Q: All mammals have non-nucleated RBCs following their birth, with one exception. Name the exception.
A: The newborn kangaroo.
The pouch baby has all the features of an out-of-womb fetus, and like fetuses of other mammals, has RBCs that are nucleated in the early stages (almost 100% on day 1), which then disappear gradually over time (mostly anucleate RBCs by postnatal day 30).
In humans, nRBCs are a feature of newborn blood, though they constitute a mere 500 nRBCs/mm3 or 0.1% of the newborn’s circulating RBCs under normal conditions. The number gradually decreases in the days that follow.
Questions:
1. Is there a selective advantage for having nucleated RBCs in the circulation during development?
2. Does the postnatal reduction in nRBCs reflect the removal of the entire nRBC by the spleen or is there selective removal (pitting) of the nucleus?