Hematopoïesis

Hematopoiesis is the complex process by which all blood cell types are formed. This continuous, regulated production originates from multipotent hematopoietic stem cells (HSCs), which reside primarily in the bone marrow. These stem cells possess the unique ability to self-renew, ensuring a lifelong supply, and to differentiate into committed progenitor cells. These progenitor cells then further mature along specific lineages, giving rise to all the distinct blood cells circulating in the body.

The process is highly organized and can be divided into several key stages. Initially, HSCs give rise to common myeloid progenitors and common lymphoid progenitors. The common myeloid progenitors subsequently differentiate into red blood cells, platelets, granulocytes (such as neutrophils, eosinophils, and basophils), and monocytes. The common lymphoid progenitors develop into lymphocytes, including B cells, T cells, and natural killer cells. This intricate developmental pathway is tightly controlled by a variety of growth factors, cytokines, and signaling molecules that dictate cell survival, proliferation, differentiation, and maturation.

During fetal development, hematopoiesis initially occurs in the yolk sac, then shifts to the liver and spleen, and finally establishes itself in the bone marrow shortly before birth. In adults, the bone marrow remains the principal site of hematopoiesis, although extramedullary hematopoiesis (hematopoiesis outside the bone marrow) can occur in certain pathological conditions. Disruptions in hematopoiesis can lead to a range of blood disorders, including anemia, leukemia, and deficiencies in immune function.