Hematopoiesis

Hematopoiesis is the continuous production of erythrocytes, platelets, granulocytes, monocytes, and lymphocytes. It begins with multipotent hematopoietic stem cells in the marrow. These cells have two defining properties: self-renewal, which preserves the scenario-cell pool, and differentiation, which generates progenitors committed to particular blood-cell lineages. A stem cell first gives rise to multipotent progenitors that branch broadly into a common myeloid progenitor and a common lymphoid progenitor. The myeloid pathway produces erythrocytes, megakaryocytes and their platelets, granulocytes, and monocytes. The lymphoid pathway produces B cells, T cells, and natural killer cells. The pathway is a useful map, not a rigid switch: the marrow adjusts cell output according to oxygen demand, infection, inflammation, bleeding, and tissue signals. The marrow therefore functions as both a factory and a reserve. A patient may have a normal circulating count for a time despite substantial injury because mature cells remain in the circulation and the marrow has stored precursor cells. Once that reserve is exhausted, more than one cell line may fall.