Hematopoesen

Hematopoesen, often spelled hematopoiesis in English, is the biological process by which all cellular components of blood are produced, developed, and maintained. In vertebrates it begins during embryonic life and continues throughout adulthood. Blood cells originate from hematopoietic stem cells (HSCs) in the bone marrow, which can self-renew and differentiate into progenitor cells that commit to either the myeloid or lymphoid lineages.

During development, hematopoiesis shifts sites: it starts in the yolk sac, then the fetal liver and spleen, and ultimately becomes bone marrow–based in the fetus near birth. In adults, the marrow of the axial skeleton and proximal limbs houses most active hematopoiesis, while extramedullary sites such as the liver and spleen can reactivate production under stress or disease.

Lineages: Myeloid differentiation yields erythrocytes, megakaryocytes (platelets), and granulocytes and monocytes; lymphoid differentiation yields T cells, B cells, and natural killer cells. Maturation is guided by cytokines and growth factors, including erythropoietin (red cells), thrombopoietin (platelets), and colony-stimulating factors (GM-CSF, G-CSF, M-CSF), as well as interleukins and signaling pathways that control proliferation and differentiation.

Clinical relevance: Abnormal hematopoiesis can cause anemia, immunodeficiency, or bleeding disorders. Malignant transformation of hematopoietic cells leads to leukemias and myelodysplastic syndromes. Therapies include supportive transfusions, growth factors, and procedures such as bone marrow or stem cell transplantation.