Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made.
Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students.
The gradual changes that occur when a cell or tissue type changes form 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) anymore.
Haematopoiesis refers to the formation of blood cellular components, including both white and red blood cell types. All cellular blood components are derived from haematopoietic stem cells, which are located within the bone marrow. In a healthy adult person, approximately 1011–1012 new blood cells are produced daily in order to maintain equilibrium levels in the peripheral circulation.
Haematopoietic stem cells (HSCs) reside in the medulla of the bone (bone marrow) and have the unique ability to give rise to all of the different mature blood cell types through differentiation into other progenitor cells. HSCs are self-renewing: when they proliferate, at least some of their daughter cells remain as HSCs, so the pool of stem cells does not become depleted over time. The daughters of HSCs are the myeloid and lymphoid progenitor cells, which cannot self-renew themselves, but differentiate into various myeloid leukocytes and lymphocytes respectively. This is one of the vital processes in the body.
There are two different lineages of leukocytes that arise from the progeny of HSCs, as well as two other lineages of non-leukocytes. Following this split in differentiation, the different subtypes undergo eventual differentiation into terminally differentiated leukocytes, which do typically do not divide anymore on their own.
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 group is diverse and 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.
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 the blood supply comes via 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, since bones and thus the bone marrow develop later, the liver functions as the main haematopoetic organ. Therefore, the liver is enlarged during development relative to its mature proportions.