Hematopoiesis is the process by which blood cells are formed. It is a continuous process that maintains an adequate supply of blood cells in the bloodstream during emergencies such as hemorrhage or infection.

In embryonic development, it occurs in the liver and spleen and then in medullary cavity red bone marrow of all bones. With increasing age, the red bone marrow is replaced by adipose tissue. In adults, hematopoietic stem cells are found in the vertebrae, pelvis, sternum, ribs, clavicles, skull, humerus and femoral bone.

In embryonic development:

  • First it occurs in the Yolk Sac
  • Then the Placenta
  • Then it occurs in the Liver and Spleen
  • Then it occurs in the Bone Marrow

Red Marrow (hematopoietic) is replaced by Yellow Marrow in the adult, in the peripheral bones. Hematopoiesis continues to occur in the axial skeleton.

Yellow Marrow may convert back to red marrow in disease or severe blood loss.

Extramedullary Hematopoiesis: Spleen and liver can revert back to being hematopoietic in some pathological conditions.

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