hematopoezy

Hematopoiesis is the physiological process by which all blood cells are produced from hematopoietic stem cells. In adults it occurs primarily in the bone marrow, while during fetal development it takes place first in the yolk sac, then the liver and spleen, before settling in the marrow. The process begins with hematopoietic stem cells, which are multipotent and capable of self-renewal. These cells differentiate into two main lineages: the myeloid lineage and the lymphoid lineage.

The myeloid lineage gives rise to red blood cells (erythrocytes), platelets (thrombocytes) through megakaryocytes, and innate immune cells such as granulocytes (neutrophils, eosinophils, basophils) and monocytes. The lymphoid lineage produces adaptive immune cells, including B cells, T cells, and natural killer (NK) cells. Erythropoiesis, the production of red blood cells, is regulated largely by erythropoietin (EPO) produced in the kidneys in response to hypoxia. Thrombopoiesis, the production of platelets, is driven by thrombopoietin (TPO). Differentiation is guided by a network of growth factors and transcription factors, such as GATA-1 and PU.1, which shape lineage commitment and maturation.

Maturation occurs within specialized bone marrow niches that support progenitor survival, proliferation, and differentiation. Hematopoiesis is tightly coordinated to maintain adequate numbers of mature blood cells for oxygen transport, hemostasis, and immune defense. Disruptions can lead to anemia, leukemias, or marrow failure, while therapeutic approaches include growth factors and bone marrow or stem cell transplantation.