Simple diagram that shows the development of different blood cells from hematopoietic stem cell to mature cells

Haematopoiesis (from Ancient Greek: haima blood; poiesis to make) (or hematopoiesis in the United States; sometimes also haematopoiesis or hemopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. In a healthy adult person, approximately 10¹¹–10¹² new blood cells are produced daily in order to maintain state levels in peripheral circulation.

Hematopoietic stem cells (HSCs)

 

Comprehensive diagram that shows the development of different blood cells from hematopoietic stem cell to mature cells

Hematopoietic stem cells (HSCs) reside in the medulla (bone marrow) and have the unique ability to give rise to all of the different mature blood cell types. 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. The other daughters of HSCs (myeloid and lymphoid progenitor cells), however can each commit to any of alternative differentiation pathways that lead to the production of one or more specific types of blood cells, but cannot self renew. This is one of the vital processes in the body.

Lineages

All blood cells are divided into three lineages.

• Erythroid cells are the oxygen carrying red blood cells. Both reticulocytes and erthroctes are functional and are released into the bllod. In fact, a reticulocyte count estimates the rate or erythropoiesis.
• Lymphoid cells are the cornerstone of the adaptive immune system. They are derived from common lymphoid progenitors. The lymphoid lineage is primarily composed of T-cells and B-cells. (a type of White blood cells)
• Myeloid cells, which include granulocytes, megakaryocytes and macrophages and are derived from common myeloid progenitors, are involved in such diverse roles as innate immunity, adaptive immunity, and blood clotting.

Granulopoiesis (or granulocytopoiesis) is hematopoiesis of granulocytes.

Locations

In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac, called blood islands. As development progresses, blood formation occurs 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 lmphoid cells occurs in secondary lymphoid organs (spleen, thymus, and lymph nodes). In children, hematopoiesis occur 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.

Extramedullary

In some cases, the liver, thymus, and spleen may resume their haematopoietic function, if necessary. This is called extramedullary hematopoieses. It may causes these organs to increase in size substantially. During fetal development, since bones and thus the bone marrow, develop later, the liver functions as the main hematopoietic organ. Therefore, the liver is enlarged during development.

Other vertebrates

In some vertebrates, hematopoiesis can occur wherever there is a loose stroma of connective tissue and slow blood supply, such as the gut, spleen, kidney or ovaries.

Maturation

As a stem cell matures it undergoes changes in gene expression that limit the cell types that it can become and moves it closer to a specific cell type. These changes can often be tracked by monitoring the presence of proteins on the surface of the cell. Each successive change moves the cell closer to the final cell type and further limits its potential to become a different cell type.