hematopoiesi

Hematopoiesis is the process by which the body's blood cells are produced. In humans, it begins during embryonic development and continues for life, with the bone marrow serving as the principal site of production in postnatal individuals. Primitive hematopoiesis occurs in the yolk sac, providing early erythroid cells, while definitive hematopoiesis arises in the fetal liver and later the spleen before the bone marrow becomes the dominant site around birth. After infancy, hematopoiesis is maintained primarily in the bone marrow, although under certain conditions, such as stress or disease, extramedullary sites like the liver or spleen can resume activity.

Hematopoietic stem cells (HSCs) reside in specialized bone marrow niches and give rise to all blood cell types through a hierarchy. HSCs differentiate into multipotent progenitors, which separate into the common myeloid progenitor (CMP) and the common lymphoid progenitor (CLP). CMPs produce erythrocytes, platelets, granulocytes, and monocytes, while CLPs generate lymphoid cells (B cells, T cells, NK cells). Individual lineages undergo further maturation under the influence of regulatory cytokines and growth factors, such as erythropoietin for red blood cells, thrombopoietin for platelets, granulocyte- and macrophage-stimulating factors, and interleukins, together with transcription factors including GATA1, PU.1, and RUNX1.

Dysregulation of hematopoiesis can lead to anemia, leukemias, and other marrow disorders. Assessment often involves blood counts and bone marrow examination. The process is tightly regulated by the microenvironment, signaling networks, and genetic programs that coordinate the production of the body's blood cells.