MEDizzy - A 54-year-old man presents with fatigue, weakness, and

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hematology

A 54-year-old man presents with fatigue, weakness, and dyspnea on exertion. Physical examination reveals conjunctival pallor, palatal petechiae, and splenomegaly. A CBC reveals profound anemia and thrombocytopenia. A bone marrow biopsy reveals agnogenic myeloid metaplasia (myelofibrosis). The patient’s splenomegaly is attributed to increased production of blood cells in the spleen. Which of the following terms indicates blood cell production outside the bone marrow in the spleen, liver, and other locations?

ExplanationHematopoiesis begins in the fetal yolk sac and later occurs predominantly in the liver and the spleen. Recent studies demonstrate that islands of hematopoiesis develop in these tissues from hemangioblasts, which are the common progenitors for both hematopoietic and endothelial cells. These islands then involute as the marrow becomes the primary site for blood cell formation by the seventh month of fetal development. Barring serious damage, such as that which occurs with myelofibrosis or radiation injury, the bone marrow remains the site of blood cell formation throughout the rest of life. In childhood, there is active hematopoiesis in the marrow spaces of the central axial skeleton (i.e., the ribs, vertebrae, and pelvis) and the extremities, extending to the wrists, the ankles, and the calvaria. With normal growth and development, hematopoiesis gradually withdraws from the periphery. This change is reversible, however; distal marrow extension can result from intensive stimulation, as occurs with severe hemolytic anemias, long-term administration of hematopoietic growth factors, and hematologic malignancies. The term medullary hematopoiesis refers to the production of blood cells in the bone marrow; the term extramedullary hematopoiesis indicates blood cell production outside the marrow in the spleen, liver, and other locations.