Haematopoiesis

Hematopoiesis is the formation of cells in the blood ( blood cells) from blood- cell- forming stem cells and is therefore also known as hematopoiesis. Many blood cells have a limited lifespan (erythrocytes: about 30-120 days, platelets: about 3-10 days), so a constant renewal is required. In an adult human billion mature blood cells are formed, therefore daily.

Spellings

There are four different versions of the word, which are believed to be correct and used in publications: hematopoiesis, haematopoiesis, hemopoiesis, and Hämopoiese. In German, the form without internal -i ( - poiesis ), in English the form with single -i ( - poiesis ) is more usual. In English also is e or ae written instead of ä ( American English: hematopoiesis or British English: haematopoiesis ). The genitive stem hematoma is hemo - preferable.

Description

Embryonic hematopoiesis

The first places the formation of blood that were found in humans and mice, are the so-called blood islands in the yolk sac. As the first mesodermal differentiation initially occur homogeneous cell aggregates, which probably consist of hemangioblasts. The outer cells of these aggregates differentiate to endothelial then, arise inside erythrocytes, so that so filled blood vessels are formed. These early, primitive ' erythrocytes contain nuclei, in contrast to later -occurring in the body. In addition, also arise first megakaryocytes and macrophages. Blood islands are already in the earliest stages of organogenesis, shortly after gastrulation, active. A little later, when the first primitive erythroid progenitor arise in the yolk sac also consistent myeloerythroide precursor, but must migrate to the liver to further differentiate. However, whether arising also true hematopoietic stem cells in the yolk sac, from which all species can arise from blood cells is unclear.

Before the subsequent hematopoietic organs thymus, spleen and bone marrow are formed temporarily takes over the function of the fetal liver as a blood cell- forming organs, in addition to other blood cell types seedless first time, mature ' erythrocytes arise. In the fetal liver hematopoietic stem cells are obtained and propagated, they mature there too, colonization of the bone marrow so that later possible. A new formation takes place but neither here nor in the later hematopoietic organs. All these institutions must therefore be colonized by hematopoietic stem cells, which have been created elsewhere and find through the bloodstream to their target organs.

These stem cells are first formed in the AGM region of the early embryo. The letters stand for aorta, gonads and mesonephros. In this region, which includes the dorsal aorta, the surrounding mesenchyme and the Urogenitalleiste, but no differentiation takes place in subsequent blood cell stages. For the AGM region could be shown that the stem cells are not formed here from hemangioblasts, but from specialized endothelial cells ( hemogenic endothelial cells ) that line the aorta. Hematopoietic stem cells are next formed in the yolk sac. Depending on the education center they appear to be functionally different, since only for those who could be shown from the AGM region, they can also generate lymphoid cells. Therefore, it is discussed that the bone marrow could be settled as a permanent organ of hematopoiesis only of stem cells from the AGM region. For the placenta has been shown in mice that there also arise hematopoietic stem cells. Another possible education center is the allantois.

In the mouse, it comes after 11.5-12.5 days after fertilization to a migration of hematopoietic stem cells from the yolk sac, AGM region and placenta via the bloodstream to the liver. For 5-6 days, this will mean a rapid proliferation of stem cells and to differentiate into various progenitor cells. Only one to two days before the end of the 20 -day gestation migrate both ordinary and progenitor cells to the bone marrow. Shortly thereafter, the stem cells make their division activity largely one and divide only very rarely. In another life, there is a migration of hematopoietic stem cells through the blood vessel system, tissues and back into the bone marrow at a low level.

The fetal spleen as the fetal liver at times a hematopoietic organ in mice from the last third of gestation until several weeks after birth. Here hardly proliferation of hematopoietic stem cells takes place, but rather their differentiation into mature blood cells.

In humans, hematopoiesis takes place until the 3rd month of the embryo in the mesenchyme of the yolk sac instead ( mesoblastic period). From the second month of embryonic hematopoiesis in the fetus passes in the liver from (hepatic period), from the 4th fetal month in the spleen and thymus ( hepatolineale period), from the 6th fetal month in the spleen and bone marrow ( lienomyelopoetische period) from 6th to 7th month, especially in the bone marrow ( myelopoietic period ), except for lymphocytes.

Adult hematopoiesis

The term, adult ', so adult hematopoiesis is misleading insofar as it already takes place from birth. The borders of the embryonic, so prenatal hematopoiesis, the term is, however, introduced and is therefore used here accordingly. After birth, hematopoiesis occurs in the bone marrow ( myelotisches system) and in the lymphatic system. Blood cells arise from stem cells, which pass into the blood after maturation in the bone marrow. A portion of the stem cells is pluripotent, meaning they can mature in both myeloid and lymphoid cells in already further differentiated, however, are determined ( committed) and can only differentiate into one or two specific directions.

Stem cells of different stages can use the on - ( ) or the absence - of different surface markers (proteins that are bound to the outside of the cell membrane ) can be distinguished from each other by immunophenotyping (). The least differentiated stem cells, long -term- repopulating cells ' (long-term repopulating cells ) are characterized, for example, in the mouse through the marker combination KIT Sca -1 CD34 - out. These cells divide only very rarely. You can in the next stage in, short term repopulating cells ' differentiation (c- kit Sca -1 CD34 ), which share frequently. From these, in turn, go, common lymphoid precursors ' (CLP, common lymphoid progenitor ) and common myeloid precursor ' (CMP, common myeloid progenitor ) out.

If immature blood cell precursors into the blood, this will be referred to as core shift.

Blood cell lines

As engraftment is defined as the branch of hematopoiesis, are, platelets, granulocytes, osteoclasts, macrophages, and mast cells from pluripotent stem cells formed in the erythrocytes. The other branch is referred to as hematopoiesis lymphopoiesis.

By type of blood cells formed further distinction:

• Erythropoiesis: formation of erythrocytes
• The thrombopoiesis: formation of platelet the megakaryopoesis: formation of megakaryocytes

• Granulopoiesis: production of granulocytes
• The lymphopoiesis: formation of lymphocytes
• The Monopoese: formation of monocytes

Etymology

The word hematopoiesis is derived from the ancient Greek αἷμα Haima "blood" ( whose tribe αἷματος in the genitive haímatos be seen, so word - formation Haemat ) and ποίησις poiesis " Make ", " activity that produces something " from. Literally, therefore, means hematopoiesis hematopoiesis. The first part of engraftment is due to μυελός myelós "Mark ".