Hematopoiesis is the production of blood cells.1
The bone marrow is the site of hematopoiesis
All blood cells arise from pluripotent hematopoietic stem cells (HSCs) in the bone marrow.
These differentiate into two main progenitor lines:
Myeloid lineage: RBCs, platelets, granulocytes, monocytes.
Lymphoid lineage: T-lymphocytes (mature in thymus), B-lymphocytes (mature in marrow/lymphoid tissue), and NK cells.
Controlled by growth factors and cytokines:
Erythropoietin (EPO): from kidney, stimulates RBC formation.
Thrombopoietin (TPO): from liver, stimulates platelet production.
Colony-stimulating factors (CSFs): regulate granulocytes, monocytes.
Interleukins (ILs): support lymphoid differentiation.
At birth: Hemopoiesis is widespread in bone marrow.
By adolescence: Restricted mostly to axial skeleton and proximal long bones.
In aging: Bone marrow cellularity decreases; fatty marrow increases, but stem cell reserves persist.
✅ Summary:
Postnatal hemopoiesis occurs mainly in the bone marrow, where hematopoietic stem cells continuously give rise to myeloid and lymphoid blood cells under the influence of growth factors. In adults, it is confined to the axial skeleton and proximal long bones, but in pathological states, the liver and spleen may reactivate as backup sites.
Would you like me to also compare postnatal hemopoiesis with fetal hemopoiesis (yolk sac → liver/spleen → marrow), so you have the developmental context?