Formation of Blood Cells

Hemopoiesis (Haematopoiesis) is the process that produces the formed elements of the blood. Hemopoiesis takes place in the red bone marrow found in the epiphyses of long bones (for example, the humerus and femur), flat bones (ribs and cranial bones), vertebrae, and the pelvis.

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Blood cells are made in the bone marrow. That's a spongy tissue located inside some bones. It contains young parent cells called stem cells.

Within the red bone marrow, hemopoietic stem cells ( hemocytoblasts) divide to produce various “blast” cells. Each of these cells matures and becomes a particular formed element.

These blood-forming stem cells can grow into all 3 types of blood cells – red cells, white cells and platelets. These blood-forming stem cells make copies of themselves, and they also produce mature blood cells. When blood cells are fully mature and functional, they leave the bone marrow and enter the blood. Healthy people have enough stem cells to keep making all the blood cells they need every day.

The process of making blood cells is called hematopoiesis.

blood cell formation, also called hematopoiesis or hemopoiesis, continuous process by which the cellular constituents of blood are replenished as needed. Blood cells are divided into three groups: the red blood cells (erythrocytes), the white blood cells (leukocytes), and the blood platelets (thrombocytes). The white blood cells are subdivided into three broad groups: granulocytes, lymphocytes, and monocytes.

Blood cells do not originate in the bloodstream itself but in specific blood-forming organs, notably the marrow of certain bones. In the human adult, the bone marrow produces all of the red blood cells, 60–70 percent of the white cells (i.e., the granulocytes), and all of the platelets. The lymphatic tissues, particularly the thymus, the spleen, and the lymph nodes, produce the lymphocytes (comprising 20–30 percent of the white cells). The reticuloendothelial tissues of the spleen, liver, lymph nodes, and other organs produce the monocytes (4–8 percent of the white cells). The platelets, which are small cellular fragments rather than complete cells, are formed from bits of the cytoplasm of the giant cells (megakaryocytes) of the bone marrow.

In the human embryo, the first site of blood formation is the yolk sac. Later in embryonic life, the liver becomes the most important red blood cell-forming organ, but it is soon succeeded by the bone marrow, which in adult life is the only source of both red blood cells and the granulocytes. Both the red and white blood cells arise through a series of complex, gradual, and successive transformations from primitive stem cells, which have the ability to form any of the precursors of a blood cell. Precursor cells are stem cells that have developed to the stage where they are committed to forming a particular kind of new blood cell.

In a normal adult the red cells of about half a litre (almost one pint) of blood are produced by the bone marrow every week. Almost 1 percent of the body’s red cells are generated each day, and the balance between red cell production and the removal of aging red cells from the circulation is precisely maintained. The rate of blood cell formation varies depending on the individual, but a typical production might average 200 billion red cells per day, 10 billion white cells per day, and 400 billion platelets per day.

Erythropoiesis

Erythropoiesis (from Greek 'erythro' meaning "red" and 'poiesis' "to make") is the process which produces red blood cells (erythrocytes), which is the development from erythropoietic stem cell for mature red blood cell.

The process of making erythrocytes, begins with the formation of proerythroblasts from hemopoietic stem cells. Over three to five days, several stages of development follow as ribosomes proliferate and hemoglobin is synthesized. Finally, the nucleus is ejected, producing the depression in the center of the cell. Young erythrocytes, called reticulocytes, still containing some ribosomes and endoplasmic reticulum, pass into the bloodstream and develop into mature erythrocytes after another one or two days.

Erythropoietin

Erythropoietin (EPO), a hormone produced mostly by the kidneys, stimulates bone marrow to produce erythrocytes (stimulates erythropoiesis). When inadequate amounts of oxygen are delivered to body cells, a condition called hypoxia, the kidneys increase EPO secretion, which in turn stimulates an increase in erythrocyte production.

The average production rate of erythrocytes in healthy individuals is two million cells per second. Normal production requires adequate amounts of iron, vitamin B ₁₂, and folic acid. Vitamin B ₁₂ and folic acid are necessary for the proper development of DNA in the erythroblasts. This DNA is responsible for the organization of the heme molecule of which iron will become a component. Proper DNA development is also necessary for erythroblast reproduction. A lack of either vitamin B ₁₂ or folic acid can result in pernicious anemia.

Erythrocyte differentiation

In the process of red blood corpuscle maturation, a cell undergoes a series of differentiations. The following stages of development all occur within the bone marrow:

A hemocytoblast, a multipotent hematopoietic stem cell, becomes
a common myeloid progenitor or a multipotent stem cell, and then
a unipotent stem cell, then
a pronormoblast, also commonly called an proerythroblast or a rubriblast.
This becomes a basophilic or early normoblast, also commonly called an erythroblast, then
a polychromatophilic or intermediate normoblast, then
an orthochromatic or late normoblast. At this stage the nucleus is expelled before the cell becomes
a reticulocyte. (These cells still contain RNA and are also called "immature red blood cells")

The cell is released from the bone marrow after Stage 7, and so in newly circulating red blood cells there are about 1% reticulocytes. After one to two days, these ultimately become "erythrocytes" or mature red blood cells.

These stages correspond to specific appearances of the cell when stained with Wright's stain and examined by light microscopy, and correspond to other biochemical changes.

In the process of maturation, a basophilic pronormoblast is converted from a cell with a large nucleus and a volume of 900 fL to an enucleated disc with a volume of 95 fL. By the reticulocyte stage, the cell has extruded its nucleus, but is still capable of producing hemoglobin.

Leukopoiesis

Leukopoiesis, the process of making leukocytes, is stimulated by various colony‐stimulating factors (CSFs), which are hormones produced by mature white blood cells. The development of each kind of white blood cell begins with the division of the hemopoietic stem cells into one of the following “blast” cells:

Myeloblasts divide to form eosinophilic, neutrophilic, or basophilic myelocytes, which lead to the development of the three kinds of granulocytes.

Monoblasts lead to the development of monocytes.

Lymphoblasts lead to the development of lymphocytes.

Thrombopoiesis

Thrombopoiesis, the process of making platelets, begins with the formation of megakaryoblasts from hemopoietic stem cells. The megakaryoblasts divide without cytokinesis to become megakaryocytes, huge cells with a large, multilobed nucleus. The megakaryocytes then fragment into segments as the plasma membrane infolds into the cytoplasm.