Myeloproliferative cells

The physiologic process of formation of blood cells is known as haematopoiesis. It proceeds through different stages starting from early embryonic life- mesoblastic stage (yolk sac), hepatic stage and myeloid ( bone marrow) stage. During embryonic and early foetal life, haematopoiesis occurs in the yolk sac (only erythroblasts) and the liver ( all blood cells). Some blood cell formation also occurs in the spleen (all blood cells), lymph nodes and thymus (most lymphocytes). Bone marrow starts producing blood cells around 3 to 4 month and by birth becomes the exclusives site of blood cell formation childhood, haematopoiesis becomes restricted to the flat bones such as sternum, ribs, iliac bones and vertebrae and proximal end of long bones. at other skeletal sites haematopoietic areas are replaced by fat cells. However when there is in creased demand for blood cells production, conversion of yellow fatty inactive marrow to red active marrow can occur. In extremely severe cases (severe chronic anaemia) resumption of haematopoitic activity in organs pother than bone marrow such as liver and spleen (extramedullary haematopoiesis) can occur.