Hematopoieesi

Hematopoieesi, also known as hematopoiesis, is the process by which all blood cells are formed. In humans, formation begins during embryonic development and, after birth, primary sites shift to the bone marrow. Blood cells arise from hematopoietic stem cells (HSCs), which are multipotent and capable of self-renewal. HSCs differentiate into multipotent progenitors, which commit to two main lineages: common myeloid progenitors (CMPs) and common lymphoid progenitors (CLPs). CMPs further give rise to megakaryocyte-erythroid progenitors (MEPs) and granulocyte-monocyte progenitors (GMPs). MEPs produce erythrocytes (red blood cells) and platelets, while GMPs generate granulocytes and monocytes. CLPs differentiate into lymphoid lineages, including T cells, B cells, and natural killer (NK) cells.

Erythropoiesis, thrombopoiesis, and leukopoiesis describe the production of red cells, platelets, and white cells, respectively, and are regulated by a network of growth factors and cytokines. Erythropoietin (EPO), produced mainly in the kidneys in response to hypoxia, stimulates erythroid maturation. Thrombopoietin (TPO) controls platelet production. Additional signals, such as granulocyte-monocyte colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), macrophage colony-stimulating factor (M-CSF), and interleukins, shape lineage commitment and maturation. Key transcription factors, including GATA-1, PU.1, and C/EBP family members, regulate lineage choice and differentiation.

During fetal life, primitive hematopoiesis occurs in the yolk sac, followed by definitive hematopoiesis in regions such as the aorta-gonad-mesonephros region, with migration to the fetal liver and, finally, to the bone marrow, which becomes the adult site of hematopoiesis. Extramedullary hematopoiesis can occur in the liver or spleen under stress. Disorders of hematopoieesi include anemia, leukemias, and bone marrow failure syndromes.