Hematopoietic tissue produces all types of blood cells, essential for health. This includes red blood cells for oxygen transport, white blood cells for fighting infections, and platelets for blood clotting. This continuous production is a fundamental biological process throughout life.
Primary Adult Locations
In adults, the primary site for hematopoiesis is the bone marrow. This soft, spongy tissue is found within the hollow spaces of bones. It exists in two forms: red bone marrow and yellow bone marrow. Red bone marrow is active hematopoietic tissue, while yellow bone marrow primarily consists of fat cells and is largely inactive in blood cell production.
Red bone marrow is predominantly located in the axial skeleton, including flat bones like the pelvis, sternum, ribs, vertebrae, and scapulae. It is also found in the ends of long bones, such as the humerus and femur. This tissue contains hematopoietic stem cells, which differentiate into all blood cell types.
The bone marrow provides a microenvironment that supports the production of billions of blood cells daily. This niche is composed of various cell types, including stromal cells, endothelial cells, and sinusoidal capillaries. These components provide necessary signals, growth factors, and structural support for hematopoietic stem cells to self-renew, proliferate, and differentiate into mature blood cells.
Developmental Locations
The location of hematopoietic tissue shifts throughout human development, particularly before birth. Blood cell production begins early in embryonic development, around the third week of gestation, within the yolk sac. The yolk sac primarily produces primitive red blood cells that facilitate oxygen delivery to the growing embryo.
As development progresses, hematopoietic activity transitions to other sites. Around the sixth week of gestation, the fetal liver becomes the main hematopoietic organ. It supports the expansion of hematopoietic stem cells and produces a wide range of blood cell types, including definitive red blood cells, myeloid, and lymphoid precursors.
Later in fetal development, the spleen and bone marrow begin to take over hematopoietic functions. The bone marrow starts producing blood cells around the 16th week of gestation, becoming the primary site by the 25th or 26th week. This progression ensures continuous blood cell supply as the embryo and fetus develop.
Extramedullary Hematopoiesis
Extramedullary hematopoiesis (EMH) refers to blood cell production outside the bone marrow. While normal during embryonic and fetal development, its occurrence in adults indicates an underlying condition where the bone marrow’s capacity is compromised. This can happen in various hematologic disorders, such as severe chronic anemias like thalassemia or myelofibrosis.
In such cases, organs active during fetal development, like the spleen and liver, can reactivate blood-forming capabilities. The spleen and liver become alternative sites for blood cell production as a compensatory mechanism. EMH can also occur in other locations, including lymph nodes, and less commonly, in the lungs or kidneys.