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 3408]