Clinical application of reticulocyte counts in dogs and cats

Reticulocytes are immature red blood cells (RBCs) that contain a reticulum network of RNA, mitochondria, and organelles, which are visible with supravital stains, such as new methylene blue (NMB) or brilliant cresyl blue (BCB) [1–4]. First recognized in 1865 by Erb using acetic acid, reticulocytes were originally named in 1891 by Erhlich using NMB to stain reticulum. In 1932, Heilmeyer and Westhaeuser described the morphology and clinical utility of reticulocytes [3]. Two types of reticulocytes, aggregate and punctate, are observed with supravital stains. The aggregate reticulocyte is larger and more immature with coarsely clumped collections of reticulum. With Romanowsky stains, they appear grayish to bluish pink [1,2]. Aggregate reticulocytes are referred to as polychromatophils or polychromatophilic red cells, and the presence of these cells on a blood smear is called polychromasia. Because aggregate reticulocytes are larger and contain incomplete hemoglobin content, they appear macrocytic on a blood smear (and result in increased mean corpuscular volume [MCV] and decreased mean corpuscular hemoglobin concentration [MCHC]). Despite the decrease in rough endoplasmic reticulum (RER), ribosomes, and mitochondria from the more immature erythroid cells, aggregate reticulocytes are more metabolically active than mature erythrocytes and synthesize up to 20% of the final hemoglobin concentration. Punctate reticulocytes, which stain similar to mature red cells with Romanowsky stains, are smaller and more mature with small sparse granules of residual RNA [1–4].