|MadSci Network: General Biology|
Why birds, amphibians and other animals have nucleated red blood cells may best be answered by a zoologist. I think it may have to do with which animals make cells intravascularly (in the blood stream) as opposed to extravascularly (in the bone marrow or other hematopoietic tissue). All cells need a nucleus for replication and maturation. Red blood cells also have a nucleus during their stages of development. Since this occurs in the bone marrow we do not normally see these nucleated red blood cells in the circulation. As the red blood cell matures and is ready to leave the bone marrow, it extrudes (spits out) its nucleus. The reason for anucleated red blood cells in humans is best explained by understanding its function. Red blood cells contain hemoglobin which carries oxygen to every cell in the body. In humans some of the smallest blood vessels (capillaries) can be so narrow that a nucleated red blood cell would have a difficult time passing through. Even an anucleate red blood cell is larger (8um) than capillaries (2-3um). The red blood cell's shape (a biconcave disc) can best accomplish this feat. More information about human red blood cells can be obtained from any Hematology textbook; found in hospital libraries, medical school book stores and colleges offering clinical laboratory courses.
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