22.5: Overview of Hematopoiesis
Hematopoiesis, or blood cell production, is a vital biological process that begins early in embryonic development and continues throughout life. This process generates the various types of cells found in blood, including red blood cells, white blood cells, and platelets from hematopoietic stem cells (HSCs).
Developmental Phases of Hematopoiesis
Initially, HSCs are formed in the embryonic yolk sac, a critical site for early blood cell production. These stem cells subsequently migrate to other fetal tissues such as the liver, spleen, thymus, and lymph nodes. During this phase, the liver becomes the primary site of hematopoiesis, taking over from the yolk sac and significantly expanding the diversity and number of blood cells produced.
After birth, hematopoiesis transitions to the red bone marrow found within the cavities of skeletal bones. In newborns and children, the red bone marrow is active in virtually all bones, facilitating a robust capacity for blood cell production. This marrow consists of a soft reticular connective tissue that provides a supportive matrix for the developing blood cells. Growth factors and cellular signals specific to needs such as infection response or blood loss direct the differentiation of HSCs into the required blood cell types, ensuring physiological needs are met.
As individuals mature, the hematopoietic activity in the long bones gradually declines, with much of the red marrow being replaced by yellow marrow, which is rich in fat cells. Consequently, in adults, active hematopoiesis is predominantly confined to axial skeleton sites like the vertebrae, sternum, ribs, and the proximal ends of the femur and humerus.
Regulation of Hematopoiesis
The regulation of hematopoiesis involves a complex interplay of growth factors, cytokines, and cellular interactions. These regulatory mechanisms ensure a balanced production of different blood cells depending on physiological demands.
