Normochromic macrocytes from a horse with regenerative anemia. The MCV and RDW both were increased due to the presence of these cells. Equine bone marrow does not release reticulocytes in health. Even when a cause of regenerative anemia with erythroid hyperplasia is present, reticulocytes are not consistently released from marrow. A few reticulocytes and nucleated red cells can be found in blood in some cases of very severe, peracute blood loss or hemolytic anemia, but the reticulocyte count is not generally useful in assessing regenerative response to anemia in horses.       The equine marrow, however, will release normochromic macrocytes under these circumstances. Macrocytes, in this context, are young RBCs that are larger than fully matured normal cells, but which stain as mature cells (i.e., are normochromic). These usually are the sole sign of regeneration in anemic horses. When numerous enough, their presence will increase the RDW and MCV above the reference range. Bone marrow examination sometimes is used when peripheral blood changes are less than clear as to the nature of the anemia. An erythroid hyperplasia (and > 5% reticulocytes in the bone marrow) is an expected response to a peripheral anemia.