietic stem cells (HSCs) reside in the bone marrow and have the unique ability to give rise to all mature blood cell types through differentiation into other progenitor cells. HSCs are self-renewing. When they proliferate, at least some daughter cells remain HSCs, so the pool of stem cells does not become depleted over time. The daughters are the myeloid and lymphoid progenitor cells, which cannot self renew but differentiate into various myeloid leukocytes and lymphocytes respectively. This is one of the body’s vital processes.
Leukocyte Lineages
Two different leukocyte lineages and two non-leukocyte lineages arise from the progeny of HSCs. Following this split in differentiation, the subtypes undergo eventual differentiation into terminally-differentiated leukocytes, which typically do not divide independently.
- The lymphocyte lineage derives from common lymphoid progenitor cells, which in turn become lymphoblasts before differentiating into T cells, B cells, and NK cells.
- Myelocytes are an offshoot of common myeloid progenitor cells, which also differentiate into the erythropoietic and magakaryotic progenitors. This diverse group differentiates into granulocytes and monocytes. Monocytes further differentiate into macrophages or dendritic cells upon reaching certain tissues.
- Megakaryocytes (the cells that produce platelets) and erythrocytes (red blood cells) are not formally considered to be leukocytes, but arise from the common myeloid progenitor cells that produce the other cellular components of blood.
Hematopoiesis in Humans : This diagram shows hematopoiesis as it occurs in humans.
Sites of Haematopoesis in Pre- and Postnatal Periods
In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac called blood islands. However, most of blood supply comes from the mother through the placenta. As development progresses, blood formation occurs primarily in the spleen, liver, and lymph nodes.
When bone marrow develops, it eventually assumes the task of forming most of the blood cells for the entire organism. However, maturation, activation, and some proliferation of lymphoid cells occurs in lymphoid organs (spleen, thymus, and lymph nodes). In children, haematopoiesis occurs in the marrow of the long bones such as the femur and tibia. In adults, it occurs mainly in the pelvis, cranium, vertebrae, and sternum.
In some cases, the liver, thymus, and spleen may resume their haematopoietic function if necessary. This is called extramedullary haematopoiesis. It may cause these organs to hypertrophy and increase in size substantially. During fetal development, the liver functions as the main haematopoetic organ since bones and marrow develop later. Therefore, the liver is enlarged during development relative to its mature proportions.
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