Haematopoiesis is the processes of blood cell production. The processes that regulate haematopoiesis and the early stages of formation of red cells (erythropoiesis), granulocytes and monocytes (myelopoiesis) and platelets (thrombopoiesis) are also discussed (Cantor and Orkin, 2002).
Site of Haematopoiesis
In the first few weeks of gestation, the yolk sac is the main site of blood production.
However, definite haematopoiesis derives from a population of stem cells first observed in the dorsal aorta termed the AGM (aorta-gonads-mesonephros) region.
These common precursors of endothelial and haemopoietic cells (haemangioblasts) are believed to seed the liver, spleen and bone marrow and from 6 weeks until 6-7 months of fetal life the liver and spleen are the major haemopoietic organs and continue to produce blood cells until about 2 weeks after birth (Lensch and Daley, 2006).
The bone marrow is the most important site from 6 – 7 months of fetal life. During normal child hood and adult life, the marrow is the only source of new blood cells. The developing cells are situated outside the bone marrow sinuses and mature cells are released into the sinus spaces, the marrow microcirculation and also into the general circulation.
In infancy all, the bone marrow is haemopoietic but during childhood there is progressive fatty replacement of marrow throughout the long bones, so that in adult life haemopoietic marrow is confined to the central skeleton and proximal ends of the femurs and humeri. The remaining fatty marrow is capable of reversion to haemopoiesis and in many disease there is also expansion of haemopoiesis down the long bones. Moreover, the liver and spleen can resume their fatal haemopoietic role (extramedullary haemopoiesis) (Taichman, 2005).
Hematopoiesis: Haemopoietic Stem and progenitor Cells
Haemopoiesis starts with a pluripotential stem cell that can give rise to the separate cell lineages (Kaush an Sky 2006). This haemopoietic stem cell is rare; perhaps I in every 20 million nucleated cells in bone marrow. Although its exact phenotype is unknown, on immunological testing, it is CD34+, CD38- and has the appearance of a small or medium sized lymphocyte. Cell differentiation occurs from the stem cell via the committed haemopoietic progenitors which are restricted in their developmental potential. (fig 1.2).
An example is the earliest detectable mixed myeloid precursor which gives rise to granulocytes, erythrocytes, monocytes and megakaryocytes and is termed CFU (colony forming unit).
The bone marrow is also the primary site of origin of lymphocytes which differentiate from a common lymphoid precursor. The stem cell has the capability for self renewal so that marrow cellularity remains constant on a normal healthy steady state. There is considerable amplification in the system: One stem cell is capable of producing about 106 mature blood cells.
Diagrammatic representation of the bone marrow pluripotent stem cell and the cell lines that arise from it. Various progenitor cells can be identified by culture in semi-solid medium by the type of colony they form. Baso, basophil, BFU, burst-forming unit: CFU, colony-forming unit, E, erythroid: eosinophil, GEMM, granulocyte, erythroid, monocyte and megakaryocyte, GM granulocyte, monocyte, Meg, megakaryocyte; NK, natural killer.