hematopoiesis

Hematopoiesis begins in the fetal yolk sac (weeks 3-8), then shifts to the liver and spleen (6 weeks-birth), and finally to the bone marrow, which becomes the primary site postnatally. In adults, active marrow is concentrated in the axial skeleton (vertebrae, sternum, pelvis, ribs) and proximal long bones. Under stress (e.g., myelofibrosis, severe anemia), extramedullary hematopoiesis can resume in the liver and spleen, causing hepatosplenomegaly.

All blood cells arise from a pluripotent hematopoietic stem cell (HSC), which differentiates into two main progenitor lines: the common myeloid progenitor (giving rise to erythrocytes, platelets, granulocytes, monocytes) and the common lymphoid progenitor (giving rise to T cells, B cells, NK cells). Key growth factors/cytokines regulate this process: erythropoietin (EPO, from kidney) drives erythropoiesis; thrombopoietin (TPO, from liver) drives megakaryocyte/platelet production; G-CSF and GM-CSF stimulate granulocyte production; IL-3 acts on multipotent progenitors.

Clinical relevance: Understanding hematopoiesis is essential for interpreting cytopenias, leukemias (clonal proliferation arrested at specific differentiation stages), and myelodysplastic syndromes. Bone marrow biopsy is used to assess hematopoietic activity in these disorders. Recombinant growth factors (epoetin alfa, filgrastim) are used therapeutically to stimulate specific cell lines in anemia or neutropenia. Myeloablative chemotherapy and radiation destroy HSCs, necessitating bone marrow or stem cell transplantation for recovery. Fanconi anemia, aplastic anemia, and marrow infiltrative processes (fibrosis, metastasis) impair normal hematopoiesis, leading to pancytopenia.