There are a number of changes which can be seen in severe anemias, which
represent immaturity.  When the body recognizes that there is a diffuse
oxygen deprivation, among many other processes, the bone marrow begins to
release immature red cells, which in themselves have diminished oxygen
carrying capacity (but diminished capacity is better than none at all!)
 
Normal RBCs shed their nucleus and the balance of cellular organelles
while still in the bone marrow - this allows them to maximize their
ability to carry oxygen, but limits their life span (you can't reproduce or
effectively repair damage without this machinery).  You can tell that an
animal is anemic by the number of nucleated red blood cells in the blood,
or those with residual organelles (reticulocytes) - these have been kicked
out to early.
 
With kindest regards,
Bruce Williams, dVM
[Posted in FML issue 3398]