|MadSci Network: Cell Biology|
Remember the human red blood cells doen't always lack a nucleus. Up until the final steps of red blood cell maturation, they do have one.
Monique Stins adds the following:
This is a very interesting question and when I was looking for an answer I found the same question posted on different sites, including an earlier one on the Mad Scientist Network. I consulted some websites (Sue Thornquist of Clinical pathology CVM, www.vet.orst.edu/clinpath/learning/vm736/avianhem.htm) and colleagues. We came up with the following explanation (facts and speculations):
Red blood cells (RBC) or erythrocytes are continually formed in the bone marrow. RBC originate from nucleated stem cells, which mature into nucleated erythroblasts, then differentiate into a-nuclear reticulocytes and finally into RBC. RBC are terminally differentiated cells (they cannot divide anymore) and are shed from the bone marrow into the blood circulation. They live approximately 120-180 days.
In contrast to mammals, RBCs in birds, reptiles and other "lower" vertebrates have a nucleus. The a-nucleated erythrocyte, as it is seen in mammals, is considered more evolutionarily "advanced". (see www.ultranet.com/~jkimball/BiologyPages/V/Vertebrates.html). The lower vertebrates (e.g., birds) are considered earlier on the evolution ladder and have a different circulatory system (see www.sciencenet.org, www.historyoftheuniverse.com/blood.html, http://library.thinkquest.org/3564/lessons/lesson3/lesson3.html ). In addition to the differences in the circulatory system, mammals have smaller end-bloodvessels (capillaries of about 3 micron in diameter) than birds. In order to squeeze through these small blood capillaries, RBC which are about 10 micron in diameter, must be very flexible. The presence of a nucleus would prevent big nucleated RBC to squeeze through these small capillaries. Therefore, during the evolutionary development, nature has found that it was better to get rid of the nucleus and also other cell organelles (e.g., endoplasmic reticulum for protein synthesis) which were not needed for their actual function as oxygen carrier.
Sue Thornquist ( www.vet.orst.edu/clinpath/learning/vm736/avianhem.htm) also thinks that the absence of the nucleus in birds is based on evolutionary differences but she’s not sure whether this theory has been proved. As homeotherms evolved, they had increased oxygen demands due to different metabolic requirements. Birds appear to have adapted to increased oxygen demands by developing a "flow-through" respiratory system (interconnecting tubes for continuous flow, rather than blind-ended alveoli) that's more efficient than mammals'. Mammals may have diverged here and developed anucleated RBC's with increased oxygen carrying capacity to adapt to the increased oxygen demands.
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