In adults, erythropoiesis begins in which location?

• A. Spleen
• B. Liver
• C. Red bone marrow
• D. Thymus

Answer: (D)

Erythropoiesis in adults happens in red bone marrow, the hematopoietic tissue within the medullary cavities of bones. This marrow is especially active in the axial skeleton—places like the vertebrae, pelvis, sternum, and ribs—and in the proximal ends of long bones in children; in adults, its activity is more localized but still in red marrow. The thymus, by contrast, is the site where T lymphocytes mature, not red blood cells. The liver and spleen can support blood cell production during fetal development and in certain disease states, but they are not the normal site of erythropoiesis in a healthy adult. So the best location is red bone marrow.