Haematopoiesis
Haematopoeisis is the process through which cells circulating in the blood form in the bone marrow. All blood cells start of as haematopoietic stem cells (HSC) that are multipotent and can be differentiate into myeloid or lymphoid lineages. These then further differentiate to produce specialized cells.
Myeloid lineage:
● Erythrocytes, Platelets, Neutrophils, Eosinophil, Basophil/Mast cells (granulocytes), Monocytes, Macrophages
Lymphoid lineage:
● Lymphocytes and natural killer cells
Site of Haematopoeisis:
In the foetus, the location for haematopoeisis changes with development as HSCs migrate to new regions. The flow chart below shows the different regions for haematopoeisis as the foetus develops:
Extraembryonic yolk sac → aorta-gonad-mesonephros → foetal liver → bone marrow
It starts in the extraembryonic yolk sac 17 days post fertilisation. HSCs then migrate to the aorta-gonad-mesonephros followed by the foetal liver and then the bone marrow where they exist in adults.
In adults, haematopoeisis occurs in the red marrow of the bone medullary cavity. Red marrow is mostly found in the axial skeleton of an adult (skull, vertebral column and thoracic cage) and has active haematopoeisis due to the rich blood supply.1,2
Haematopoeisis Process:
The process of haematopoeisis, as shown in the flowchart below, starts off soon post fertilisation and continues throughout life. As the HSC commit to a lineage, they develop from stem cells to multipotent progenitor cells. These then divide into common myeloid progenitor and common lymphoid progenitor to commit to the specific lineages. The common lymphoid lineage goes on to form T lymphocytes, B lymphocytes and natural killer cells. The common myeloid lymphoid further divides into Megakaryocyte-Erythroid progenitor, which develops into erythrocytes and platelets, and Granulocyte Monocyte progenitor which goes on to form eosinophils, neutrophils, monocytes and basophils/mast cells.1,2
A visual detailed summary of the haematopoiesis process is shown below: (This image was created by Servier Medical Art. It was edited to include labels by the HaemHelper team. Link to license: https://creativecommons.org/licenses/by/3.0/legalcode).
The mature cells, as shown in the flowchart, develop from progenitor cells with the aid of extrinsic and intrinsic factors. Intrinsic factors include transcription factors and epigenetic regulation. Extrinsic factors include cellular growth factors and soluble growth factors found in the environment of the bone marrow controlled by stimulus. The following table shows the different cell types along with their growth factors and progenitor cells1,2:
References
1. Jagannathan-Bogdan, M. and I. Zon, L., 2013. Haematopoiesis. Development, 140(12), pp.2463–2467.
2. Hoffbrand V, Moss P. Hoffbrand's essential haematology. Oxford: John Wiley & Sons, Ltd; 2016.